SCIENTIFIC PAPER

Photon activation therapy: a Monte Carlo study on doseenhancement by various sources and activation media

Mahdi Bakhshabadi • Mahdi Ghorbani •

Ali Soleimani Meigooni

Received: 13 November 2012 / Accepted: 28 July 2013 / Published online: 11 August 2013

� Australasian College of Physical Scientists and Engineers in Medicine 2013

Abstract In the present study, a number of brachytherapy

sources and activation media were simulated using

MCNPX code and the results were analyzed based on the

dose enhancement factor values. Furthermore, two new

brachytherapy sources (131Cs and a hypothetical 170Tm)

were evaluated for their application in photon activation

therapy (PAT). 125I, 103Pd, 131Cs and hypothetical 170Tm

brachytherapy sources were simulated in water and their

dose rate constant and the radial dose functions were

compared with previously published data. The sources

were then simulated in a soft tissue phantom which was

composed of Ag, I, Pt or Au as activation media uniformly

distributed in the tumour volume. These simulations were

performed using the MCNPX code, and dose enhancement

factor (DEF) was obtained for 7, 18 and 30 mg/ml con-

centrations of the activation media. Each source, activation

medium and concentration was evaluated separately in a

separate simulation. The calculated dose rate constant and

radial dose functions were in agreement with the published

data for the aforementioned sources. The maximum DEF

was found to be 5.58 for a combination of the 170Tm source

with 30 mg/ml concentration of I. The DEFs for 131Cs and170Tm sources for all the four activation media were higher

than those for other sources and activation media. From

this point of view, these two sources can be more useful in

photon activation therapy with photon emitter sources.

Furthermore, 131Cs and 170Tm brachytherapy sources can

be proposed as new options for use in the field of PAT.

Keywords Brachytherapy � Photon activation

therapy � Dose enhancement � TG-43 parameters

Introduction

In photon activation therapy (PAT), the energy deposition

within a tumour may be increased by provoking the

emission of Auger electrons through photoelectric effect,

by addition of high Z elements [1]. The binding energy of

the activation element within the tumour is closely matched

with the incident photon energy to maximize the produc-

tion of free electrons and, subsequently, enhancement of

the absorption energy [2].

When an atom is ionized, the excited atom returns to

stable state via radiative and nonradiative processes. The

radiative processes result in emission of characteristic

X-ray photons, while the nonradiative transitions result in

ejection of atomic electrons, via Auger, Coster–Kronig and

super Coster–Kronig processes. The ejected electrons

through nonradiative processes, are denoted as Auger

electrons. Furthermore, physical decay of the radionuclide

by either orbital electron capture or internal conversion will

also result in inner atomic shell ionization. Such radio-

nuclides are known as Auger emitters, of which 125I is one

that is commonly used in brachytherapy [3].

For photon activation therapy, it is feasible to use var-

ious sources and activation media. There are studies in

which the synchrotron X-rays were used for the purpose of

photon activation therapy [4–6]. The possibility of tuning

M. Bakhshabadi

North Khorasan University of Medical Sciences, Bojnurd, Iran

M. Ghorbani (&)

Medical Physics Department, Faculty of Medicine, Mashhad

University of Medical Sciences, Pardis-e-Daneshgah, Vakil

Abad Blvd., 9177948564 Mashhad, Iran

e-mail: mhdghorbani@gmail.com

A. S. Meigooni

Comprehensive Cancer Centers of Nevada, Las Vegas, NV, USA

123

Australas Phys Eng Sci Med (2013) 36:301–311

DOI 10.1007/s13246-013-0214-0

the energy of X-rays to the K-edge energy make a syn-

chrotron a great tool for photon activation therapy. How-

ever, synchrotrons are not as widely available as other

radiation sources [7]. Brachytherapy sources may be an

alternative for photon activation therapy. Since brachy-

therapy provides much greater localization of radiation

dose to the tumour, compared to external beam therapy

sources, use of brachytherapy sources in photon activation

therapy can decrease normal tissue radiation dose, and

therefore improve the therapeutic outcomes compared to

external radiotherapy sources [1].

In a study by Nath et al. [8], the dependence of IUdR

radiosensitization on photon energy was investigated

through an in vitro study irradiating Chinese hamster cells at

dose rate of 0.72 Gy/h. IUdR radiosensitization was com-

pared for 226Ra, 241Am and 125I sources. The results have

indicated that IUdR produces significant radiosensitization

with all three sources at 0.72 Gy/h irradiation dose rate. This

is due to an increase in IUdR radiosensitization just above the

K-edge energy of iodine atoms with photon induced Auger

electrons. Greater radiosensitization was observed for 241Am

photons compared to 226Ra on the higher energy range and to125I on the lower energy range. The results suggested that a

combination of IUdR with low energy emitting sources may

have a clinical potential for photon activation therapy.

