Investigating the Implications of a Variable RBE on Proton DoseFractionation Across a Clinical Pencil Beam Scanned Spread-OutBragg PeakMarshall, T., Chaudhary, P., Michaelidesova, A., Vachelova, J., Davidkova, M., Vondracek, V., ... Prise, K.(2016). Investigating the Implications of a Variable RBE on Proton Dose Fractionation Across a Clinical PencilBeam Scanned Spread-Out Bragg Peak. International Journal of Radiation: Oncology - Biology - Physics, 95(1),70-77. https://doi.org/10.1016/j.ijrobp.2016.02.029

Published in:International Journal of Radiation: Oncology - Biology - Physics

Document Version:Publisher's PDF, also known as Version of record

Queen's University Belfast - Research Portal:Link to publication record in Queen's University Belfast Research Portal

Publisher rights2016 The Authors. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/), which permitsunrestricted use, distribution and reproduction in any medium, provided the author and source are cited.

General rightsCopyright for the publications made accessible via the Queen's University Belfast Research Portal is retained by the author(s) and / or othercopyright owners and it is a condition of accessing these publications that users recognise and abide by the legal requirements associatedwith these rights.

Take down policyThe Research Portal is Queen's institutional repository that provides access to Queen's research output. Every effort has been made toensure that content in the Research Portal does not infringe any person's rights, or applicable UK laws. If you discover content in theResearch Portal that you believe breaches copyright or violates any law, please contact openaccess@qub.ac.uk.

Download date:04. Feb. 2019

International Journal of

Radiation Oncology

biology physics

www.redjournal.org

RBE and Particle Therapy Biology

Investigating the Implications of a Variable RBEon Proton Dose Fractionation Across a ClinicalPencil Beam Scanned Spread-Out Bragg PeakThomas I. Marshall, MSci,* Pankaj Chaudhary, PhD,*Anna Michaelidesova, ING,y,z,x Jana Vachelova, BC,y

Marie Davıdkova, PhD,y Vladimir Vondra�cek, MGR,z

Giuseppe Schettino, PhD,k and Kevin M. Prise, PhD*

*Centre for Cancer Research and Cell Biology, Queen’s University, Belfast, UK, yDepartment ofRadiation Dosimetry, Nuclear Physics Institute CAS and zProton Therapy Center Czech, Prague, CzechRepublic, xDepartment of Dosimetry and Application of Ionizing Radiation, Faculty of NuclearSciences and Physical Engineering, Czech Technical University in Prague, Prague, Czech Republic, andkRadiation Dosimetry, National Physical Laboratory, Teddington, UK

Received Sep 30, 2015, and in revised form Jan 27, 2016. Accepted for publication Feb 8, 2016.

Summary

Clinical adoption of a con-stant cell killing relativebiological effectiveness(RBE) for acute exposuresunderestimates the effect ofincreased linear energytransfer (LET) in the distalregions of clinical protonbeams. Experimental data forthe impact of dose fraction-ation in such scenarios re-mains limited. Toward distalregions of the spread-outBragg peak, we found an

Reprint requests to: Giuseppe Schettino, Ph

oratory, Hampton Rd, Teddington, Middlesex,

giuseppe.schettino@npl.co.uk

T. I. Marshall and P. Chaudhary contributed

Supported by funding provided by Medic

through grant no. G 1100014/1 and Engineeri

Research Council program Advanced Strategies

Laser grant no EP/K022415/1. TM is supp

Int J Radiation Oncol Biol Phys, Vol. 95, No. 1

0360-3016/� 2016 The Authors. Published by E

4.0/).

http://dx.doi.org/10.1016/j.ijrobp.2016.02.029

Purpose: To investigate the clinical implications of a variable relative biological effec-tiveness (RBE) on proton dose fractionation. Using acute exposures, the current clin-ical adoption of a generic, constant cell killing RBE has been shown to underestimatethe effect of the sharp increase in linear energy transfer (LET) in the distal regions ofthe spread-out Bragg peak (SOBP). However, experimental data for the impact of dosefractionation in such scenarios are still limited.Methods and Materials: Human fibroblasts (AG01522) at 4 key depth positions on aclinical SOBP of maximum energy 219.65 MeV were subjected to various fractionationregimens with an interfraction period of 24 hours at Proton Therapy Center in Prague,Czech Republic. Cell killing RBE variations were measured using standard clonogenicassays and were further validated using Monte Carlo simulations and parameterized us-ing a linear quadratic formalism.Results: Significant variations in the cell killingRBE for fractionated exposures along theproton dose profile were observed. RBE increased sharply toward the distal position, cor-responding to a reduction in cell sparing effectiveness of fractionated proton exposures athigher LET. The effect was more pronounced at smaller doses per fraction. Experimental