In a photon activation therapy study by Young et al. [2], a

dose enhancement by silver tetraphenyl sulfonato porphyrin

(AgTPPS4) agent with an 125I brachytherapy source was

quantified. The dose in the tumour volume was calculated

for various concentrations of AgTPPS4, ranging

0–20 mmol/kg, using the EGS4 Monte Carlo code. There

was a linear increase in tumour dose with corresponding

increase in AgTPPS4 concentration. In vitro toxicity studies

have shown that TPPS4 porphyrin derivatives were cyto-

toxic at concentration levels needed to have significant

brachytherapy dose enhancement. However, the toxicity

effect was not due to presence of silver atoms in the

structure of AgTPPS4 Ranjbar et al. [9], have investigated

the dose enhancement by gold nanoparticles when the tissue

is irradiated by a monoenergetic photon beam. MCNPX

code was used for this purpose and a phantom model was

developed and verified by comparison of the obtained depth

dose data with corresponding published values. Following

this verification, four various concentrations of gold nano-

particles were introduced in the tumour inside a phantom.

Monoenergetic synchrotron X-rays were simulated as the

irradiation source. The optimum energy for dose enhance-

ment was found to be around 83–90 keV for all concen-

trations. Dose enhancement factor was increased linearly

with concentration of the activation elements. In a study by

Karnas et al. [10], DNA damage due to Auger electrons

from 127I atoms, when irradiated by external X-rays was

investigated. In this study DNA double-strand breaks

(DSBs) were evaluated by Monte Carlo simulations and the

results were compared with measured data. The results of

Monte Carlo simulations were in good agreement with

measurements using comet assay. The results have shown

that the Auger electrons produced approximately 0.03

DSBs per vacancy in N-shell and 0.3 DSBs per vacancy in

K-shell or L-shell. It was concluded that Auger electrons

from iodine produced a modest increase in the number of

strand breaks. The order of the increase was 10 % but the

DSBs were very difficult to be repaired or potentially prone

to misrepair because of their complex nature. Based on the

results of this study, it was proposed that the DNA damage

may have consequences in cell survival and will be

exploitable in photon activation therapy of tumours sensi-

tized with iodine using kilovoltage X-rays. In a different

study by Moiseenko et al. [11], nucleosomal DNA damage

from Auger electrons due to the incorporation of IUdR was

modeled by Monte Carlo simulations. DSB production

following vacancy in various atomic shells of iodine was

estimated. The results have also indicated that the proba-

bility of an Auger electron cascade leading to at least one

DSB depends on the shell of initial vacancy production. The

probability was approximately equal to 0.35 for K and L

shells and 0.02 for the N shell. Furthermore, DSBs were

predominantly produced in a nucleosome containing iodine.

Approximately 14 % of DSBs were produced due to a

vacancy in the L1 orbital. The results provide a basis for

photon activation therapy using kilovoltage X-rays when

the tumour is sensitized by IUdR. In another study, as a Ph.

D. thesis research by Stephens [12], radioactive 161Ho has

been successfully created and characterized for photon

activation therapy. This radioisotope emits K-edge X-rays

of 45 and 52 keV which are just above the K-edge of iodine

(33 keV). In this research the survival of human colorectal

cancer (HT-29) cells with and without presence of IUdR

was evaluated following irradiation of the cells with pho-

tons emitted by 137Cs, 125I and 161Ho radioactive sources

and 300 kVp X-rays. The results have indicated that full

efficacy of a combination of using 161Ho source and IUdR

activation medium, which was not exploited for many

years, can be realized. Furthermore, in vivo and clinical

studies on this combination have shown the potential to

save lives and thus are of importance for consideration. In a

study by Young et al. [13], the tumour dose was enhanced

while minimizing the absorbed dose by the surrounding

normal tissues. To achieve this goal a 125I brachytherapy

source was used. The number of photoelectric interactions

within the tumour was increased through the introduction of

a silver compound to take advantage of the K-edge effect.

X-ray fluorescence excitation spectroscopy method and

clinically accepted calculation techniques were utilized to

estimate the absorbed dose enhancement by the presence of

7.5 mM of silver in a water phantom. There was excellent

302 Australas Phys Eng Sci Med (2013) 36:301–311

123

agreement between the Monte Carlo and experimental

results. The results of this study have indicated that the

K-edge enhancement effect in brachytherapy with 125I

sources is possible with further development of nontoxic

silver compounds.

As previously mentioned, there are various Monte Carlo

and cellular studies on photon activation therapy by various

radiation sources and different activation media. However,

to the best of our knowledge, in most of these studies, a single

source and no activation medium was evaluated and, there is

no consensus study comparing various combinations of

radiation sources and activation media. In the present study,

various brachytherapy sources and activation media com-

binations were simulated by MCNPX code and the results of

dose enhancement factors were compared with the aim of

selecting the best source and media combination. Further-

more, in the process 131Cs and 170Tm sources were added to

the list of the sources used in photon activation therapy.

Materials and methods

Radioactive sources

Three brachytherapy sources and a hypothetical source

were evaluated in the present study: 125I, 103Pd, 131Cs and

hypothetical 170Tm source. 125I and 103Pd nuclides have

been previously used in photon activation therapy studies

[1, 8]. However, the 131Cs and 170Tm sources are presented

in this study as new options for photon activation therapy.

The physical characteristics and the source models used in

this study are listed in Table 1.

A schematic diagram of the 125I, 103Pd, 131Cs and

hypothetical 170Tm sources used in this study are illustrated

in Fig. 1. In this figure, the dimensions of various sources’

parts including the active core, encapsulation and source

cable were mentioned.

We have used an IsoAid Advantage 125I source (model

IA1-125A) in our simulations. This source model includes

a cylindrical silver X-ray marker with 3 mm length and

0.5 mm diameter which is coated with AgI within

0.001 mm thick. The AgI coating includes 125I isotope

uniformly distributed on the cylindrical surface of the

marker as well as two ends. The marker and coating

assembly is encapsulation within 0.05 mm thickness tita-

nium. The overall length and diameter of the source is 4.5

and 0.8 mm respectively [14, 18].

The IsoAid Advantage 103Pd source (model IAPd-103A)

is composed of a cylindrical silver X-ray marker with

1.25 mm length and 0.5 mm diameter. Two spherical

pellets made of polystyrene with 0.5 mm diameters and

mass density of 1.2 g/cm3 are placed at each end of the

Table 1 Characteristics of the

four source models used in this

study

Source type Half-life (days) Source model Reference

125I 59.40 IsoAid Advantages (model IA1-125A) Meigooni et al. [14]103Pd 16.991 IsoAid Advantages (model IAPd-103A) Meigooni et al. [15]131Cs 9.7 IsoRay (model CS-1) Murphy et al. [16]170Tm 128.6 Flexisource (hypothetical) Ballester et al. [17]

Fig. 1 Geometry of the four source models used in this study: a IsoAid Advantage 125I source [14], b IsoAid Advantage 103Pd source [15],

c IsoRay 131Cs source [16], d hypothetical Flexisource 170Tm source [17]

Australas Phys Eng Sci Med (2013) 36:301–311 303

123

marker. 103Pd isotope is absorbed uniformly throughout the

volume of the polystyrene pellets. The source has a tita-

nium capsule with 4.5 mm length and 0.8 mm outer

diameter. The wall thickness of the capsule is 0.05 mm.

The thickness of the two end caps is 0.35 mm. The

effective active length of the source, Leff, was taken as

3.61 mm. This length is the maximum distance between

the proximal and distal ends of the active pellets (Fig. 1)

[15, 19].

In the IsoRay 131Cs source (model CS-1) the capsule is a

titanium tube with outer and inner diameters of 0.8 and

0.7 mm, respectively. There is also a central X-ray gold

marker with 0.25 mm diameter. The gold marker is sur-

rounded by a glass/ceramic tube with 0.4 and 0.65 mm

inside and outside diameters, respectively. The active 131Cs

source is uniformly distributed throughout the outer

0.11 mm thickness of the glass/ceramic complex. The

overall length of the source complex is 4.7 mm, including a

4.5 mm long tube and two 0.1 mm thick caps on the ends

(Fig. 1) [16].

Since there were no commercial 120 Tm brachytherapy

sources available, a hypothetical one was simulated and

used for the photon activation study. Our selection of this

hypothetical source was based on a study by Ballester

et al. on 170Tm radionuclide proposed for use in medical

application [17]. The source geometry is similar to the

commercially available Flexisource 192Ir HDR source

[20]. However, in the hypothetical 170Tm source, the

active core is composed of 170Tm radionuclide. The active

core of the hypothetical 170Tm source is a pure 170Tm

cylinder (density = 9.3 g/cm3). The core’s active length

is 3.5 mm and its diameter is 0.6 mm. The active core has

a 304-type stainless steel capsule (density: 8.0 g/cm3).

The outer diameter of the capsule is 0.85 mm, having a

total length of 4.6 mm. The source cable was simulated as

a 304 stainless steel cylinder with 5 mm length and

0.5 mm diameter (Fig. 1) [17]. The gamma and X-rays of

the 170Tm source were defined in the simulations, but the

beta emissions by 170Tm radionuclide were ignored in this

study.