D, National Physical Lab-

TW11 0LW, UK. E-mail:

equally to this study.

al Research Council, UK

ng and Physical Sciences

for Accelerating Ions with

orted by Department of

Education and Learning, Northern Ireland Studentship. A.M., J.V. and

M.D. greatly appreciate financial support of the Czech Science Foundation

in the frame of the Center of Excellence. Financial support was also

provided by the Czech Science Foundation grant no. P108/12/G108.

Conflict of interest: none

Supplementary material for this article is available online at

www.redjournal.org.

, pp. 70e77, 2016lsevier Inc. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/

Volume 95 � Number 1 � 2016 Fractionated proton therapy RBE 71

increased RBE correspond-

ing during fractionated pro-ton exposures at higher LETand lower dose per fraction.This increase in RBE resultsin considerable deviationfrom clinically predictedisoeffective regimens.

survival fractions were adequately predicted using a linear quadratic formalism assumingfull repair between fractions. Data were also used to validate a parameterized variableRBEmodel based on linear a parameter responsewith LET that showed considerable de-viations from clinically predicted isoeffective fractionation regimens.Conclusions: The RBE-weighted absorbed dose calculated using the clinically adoptedgeneric RBE of 1.1 significantly underestimates the biological effective dose from var-iable RBE, particularly in fractionation regimens with low doses per fraction. Coupledwith an increase in effective range in fractionated exposures, our study provides anRBE dataset that can be used by the modeling community for the optimization of frac-tionated proton therapy.� 2016 The Authors. Published by Elsevier Inc. This is an openaccess article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

Introduction

Proton therapy is a rapidly advancing form of externalbeam radiation therapy and has been established as analternative to photon-based modalities for specific cancertypes (1). The motivation behind the adoption of thera-peutic protons lies in their inherent physical advantagesexpressed over more conventionally used X-ray techniques.The inverted Bragg peak depth-dose profile allows foreffective treatment of a tumor region while significantlysparing surrounding healthy tissues, with the superpositionof several peaks of discrete energies treating extended re-gions using a spread-out Bragg peak (SOBP) (2-4). Thisincreased sparing of healthy tissues offers additional ad-vantages in the treatment of pediatric patients, in whom therisk of secondary cancers and late morbidity is significantlyhigher (5). Additionally, the rapid distal falloff in dose isideal in the treatment of tumors located near organs at risk(OAR). Further to the advantages offered by the depth-doseprofile, the increased linear energy transfer (LET) of pro-tons in comparison with X-rays results in an increasedbiological effectiveness for cell killing. Particularly forregions surrounding the Bragg peak, the more localizedpattern of energy deposition enhances biological damage,primarily through more complex DNA lesions (6, 7). Toaccount for such an increase in effectiveness, the concept ofrelative biological effectiveness (RBE), defined as the ratioof photon to particle doses to induce an equivalent bio-logical effect (8), is adopted in treatment planning to scalephysical dose into RBE-weighted absorbed dose: DRBE, theproduct of physical dose and RBE (9, 10).

The estimation of proton beam RBE compared withenergetic X-rays presents a key issue in radiation therapybecause any uncertainty in the RBE transfers directly to anuncertainty in the biologically effective dose delivered to apatient. Considering the necessity of a 3.5% dose toler-ance, characterization of proton RBE in a clinical settingis pertinent to the optimal delivery of proton radiationtherapy (11). A lack of strong datasets has seen the clin-ical adoption of a fixed, generic RBE value of 1.1regardless of evidence for changes in biological effec-tiveness as a function of energy modulation, beam size,

cellular radiosensitivity, or SOBP size and position(12-15). In addition, a fixed RBE during fractionated ex-posures disregards any effects resulting from the variationof dose per fraction and the total number of fractionsdelivered in relation to the LET (16).