Activation media used

In photon activation therapy, the Auger electrons have a

dominant role in transferring the radiation energy to the

DNA molecules. Production of these electrons originates

from vacancies in the atomic K, L shells. In this study, four

activation media were used in the target volume: Ag, I, Pt

and Au. These elements were selected based on previous

studies in the field of photon activation therapy [1, 8, 9,

21]. With the purpose of a comparison, the orbital energy

corresponding to K and L shells for Ag, I, Pt and Au ele-

ments are listed in Table 2. The data presented in this table

will be useful in the comparison of dose enhancements

related to these elements, with the purpose of evaluation of

the dependence of dose enhancement on the type of acti-

vation medium.

Monte Carlo simulations of the sources

Dosimetric parameters of the sources were determined

according to the TG-43U1 protocol [23]. Following this

protocol, the dose rate distribution around a brachytherapy

source is determined from the following formalism:

_Dðr; hÞ ¼ SkKGðr; hÞ

Gðr0; h0ÞgðrÞFðr; hÞ ð1Þ

where Sk is the air kerma strength of the source; K is the

dose rate constant at reference point of (1 cm, p/2); G(r, h)

is the geometry function; g(r) is the radial dose function;

and F(r, h) is the anisotropy function of the source. Details

on the definition of the above quantities can be found in the

update of task group No. 43 report (TG-43U1) by Ameri-

can Association of Physicists in Medicine (AAPM) [23]

and previous publications on TG-43 parameter determina-

tions for various brachytherapy sources [24–26], therefore

they are not repeated here. MCNPX (version 2.4.0) Monte

Carlo code was used. This code is a general purpose Monte

Carlo code which is used for transport of electrons, pho-

tons, neutrons and other particles in various radiation

problems. It includes a tool for geometry modeling and

various tallies to score particle flux, particle current, energy

deposition and other radiation physics quantities [27]. The

dose rate constant and radial dose function were calculated

through simulations of the source and were compared with

the previously published data. For this purpose, the source

geometries, including before active core, capsule, cable,

etc. were defined in the simulations and the dose values

were obtained in a water phantom.

In the calculation of the air kerma strength, the source

was positioned in vacuum and a number of toroid cells

containing dry air were defined at distances ranging from

5–10 cm from the source. The thickness of each toroid cell

Table 2 K-edge and L-edge energy (keV) for the four activation

media used in this study

Activation

media

Atomic

number

K-edge L-edge

L-I L-II L-III

Ag 47 25.5140 3.8058 3.5237 3.3511

I 53 33.1694 5.1881 4.8521 4.5571

Pt 78 78.3948 13.8799 13.2726 11.5637

Au 79 80.7249 14.3528 13.7336 11.9187

The data were adopted from [22]

304 Australas Phys Eng Sci Med (2013) 36:301–311

123

was 0.1 cm. Air kerma was scored in the tally cells using

F6 tally following scoring 7 9 107 photons.

The dose rate constant was calculated by dividing dose

rate at reference point (r0 = 1, h0 = 908) by air kerma

strength. Dose rate at the reference point was obtained

using *F8 tally in a water phantom with radius of 15 cm.

The input file was run for 107 photons and the related error

in the tally cell in the reference point was less than 3.32 %

for the four sources.

The radial dose function was obtained at radial distances

0.2–10 cm from the source in a water phantom with 15 cm

radius. For this purpose, *F4 tally was calculated in toroid

cells defined at these distances. The thickness of the toroid

cells was 0.1 cm and they had not overlapping with each

other. *F4 tally scores photon fluence. The fluence values

were then converted to absorbed dose by multiplication of

fluence values to the corresponding mass energy absorption

coefficients for water extracted from NIST database [28].

Since the mass energy absorption coefficient depends on

the photon energy, photon fluence in various energy bins

were obtained and then multiplied by the corresponding

mass energy absorption coefficient at that energy bin. The

total dose then was calculated by summing the dose frac-

tions in these energy bins. It was possible to use *F8 tally

to calculate the energy deposition directly, but *F4 tally

was applied to minimize the running time needed for the

simulations. Maximum Monte Carlo statistical error in the

tally cells was 2.59 % following transport of 1.5 9 108

primary photons.

The photon spectra including the emitted photon ener-

gies and the related probabilities used in the definition of

the four sources are listed in Table 3. The data in this table

were adopted from [17, 23, 29, 30].

Dose enhancement calculations

The dose enhancement in the tumour was obtained by

calculation of dose enhancement factor (DEF). Dose

enhancement factor was calculated from the following

definition:

DEF values were obtained on a transverse plane inside

the tumour volume and the maximum and averaged values

were reported as DEF for that specific source and activa-

tion media. The source was positioned inside a spherical

phantom with 15 cm radius. The phantom contained soft

tissue. The composition for soft tissue was defined as

outlined by ICRU report, 44 [31]. Based on this report the

soft tissue composition was: 76.2 % oxygen, 10.1 %

Table 3 Energy spectrum data for the 125I, 103Pd, 131Cs and 170Tm radionuclides used in the present study

125I (TG-43U1 [23]) 103Pd (Rivard [29]) 131Cs (Rivard [30]) 170Tm (Ballester et al. [17])

Energy

(keV)

Photons per

disintegration

Energy

(keV)

Photons per

disintegration

Energy

(keV)

Photons per

disintegration

Energy

(keV)

Photons per

disintegration

27.202 0.406 20.074 0.224 4.11 0.086 6.95 0.000276

27.472 0.757 20.216 0.423 29.461 0.211 7.42 0.0292

30.98 0.202 22.717 0.104 29.782 0.389 48.22 0.000291

31.71 0.0439 23.312 0.0194 33.562 0.0363 49.13 0.000513

35.492 0.0668 39.755 0.000683 33.624 0.0702 51.35 0.0097

62.51 0.0000104 34.419 0.0213 52.39 0.0169

294.95 0.000028 55.48 0.0000546

357.46 0.000221 55.67 0.0001055

497.054 0.0000401 57.14 0.0000354

59.16 0.00185

59.38 0.00356

60.96 0.001205

78.7 0.000035

84.26 0.0248

DEF ¼ Dose in a point with presence of activation media

Dose in the same point without presence of activation mediað2Þ

Australas Phys Eng Sci Med (2013) 36:301–311 305

123

hydrogen, 11.1 % carbon and 2.6 % nitrogen. The central

part of the phantom was defined as the tumour. The tumour

was a sphere with 1.5 cm radius. The compositions of soft

tissue phantom and tumour were the same, but with the

difference that the composition of activation media was

added to that of soft tissue in the case of tumour containing

activation media. Dose values were then calculated in the

absence and presence of activation media inside the

tumour. DEF for the four activation media (Ag, I, Pt and

Au) with concentrations of 7, 18 and 30 mg/ml tumour was

obtained. Each source, activation medium and concentra-

tion was evaluated separately in a separate simulation. A

schematic diagram illustrating the position of source,

tumour and phantom can be seen in Fig. 2. Energy depo-

sition in tally cells (MeV) was calculated using *F8 tally.

For this purpose, this tally type was scored for electrons

before and after loading of activation media in the tumour

in the simulations. Since the mass of tally cells were dif-

ferent for various concentrations and activation media, the

energy deposition was then divided by the corresponding

mass to obtain energy deposition in term of MeV/g. Energy

cut off of 1–10 keV was defined for both photons and

electrons in the simulations for obtaining dose enhance-

ments. The energy cut offs for the sources were the same as

those values defined in simulation of the sources for

obtaining TG-43 parameters. Furthermore, the effect of

energy cut off on the value of dose enhancement was

evaluated for a number of situations. For this purpose, dose

enhancement for 103Pd and 170Tm sources in presence of

Ag in the tumour with various concentrations was com-

pared for 10 and 1 keV cases. Toroid cells with 0.1 cm

thickness were defined as the tally cells at radial distances

of 0.4–6.4 cm from the source center on the transverse

plane. This amount of thickness was defined to avoid

overlapping of the toruses. The number of photons trans-

ported in dose enhancement calculations in each input file

was 1.5 9 108 and the maximum statistical error in the

tally cells was 18.12 %. However, the maximum MC error

was on average 3.43 %. With respect to these values it

should be mentioned that, since there were four sources,

four activation media, and three concentrations, the total

number of forty-eight simulations were performed in DE

calculations. The maximum MC error in each simulation

was noted and the maximum (18.12 %) and average

(3.43 %) of these maximum values were reported.

Results

Dose rate constant and radial dose function

As it is mentioned in the materials and methods section,

the dose rate constant and radial dose function of the 125I,103Pd, 131Cs and hypothetical 170Tm sources were

obtained and compared with the previously published

data for the source models. The results of dose rate

constants are presented in Table 4. The corresponding

previously published data can be considered in the table

as well. Additionally, the percentage differences of our

calculated values and the published data at each specific

point were listed in this table. The radial dose function

values from the calculations, previously reported data and

the percentage differences between the two data sets are

listed in Table 5.

As is evident from the data in Table 4, the maximum

discrepancy between two sets of dose rate constants is

related to 131Cs source which is equal 9.91 %.