As a key radiation therapy strategy, fractionation aims tomaximize dose delivery to a treatment region whileallowing healthy tissues time to repair by splitting the totaldose into smaller fractions, with 1 or more rest periodsbetween each delivery. Having reported a sharp rise in RBEtoward the high LET regions of a clinical SOBP for acuteexposures (12), the authors aim to elucidate the additionaleffects of a variable proton RBE on a fractionated regimen.The use of a variable RBE may see significant deviationsfrom current clinical assumptions, and may obscure thepotential therapeutic advantages of proton radiation therapywhen delivering fractionated regimens based on theextrapolation of clinical experience with photons (17). Theunique depth-dose deposition characteristics of protonsmay present opportunities for the shortening of clinicalfractionation schedules through hypofractionation and havebeen investigated through several clinical trials (18-20). Asa result, comparisons with the widely accepted Interna-tional Atomic Energy Agency standard fractionationregimen of 2 Gy per fraction photon irradiation will provideuseful insights for the discussion about the adoption ofmodified fractionation schemes (10).

For this study, the cell killing RBE of various protonfractionation regimens in normal human skin fibroblast(AG01522) cells were investigated. The pencil-beam scan-ning clinical beam of maximum energy 219.65 MeV atProton Therapy Center, Prague, Czech Republic has beenpreviously used in the treatment of a range of tumor sitesincluding head and neck, brain, and prostate.

Cells were exposed at the positions of key features on aclinical dose profile: at the proton entrance and at theproximal-SOBP, central-SOBP, and distal-SOBP regions.These clinically relevant exposure conditions allowed theinvestigation of a wide range of clinical LET values. Theeffect of proton fractionation on cell survival was investi-gated by delivering up to 3 fractions to cell monolayerswith an interfraction rest period of 24 hours.

Marshall et al. International Journal of Radiation Oncology � Biology � Physics72

Methods and Materials

Cell culture

AG01522 cells were maintained in a-modified mini-mum essential medium (Sigma Aldrich, St. Louis, MO)supplementedwith 20% fetal bovine serum and 1%penicillin-streptomycin (Gibco, Life Technologies Carlsbad, CA). Allcells were incubated in 5% CO2 with 95% humidity at 37�C.

Proton irradiation, dosimetry, and simulation

Cells were exposed at various positions along a clinicalSOBP generated by pencil scanning beam of maximumenergy 219.65 MeV, generated by an IBA Protheus 230cyclotron at Proton Therapy Center Prague, Czech Re-public. Up to 3 fractions (of the same dose per fraction)were delivered to cells with an interfraction rest period of24 hours. The full details are outlined in SupplementaryInformation (available online at www.redjournal.org).

Acute

X-ray

1A

C

0.1

0.01

0.0010 1 2 3 4

0 1

1

0.1

0.01

0.001

Surv

ival

Fra

ctio

n

Surv

ival

Fra

ctio

n

EntranceProximalCentralDistal

Dose (Gy)

Dose

X-rayEntranceProximalCentralDistal

Fig. 1. Clonogenic survival data at experimental positions inspread-out Bragg peak for AG01522 cells alongside reference 2survival after irradiated dose delivered under each fractionatiodelivered in a single (A), double (B), and triple (C) exposure aerror of the mean with fits obtained using the linear quadratic m

Clonogenic assay

Cells were incubated in full media for 24 hours before thedelivery of each fraction. After the delivery of the finalfraction, cells were immediately trypsinized, counted, andseeded onto 6-well plates in duplicate with sufficient densityto obtain w50 macroscopic colonies per well. Plates werethen incubated in 5% CO2 with 95% humidity at 37�C for12 days to allow macroscopic colony formation. Colonieswere fixed and stained using 0.5% crystal violet dye in 95%methanol in water for 30 minutes at room temperature, thengently rinsed in water and air dried. Colonies consisting of atleast 50 cells were scored as viable.

Data analysis and simulation

Cell survival was described using a linear quadraticformalism, where for acute exposures the surviving fraction(SF) of cells after receiving an acute dose D is given by:

SFacuteZexp�� aD� bD2

� ð1Þ

Double

Triple

B

2 3 4

0 1 2 3 4

1

0.1

0.01

0.001

Surv

ival

Fra

ctio

n

Dose (Gy)

(Gy)

X-rayEntranceProximalCentralDistal

the entrance, proximal, central, and distal regions of the25 kVp X-ray curves. Survival curves indicate overall celln regimen. (A) Cell survival as a function of total doset the 4 experimental positions. Error bars indicate standardodel.