From this table it is evident that there are a number of

points with negative percentage values. Since the percentage

Fig. 2 A schematic geometry illustrating position of source, tumour

and soft tissue phantom and the related dimensions

Table 4 Dose rate constant

(cGy h-1 U-1) for the four

brachytherapy sources obtained

in this study and from the other

studies

Source type This study Other studies Reference Difference (%)

125I 1.03 0.98 Meigooni et al. [14] 4.78103Pd 0.701 0.709 Sowards [32] -1.16131Cs 1.006 0.915 Murphy et al. [16] 9.91

Hypothetical 170Tm 1.20 1.23 Ballester et al. [17] -2.8

306 Australas Phys Eng Sci Med (2013) 36:301–311

123

difference between our values and those from other

studies was calculated, these data points are related to the

points in which the data from this study was less than that

from the other study. As it is evident from the data in

Table 5, the maximum difference between the radial dose

function values from the simulations and accepted values

amounts to 13.99 %, which is related to the distance of

6 cm from the 170Tm source. The reason for this level of

discrepancy for 170Tm source can be related to the fact

that we have ignored the beta emissions by 170Tm source,

while they were considered in the previous study on this

source.

Dose enhancement

The maximum dose enhancement factors in the transverse

plane in the tumour for the mentioned sources and

activation media are presented in Table 6. This table also

includes the averaged DEFs in the transverse plane in the

tumour.

As it can be considered from the values of Table 6, the

maximum dose enhancement factor is related to 170Tm

source and I media combination which amounts to 5.58.

The average value of dose enhancement is highest for the

source 170Tm and I combination. However, DEF for170Tm source for other materials and for 131Cs source for

Pt and Au are close to this value. DEF for 125I and 103Pd

sources with Pt and Au are relatively higher than those for

Ag and I.

The evaluation of the effect of energy cut off on dose

enhancement in the case of 103Pd and 170Tm sources in the

presence of Ag shows that the dose enhancement was

unchanged when 10 or 1 keV energy cutoffs were used for

the particles.

Table 5 Radial dose functions for the four brachytherapy source models compared with other studies

r (cm) 125I 103Pd 131Cs Hypothetical 170Tm

Present

study

Meigooni

et al. [14]

Diff.

(%)

Present

study

Sowards

[32]

Diff.

(%)

Present

study

Murphy

et al. [16]

Diff.

(%)

Present

study

Ballester (Personal

communication)

Diff.

(%)