Volume 95 � Number 1 � 2016 Fractionated proton therapy RBE 73

with fitting parameters a and b. Additionally, cell survivalafter a fractionated regimen of n fractions and dose perfraction d is described as follows:

SFfracZexp�� a n d� b n d2

� ð2ÞUsing the definition of RBE calculated relative to 225

kVp X-rays (DX rays/Dprotons at isoeffect where D denotesacute dose), it is possible to obtain analytic equations forthe RBE as a function of the radiation dose in acute andfractionated regimens, where

RBEacuteZe aX þ

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffia2X þ 4 bX

�aP DP þ bP D

2P

�q

2bX DP

ð3Þ

and

RBEfracZaP þ bPdPaX þ bX dX

ð4Þ

where X and P subscripts denote parameters correspondingto X-ray and proton exposures, respectively. Nonlinearregression analysis was performed on survival curves usingGraphPad Prism version 6.0f. A detailed description of thesimulation parameters and toolkit used is provided inSupplementary Information (available online at www.redjournal.org).

Entrance

Distal

SingleDoubleTriple

Dose (Gy)

Surv

ival

Fra

ctio

n

1

0.1

0.01

0.0010 1 2 3

3

4

SingleDoubleTriple

Dose (Gy)

Surv

ival

Fra

ctio

n

1

0.1

0.01

0.0010 1 2 4

SingleDoubleTriple

Dose

Surv

ival

Fra

ctio

n

1

0.1

0.01

0.0010 1

A B

D

Fig. 2. Clonogenic survival data under the 3 fractionation refractions for AG01522 cells alongside reference 225 kVp X-ray cexperimental position at the entrance (A), proximal (B), central1.68, 2.45, and 7.5 keV/mm, respectively. X-ray response is descwith fits obtained using the linear quadratic model (full detailswww.redjournal.org).

Results

Cell survival by fractionation regimen

Figure 1 details the cell survival under the various protonfractionation regimens alongside reference X-ray survivalcurves. It is evident that for all fractionation regimens, cellsurvival curves become consistently steeper toward moredistal positions and remain steeper than the X-ray curves inall cases. With the introduction of more fractions, the levelof cell sparing increases across all positions but variesalong the SOBP, with the most distal positions seeing theleast amount of sparing. The fold decrease (ie, SFdistal/SFproximal at 3.6 Gy) in survival between the proximal anddistal positions is 3.7 � 1.0 and 3.8 � 0.8 for the single-fraction and double-fraction regimens but is increased to6.1 � 1.3 for a triple-fraction regimen, where a total dose of3.6 Gy is delivered.

Cell survival by SOBP position

Figure 2 details the cell survival at the various experi-mental SOBP positions alongside reference X-ray survival

Proximal Central

X-ray

(Gy)2 3 4

SingleDoubleTriple

Dose (Gy)

Surv

ival

Fra

ctio

n

1

0.1

0.01

0.0010 1 2 3 4

SingleDoubleTriple

Dose (Gy)

Surv

ival

Fra

ctio

n

1

0.1

0.01

0.0010 1 2 3

C

E

4

gimens delivering total dose in single, double, and tripleurves. Survival curves indicate overall cell survival at each(C), and distal (D) regions of the SOBP with LET Z 0.63,ribed in (E). Error bars indicate standard error of the meanoutlined in Supplementary Information; available online at

Marshall et al. International Journal of Radiation Oncology � Biology � Physics74

curves. Again, in all cases the increased level of cellsparing with increasing number of fractions is evident: thefold increase in cell survival between triple-fraction andsingle-fraction regimens for the proximal and central po-sitions at 3.6 Gy is 2.59 � 0.27 and 2.0 � 0.4, respectively.However, the effect of fractionation is less evident in thepositions with higher LET, with cell survival curveseffectively overlapping regardless of fractionation regimenat the distal position, with fold increase in cell survival of1.6 � 0.27 between single-fraction and triple-fractionregimens at 3.6 Gy.

Cell survival by fraction size

Figure 3 shows survival data after delivering single, double.and triple fractions of 1.2, 0.8, 0.6, and 0.3 Gy per fractionalongside reference X-ray data. For all fraction sizes,fractionation of the proximal and central positions allowedsignificantly more cell sparing than the distal region, wheresurvival curves were significantly steeper. Adoption of alinear quadratic formalism to predict fractionated response

X-ray

A

C

0

1

0.1

0.01

0.0011 2 3 4

ProximalCentralDistal

Dose (Gy)

Dose (Gy)

Surv

ival

Fra

ctio

n

0.0 0.5 1.0 1.5 2.0

X-ray

1

0.1

0.01

0.001

ProximalCentralDistal

Surv

ival

Fra

ctio

n

Fig. 3. Clonogenic survival data at experimental positions inBragg peak for AG01522 cells alongside reference 225 kVp X1.2 Gy, (B) 0.8 Gy, (C) 0.6 Gy, and (D) 0.3 Gy per fraction formean, solid lines represent fits obtained using the fractionated linacute exposures.

based on the cell response parameters of a single fractionappears suitable, matching experimental data points closelyacross all data sets. The comparison of experimental versusanalytically obtained survival for single, double, and tripleexposures yields Pearson’s correlation coefficients >0.975(P<.0001), indicating an excellent degree of correlation(Fig. E2; available online at www.redjournal.org).

The clinical implications of a variable RBE undervarious fractionation regimens

A strong linear relationship of the proton a parameter apwith LET (Fig. E3; available online at www.redjournal.org)allows the parameterization of RBE in acute and fraction-ated regimens by substituting the expression

apZax þ lLET ð5Þin equations 3 and 4, where ap can be described in terms ofthe a parameter for X-ray exposure ax, proton LET and thelinear gradient of the acute cell response (l Z 0.0883,characteristic for the cell line used). Inasmuch as no sig-nificant difference or relationship between proton b

B

DDose (Gy)

Dose (Gy)

X-rays

1

0.1

0.01

0.0010.0

0.0 0.2 0.4 0.6 0.8 1.0

0.5 1.0 1.5 2.0 2.5 3.0

ProximalCentralDistal

Surv

ival

Fra

ctio

n

X-ray

1

0.1

0.01

0.001

ProximalCentralDistal

Surv

ival

Fra

ctio

n

the proximal, central, and distal regions of the spread-out-ray curves. Survival curves indicate cell survival for (A)up to 3 fractions. Error bars indicate standard error of theear quadratic model (equation 2), and dotted lines are for the

6Clinical (RBE = 1.1)AcuteFractionated

Water Depth (cm)

D RBE

(Gy

RBE)

5

5

4

3

10 15 20 25 30 35

2

1

00

Fig. 5. Spread-out Bragg peak (SOBP) relative biologi-cally effective (RBE)-weighted absorbed dose (DRBE)profile comparing analytically obtained values whendelivering an SOBP plateau dose of 3.6, 2.4, 1.8, and0.8 Gy in both acute (dashed line) and fractionated (dottedline) regimens. Clinically assumed DRBE values using ageneric value of RBE Z 1.1 are marked in solid line. RBEvalues obtained are for 225 kVp X-ray doses of the same

Volume 95 � Number 1 � 2016 Fractionated proton therapy RBE 75

parameters bp and LET was observed, the parameterized bvalues were assumed to be constant and equivalent to thosefor the X-ray response.This is in agreement with publishedwork (21).

Parameterized RBE values as a function of proton doseand depth (ie, LET) for acute and fractionated regimens areshown in Figure 4. In agreement with literature data, RBEfor acute exposure increases slowly in the SOBP regionbefore rising sharply at the distal dose falloff. In the SOBPregion there is also a small but significant increase in RBEas the total dose is reduced (from RBE Z 1.12 for 3.6 Gyto RBE Z 1.21 for 0.8 Gy). Under fractionated regimens,similar patterns for RBE with depth are observed, althoughthe data indicate a smaller RBE increase in the SOBP re-gion as a consequence of reducing the dose per fraction(from RBE Z 1.17 for 3.6 Gy/fraction to RBE Z 1.24 for0.8 Gy/fraction).

The clinical implication of these RBE increases forfractionated exposures is highlighted in Figure 5, where theexperimental RBE-weighted absorbed dose DRBE for acuteand fractionated exposures for various dose sizes is pre-sented alongside the clinically assumed profiles. Acute

A

B

1.2

1.0

0.8 Dose3.6 Gy2.4 Gy1.8 Gy1.2 Gy0.8 Gy

Dose3.6 Gy / fraction2.4 Gy / fraction1.8 Gy / fraction1.2 Gy / fraction0.8 Gy / fraction

0.6

0.4

0.2

0.0 0.0

0.5

1.0

1.5

2.0 RBERBE

2.5

3.0

3.5

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

0 5 10 15 20 25 30 35

0 5 10 15 20 25 30 35

1.2

1.0

0.8

0.6

0.4

0.2

0.0

Water Depth (cm)

Rela

tive

Dos

eRe

lati

ve D

ose

Water Depth (cm)