0.2 1.070 1.066 0.38 1.189 1.234 -3.68 0.957 – – – – –

0.3 1.069 1.065 0.36 1.272 1.296 -1.88 0.978 – – – – –

0.4 1.065 1.056 0.86 1.273 1.290 -1.30 0.989 – – 0.896 0.968 -7.44

0.5 1.055 1.048 0.69 1.244 1.260 -1.28 0.996 1.003 -0.71 0.917 – –

0.6 1.047 1.041 0.61 1.201 1.213 -0.95 1.001 – – 0.935 0.972 -3.76

0.7 1.038 1.042 -0.41 1.154 1.160 -0.51 1.004 – – 0.952 – –

0.8 1.027 1.027 -0.01 1.102 1.106 -0.38 1.003 – – 0.968 0.985 -1.73

0.9 1.014 1.013 0.09 1.050 1.053 -0.28 1.003 – – 0.985 – –

1 1.000 1.000 0.00 1.000 1.000 0.00 1.000 1.000 0.00 1.000 1.000 0.00

1.5 0.925 0.923 0.18 0.766 0.768 -0.31 0.974 – – 1.071 1.034 3.58

2 0.841 0.834 0.86 0.576 0.576 -0.02 0.930 0.923 0.74 1.130 1.065 6.15

2.5 0.756 0.750 0.85 0.428 0.429 -0.25 0.875 – – 1.178 – –

3 0.675 0.669 0.89 0.317 0.318 -0.47 0.813 0.806 0.87 1.218 1.108 9.89

3.5 0.598 0.592 1.02 0.233 0.233 0.07 0.751 – – 1.251 – –

4 0.528 0.523 1.01 0.171 0.173 -1.10 0.687 0.679 1.19 1.273 1.131 12.59

4.5 0.462 0.462 -0.10 0.126 0.127 -0.82 0.627 – – 1.290 – –

5 0.406 0.399 1.63 0.093 0.092 0.68 0.568 0.558 1.73 1.295 1.138 13.78

5.5 0.355 0.353 0.54 0.067 0.069 -2.75 0.512 – – 1.292 – –

6 0.309 0.305 1.28 0.050 0.050 -0.83 0.460 0.454 1.36 1.288 1.130 13.99

6.5 0.272 0.269 0.99 0.036 0.037 -2.16 0.411 – – 1.272 – –

7 0.234 0.222 5.52 0.027 0.028 -4.11 0.369 0.361 2.27 1.253 1.112 12.68

7.5 0.202 0.189 6.97 0.021 0.020 2.67 0.326 – – 1.228 – –

8 0.175 0.163 7.31 0.015 0.015 -2.04 0.291 – – 1.203 1.082 11.15

8.5 0.152 0.138 10.34 0.011 0.011 1.87 0.256 – – – – –

9 0.131 0.126 3.81 0.009 0.008 8.19 0.229 – – – – –

9.5 0.113 0.105 7.80 0.006 0.006 3.43 0.202 – – – – –

10 0.097 0.090 7.93 0.005 0.005 0.22 0.179 – – – – –

Australas Phys Eng Sci Med (2013) 36:301–311 307

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Plot of DEF and dose rate (cGy mCi-1 h-1) versus

radial distance from the source for Au with 7, 18 and

30 mg/ml concentrations in the case of the four sources

used in this study was illustrated in Fig. 3. The minimum

value of dose enhancement factor in the cells outside the

tumour was 0.04. The value is related to 103Pd source when

Au element with 30 mg/ml is used as the activation media.

The average value of DEF outside the tumour for the 125I,103Pd, 131Cs and hypothetical 170Tm sources for all media

are 0.54, 0.43, 0.59 and 0.88 respectively.

Discussions and conclusions

Three brachytherapy sources (125I, 103Pd and 131Cs) and a

hypothetical 170Tm source were simulated and dose rate

constant and radial dose function were obtained and com-

pared with the published data for the sources. The dosi-

metric values from the present study are in agreement with

the previously published data for these source models,

validating the simulations.

The brachytherapy sources with four activation agents

(Ag, I, Pt and Au) were studied to determine the dose

enhancement in photon activation therapy. Two brachy-

therapy sources, 131Cs and 170Tm, were added to the list of

the sources and were evaluated for use in photon activation

therapy applications. From all the possible situations for

combinations of these sources and media (Table 6), the

hypothetical 170Tm source when used with I medium loa-

ded in the tumour had the highest dose enhancement factor

of 5.58. The 131Cs source shows a DEF value (4.82) close

to this value which is related to 30 mg/ml concentration of

Au activation media.

By comparing various sources from a DEF point of view

(Table 6), DEF for 131Cs and hypothetical 170Tm sources

are higher than those for 125I and 103Pd sources. Although

there are minor differences between the DEFs for 131Cs and170Tm sources, these differences are not high and can be

relatively considered equal. When comparing various

media (Table 6), DEF values were higher for Pt and Au

media with 125I and 103Pd sources, and for Ag, Pt and Au

with 131Cs source. The DEF values for various media for

hypothetical 170Tm source were relatively equal. By

keeping in mind the sources’ energies, atomic numbers and

orbital edge energies of the activation media, it is not easy

to find simple rules for description of increasing trends of

DEFs for various cases. However, performing a study to

determine the spectrum of electrons and photons at the

surface of the source capsule and at various depths in the

phantom may be useful.

As it can be seen from the data in Table 6, neglecting

some exceptions, DEF increases with concentration of the

activation media. This fact can be justified with consider-

ing that with increased concentration, the number of acti-

vation element atoms present in the limited tumour volume

increases. Thus, the probability of interaction of source

photons increases with activation element concentration

leading to a higher dose enhancement.

From the data in Fig. 3 it can be noticed that both DEF

and dose rate have decreasing trends inside the tumour.

However, dose rate (cGy mCi-1 h-1) decreases with

steeper trends inside the tumour for all the four sources in

presence of Au media in the tumour. The decreasing trend

in DEF in the tumour can be due to the fact that a number

of low energy photons are absorbed at first few millimeters

in the tumour. Therefore, the photon beams will have

Table 6 Maximum and average dose enhancement factor in tumour for 125I, 103Pd, 131Cs and hypothetical 170Tm sources and various con-