Fig. 4. Parameterized relative biologically effective (RBE)values superimposed over spread-out Bragg peak dose-depthprofiles for plateau doses of 3.6, 2.4, 1.8, and 0.8 Gy in acute(A) and fractionated (B) regimens. RBE values are calculatedusing equations 3, 4, and 5 and X-ray data from Table E1(available online at www.redjournal.org).

fraction size.

deliveries see significant increases in delivered DRBE versusclinical assumptions (RBE Z 1.1), particularly for smallerdoses and in the distal region. Fractionation increases thiseffect in the SOBP region, seeing increases of 8.3% to12.1% in integral DRBE over the clinical case in comparisonwith 4.6% to 10.6% for the acute delivery of the samedoses. The percentage increase is higher for smaller dosesper fraction. The greatest difference between the experi-mental and clinically assumed DRBE lies in the distal dosefalloff region, as shown in Figure E4 (available online atwww.redjournal.org). The increase in effective range (pointof 80% peak DRBE) over the clinically assumed DRBE

profile shows a marked difference (w7%) between acuteand fractionated delivery toward higher SOBP doses(0.82 mm vs 0.88 mm at 3.6 Gy) (Fig. E5; available onlineat www.redjournal.org) before converging at lower dosesper fraction.

EQD2 of fractionated proton regimens

The parameterized RBE for single fractions of protonradiation is shown in Figure 6A. Consistent with the sharprise in LET, the highest RBE is found toward the moredistal positions of the SOBP. For each position, RBE ishigher for lower doses. The nonuniformity in biologicaleffect when using a variable RBE is compounded undermultifraction regimens, under low dose per fraction and inthe low-dose regions of the proton dose profile inparticular. As a result, it is useful to quantify the impactof a variable RBE on a typical clinical fractionationschedule.

A B2.5

2.0

1.5

1.0

0.5

0.00 1 2 3 4 5

110

100

90

80

70

60

50

400 5 10 15 20

Entrance

RBE

Equi

vale

nt P

hoto

n Do

sein

2 G

y Fr

acti

ons

(Gy)

Proximal

Central

Distal

Total Proton Dose (Gy) Proton Dose per Fraction (Gy)

EntranceProximalCentralDistalFixed RBE

Fig. 6. (A) Measurement of cell killing RBE relative to 225 kVp X-rays as a function of acute proton dose at the 4experimental positions on the SOBP. (B) Dose per fraction required to deliver a regimen equieffective to the conventionaldelivery of 35 fractions of 2 Gy X-rays. All data derived from proton a and b values obtained by the linear quadratic model.

Marshall et al. International Journal of Radiation Oncology � Biology � Physics76

Figure 6B outlines the equivalent photon dose in 2-Gyfractions (EQD2) for regimens considered isoeffectiveusing a constant RBE of 1.1 to deliver a clinically relevantEQD21.1 of 70 Gy to AG01522 cells. Details for EQD2calculations are outlined in Supplementary Information(available online at www.redjournal.org). By incorpo-rating a variable RBE using the same regimens, the pre-dicted equivalent doses in all experimental positionsdeviate from those using the clinical assumption with anincrease in predicted equivalent dose in hyperfractionatedregimens and a reduction in the case of hypofractionatedregimens. Notably, equivalent doses in the distal positionare underestimated clinically for all fraction sizes.

Discussion

The lack of robust experimental data exploring fractionatedproton radiation presents a substantial opportunity to gaininsight into the effects of an inhomogeneous cell responsealong clinical dose profiles. Although the sole use of theAG01522 cell line is not a comprehensive representation,this study provides useful reference data and highlights aninteresting trend of RBE as a function of LET and fractionsize, examining also the potential clinical implications. Thefindings from this report indicate a significant increase inRBE over the acute delivery of protons, where the sametotal physical doses are delivered in fractionated regimens.This is particularly evident toward the distal dose falloff(Fig. 5), where high RBE values have been reported foracute exposures (21-24). Such an increase in effectivenessof fractionated exposures is proportional to the LET andinversely proportional to the dose per fraction delivered.Previously reported in vivo experiments (25, 26) haveindicated a constant RBE with fractions for the middle ofthe SOBP but have acknowledged that the end of the SOBPwas w1.14 more effective also for fractionated exposures.

The data presented further highlight the inadequacy ofextrapolating the cell response from X-ray radiation in theform of a generic, fixed RBE value of 1.1 and outline thedifficulties in delivering isoeffective doses to treatmentregions in terms of biologically effective beam range anddose.

Exposures to low-LET regions of the SOBP appear toproduce cell survival levels similar to those of X-rays, withsteeper and more linear survival curves correlating stronglywith the increasing LET toward more distal regions. Thisincrease in RBE with LET supports the hypothesis of morecomplex damage and has been observed in several previousstudies for various cell lines (27, 28), and in vivo mousemodels (25, 26) with the same linear relationship betweenap and LET having been previously reported throughextensive analysis of current radiobiological data byWedenberg et al (29).

The adoption of a linear quadratic formalism has suc-cessfully been used for clinical schedules using low LETradiation and closely describes the experimental data (30).The use of an experimental interfractional rest period of24 hours reflects current clinical practice and appears to beadequate for the complete repair for sublethal damage forthe AG01522 normal fibroblast cells. Complete repair of thefibroblast cells under these experimental conditions (eg,dose, LET) has previously been observed by the authorsusing immunofluorescence techniques to investigate DNAdamage (31). Cells associated with more erroneous repair orexposure to higher LET radiation may promote incompleterepair between fractions, compounding sublethal repair tosee higher than expected toxicity (32). The applicability ofthis approach for the full proton LET range and to tumorcells presents an avenue for further investigation.

Given the perception of protons as “low LET” radiation,there is a natural motivation to alter proton deliveries based onclinical experience with photons, with an advantageous dosedeposition profile providing an incentive to deliver higher

Volume 95 � Number 1 � 2016 Fractionated proton therapy RBE 77

doses per fraction. Isoeffect calculations in this study have,however, outlined how the strong dependence of RBE on theproton LET component must be taken into consideration,particularly whenvarying dose per fraction. Additionally, thisvariation in RBE must be noted when comparing photon andproton schedules in the evaluation of clinical trials. TheAG01522 cells in this study (a/bZ 6.4Gy) provide an insightinto the behavior of late-responding tissues under variousfractionation schedules. The increased effectiveness in thedistal region for hyperfractionated exposures may reducethe therapeutic benefits of fractionation by counteracting thedifferential response with rapidly growing tumors. Addition-ally, the movement toward hypofractionation sees the poten-tial for overestimation of effective dose delivered under theassumption of a constant RBE for all but the most distalregions of the SOBP.

The inhomogeneous cell killing response observedacross the SOBP, the further effect of the different frac-tionation regimens on biological effectiveness, the effectiverange increases in the order of 1 mm, and the lack ofconfidence in predicting isoeffective treatments show sig-nificant limitations in the use of a generic, fixed value ofRBE of 1.1. The experimental RBE variations and theirimplications for fractionated proton radiation therapyobserved in this study support the incorporation of a vari-able RBE in the planning of clinical treatments. This studyprovides a dataset from primary human cells that can beused for assessing optimization strategies for fractionatedproton radiation therapy in line with similar studies onvariable RBE in treatment planning (17).

References

1. Suit HD, Goitein M, Munzenrider J, et al. Increased efficacy of ra-

diation therapy by use of proton beam. Strahlenther Onkol 1990;166:

40-44.

2. Karger CP, Jakel O. Aktueller Stand und neue Entwicklungen in der

Ionentherapie. Strahlenther Onkol 2007;183:295-300.

3. Fokas E, Kraft G, An H, et al. Ion beam radiobiology and cancer:

Time to update ourselves. Biochim Biophys Acta 2009;1796:216-229.

4. Weber U, Kraft G. Design and construction of a ripple filter for a

smoothed depth dose distribution in conformal particle therapy. Phys

Med Biol 1999;44:2765-2775.

5. Loeffler JS, Durante M. Charged particle therapy: Optimization,

challenges and future directions. Nat Rev Clin Oncol 2013;10:411-

424.

6. Goodhead DT. Energy deposition stochastics and track structure: What

about the target? Radiat Prot Dosimetry 2006;122:3-15.

7. Paganetti H, Niemierko A, Ancukiewicz M, et al. Relative biological

effectiveness (RBE) values for proton beam therapy. Int J Radiat

Oncol Biol Phys 2002;53:407-421.

8. Levin WP, Kooy H, Loeffler JS, et al. Proton beam therapy. Br J

Cancer 2005;93:849-854.

9. Prescribing, recording, and reporting proton-beam therapy. J ICRU

2007;7:NP.

10. International Atomic Energy Agency, International Commission on

Radiation Units and Measurements (ICRU). Relative biological

effectiveness in ion beam therapy; 2008;1-165.

11. Brahme A. Development of radiation therapy optimization. Acta

Oncol 2000;39:579-595.