centrations of Ag, I, Pt and Au media

125I 103Pd 131Cs Hypothetical170Tm

Concentration (mg/ml) 7 18 30 7 18 30 7 18 30 7 18 30

Maximum dose enhancement factor

Activation media

Ag 1.73 2.63 3.35 1.18 1.44 1.67 2.05 3.38 4.44 2.02 3.50 5.02

I 1.35 1.85 2.32 1.26 1.60 1.88 1.47 2.12 2.74 2.17 3.85 5.58

Pt 2.01 3.18 4.03 1.72 2.31 2.48 2.14 3.57 4.73 1.97 3.42 4.89

Au 2.04 3.24 4.08 1.73 2.32 2.49 2.17 3.64 4.82 2.00 3.48 4.98

Average dose enhancement factor

Activation media

Ag 1.59 2.08 2.30 1.12 1.24 1.31 1.88 2.65 3.03 2.01 3.41 4.69

I 1.29 1.64 1.91 1.16 1.30 1.35 1.40 1.88 2.26 2.14 3.69 5.11

Pt 1.74 2.29 2.45 1.35 1.33 1.10 1.91 2.75 3.17 1.94 3.26 4.49

Au 1.76 2.31 2.46 1.35 1.32 1.09 1.94 2.79 3.20 1.97 3.32 4.56

The tumour was a 1.5 radius sphere with the source located at its center

308 Australas Phys Eng Sci Med (2013) 36:301–311

123

higher average energy at steeper distances and because of

inverse dependence of probability of photoelectric effect

with photon energy, the DEF decreases with distance in the

tumour. In the present study, we have only calculated DEF

in a transverse plane relative to longitudinal axes of the

sources. In this case, the angle theta will be equal to 908. It

is predicted that DEF may show angular dependence. This

effect was not evaluated here but can be a subject for

further studies in this field. It should be noticed from the

data in Fig. 3 that while the dose rate was plotted for dose

rate of 0.00–18.00 cGy mCi-1 h-1 range on vertical axis

for 125I, 103Pd and 131Cs sources, the dose rate range on

vertical axis for hypothetical 170Tm source is

0.00–1.00 cGy mCi-1 h-1. The reason is that for the same

activity and time for the four sources, the dose rate values

for 170Tm source were considerably less than those for

other sources. This can be related to the low photon yields

for 170Tm radionuclide. This figure was only plotted for

Au, but it is predicted that similar trends will be seen for

other media. Dose rate value in tumour is higher for higher

concentrations for all four sources. Higher dose enhance-

ment by 170Tm source is an advantage of this source over

the other ones, but when comparing dose rate values, this

source shows lower dose rates and thus this effect can be a

disadvantage of this source compared to 125I, 103Pd and131Cs sources. In other words, with the same activity and

time used for the four sources, 125I, 103Pd and 131Cs sources

shows higher dose rate values in presence of the activation

media. This can be an advantage of these sources when

compared with 170Tm.

The results of dose enhancement factors outside the

tumour, which are less than unity, indicate that the absor-

bed dose is reduced in the tissue outside the tumour when

the tumour is loaded by the activation media. The dose

reduction amounts to a value of 96 % in some cases (DEF

of 0.04). This is because of the shielding effect of the

activation media in the tumour which absorbs the source

radiation, thereby reducing the number of photons that

escape the tumour. This fact is considered as an advantage

of photon activation therapy, since the presence of acti-

vation media increases the dose inside the tumour, and

reduces the dose to tissues outside the tumour. However,

this effect has been studied in the present work only for the

points on a transverse line crossing the tumour. Calculation

Fig. 3 Dose enhancement factor and dose rate (cGy mCi-1 h-1) for 7, 18, 30 mg/ml concentrations of Au: a 125I source, b 103Pd source, c 131Cs

source, d hypothetical 170Tm source

Australas Phys Eng Sci Med (2013) 36:301–311 309

123

of the dose reduction on other parts of the tissues outside

tumour is a subject of future study. Furthermore, in a real

situation the activation media will be absorbed in some

extent by the normal tissues. This will result to enhance-

ment of dose in these tissues. In fact the dose in normal

tissues will be affected by both the shielding effect of

tumour and the dose enhancement in these normal tissues.

The 170Tm source has a longer half-life (128.6 days)

compared to the other sources. This may be an advantage in

photon activation therapy because a source with longer half-

life will need to fewer source exchanges, calibrations and

commissioning. Less source exchanges will be more cost-

effective for brachytherapy departments. On the other hand,

considering the low yield values of useful photons with 170Tm

source (Table 3), it will be a need for a source with higher

activity to compensate the low yields of this radioisotope.

As a future study, obtaining the DEF values for 145Sm,241Am and 161Ho sources and the four activation media of

Ag, I, Pt and Au will be useful. However, it should be

noticed that some of these radionuclides are not currently

available as standard brachytherapy sources. In the present

study, only physical effect (DEF) of presence of activation

media in tumour was evaluated in photon activation therapy

with various brachytherapy sources. Since the biological

response of the cells for a specific source and activation

media differs from the physical response (DEF), it is also

suggested to evaluate the response of the human cells to

various activation media and brachytherapy source combi-

nations in future studies on photon activation therapy.

Acknowledgments The authors are thankful to Dr. Facundo Bal-

lester for his help to review the manuscript. The authors would also

like to thank North Khorasan University of Medical Sciences for

funding this work.

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