12. Chaudhary P, Marshall TI, Perozziello FM, et al. Relative biological

effectiveness variation along monoenergetic and modulated Bragg

peaks of a 62-MeV therapeutic proton beam: A preclinical assessment.

Int J Radiat Oncol Biol Phys 2014;90:27-35.

13. Guan F, Bronk L, Titt U, et al. Spatial mapping of the biologic

effectiveness of scanned particle beams: Towards biologically opti-

mized particle therapy. Sci Rep 2015;5:9850.

14. Calugaru V, Nauraye C, Noel G, et al. Radiobiological characteriza-

tion of two therapeutic proton beams with different initial energy

spectra used at the Institut Curie Proton Therapy Center in Orsay. Int J

Radiat Oncol Biol Phys 2011;81:1136-1143.

15. Paganetti H. Relative biological effectiveness (RBE) values for proton

beam therapy: Variations as a function of biological endpoint, dose,

and linear energy transfer. Phys Med Biol 2014;59:R419-R472.

16. Mohan R, Mahajan A, Minsky BD. New strategies in radiation ther-

apy: Exploiting the full potential of protons. Clin Cancer Res 2013;19:

6338-6343.

17. Dasu A, Toma-Dasu I. Impact of variable RBE on proton fraction-

ation. Med Phys 2013;40:011705.

18. Habl G, Hatiboglu G, Edler L, et al. Ion prostate irradiation (IPI): A

pilot study to establish the safety and feasibility of primary hypo-

fractionated irradiation of the prostate with protons and carbon ions in

a raster scan technique. BMC Cancer 2014;14:202.

19. Newhauser W, Zhang R, Jones T, et al. Reducing the cost of proton

radiation therapy: The feasibility of a streamlined treatment technique

for prostate cancer. Cancers (Basel) 2015;7:688-705.

20. Wang Y, Efstathiou JA, Lu HM, et al. Hypofractionated proton therapy

for prostate cancer: Dose delivery uncertainty due to interfractional

motion. Med Phys 2013;40:071714.

21. Wilkens JJ, Oelfke U. A phenomenological model for the relative

biological effectiveness in therapeutic proton beams. Phys Med Biol

2004;49:2811-2825.

22. Belli M, Bettega D, Calzolari P, et al. Inactivation of human normal

and tumour cells irradiated with low energy protons. Int J Radiat Biol

2000;76:831-839.

23. Robertson JB, Williams JR, Schmidt RA, et al. Radiobiological

studies of a high-energy modulated proton beam utilizing cultured

mammalian cells. Cancer 1975;35:1664-1677.

24. Courdi A, Brassart N, Herault J, et al. The depth-dependent radiation

response of human melanoma cells exposed to 65 MeV protons. Br J

Radiol 1994;67:800-804.

25. Gueulette J, Bohm L, Slabbert JP, et al. Proton relative biological

effectiveness (RBE) for survival in mice after thoracic irradiation with

fractionated doses. Int J Radiat Oncol Biol Phys 2000;47:1051-1058.

26. Gueulette J, Slabbert JP, Bohm L, et al. Proton RBE for early intestinal

tolerance in mice after fractionated irradiation. Radiother Oncol 2001;

61:177-184.

27. Friedrich T, Scholz U, Elsasser T, et al. Systematic analysis of RBE

and related quantities using a database of cell survival experiments

with ion beam irradiation. J Radiat Res 2013;54:494-514.

28. Britten RA, Nazaryan V, Davis LK, et al. Variations in the RBE for

cell killing along the depth-dose profile of a modulated proton therapy

beam. Radiat Res 2013;179:21-28.

29. Wedenberg M, Lind BK, Hardemark B. A model for the relative

biological effectiveness of protons: The tissue specific parameter a/bof photons is a predictor for the sensitivity to LET changes. Acta

Oncol 2013;52:580-588.

30. Brenner DJ. The linear-quadratic model is an appropriate methodol-

ogy for determining isoeffective doses at large doses per fraction.

Semin Radiat Oncol 2008;18:234-239.

31. Chaudhary P, Marshall TI, Currell FJ, et al. Variations in the pro-

cessing of DNA double-strand breaks along 60-MeV therapeutic

proton beams. Int J Radiat Oncol Biol Phys 2016;95:86-94.

32. Antonelli F, Bettega D, Calzolari P, et al. Inactivation of human cells

exposed to fractionated doses of low energy protons: Relationship

between cell sensitivity and recovery efficiency. J Radiat Res 2001;42:

347-359.