The Challenges of Small Field Dosimetry

R. Ahmad Razali, P. Hadjitheodorou, L. Jambi, M. Marshall, J. Wardle

MSc Medical PhysicsMay 2013

What are small fields? Uses of Small Fields. Challenges:

◦ CPE & Penumbra.◦ Monte Carlo Modelling.

Dosimeters. Conclusions.

Outline

40x40 cm² - 4x4 cm² 4x4 cm² - 0.3x0.3 cm²

What are small fields?

What are small fields?

Aspradakis M., ByrneWinston J., (2011), Small field dosimetry: challenges and progress, Medical Physics Web, accessed 17 March 2013,

http://medicalphysicsweb.org/cws/article/opinion/45334

Intensity Modulated Radiotherapy

Helical Tomotherapy GammaKnife CyberKnife SRS & SRT

Uses of Small Fields.

http://cdn.medgadget.com/img/45ndftgq.jpg

IMRT

http://aventuracomprehensivecancercenter.com/our-services/radiation/imrt.dothttp://radiationoncology.emory.edu/

Helical Tomotherapy

http://www.cityofhope.org/patient_care/treatments/radiation-oncology/Pages/helical-tomotherapy.aspxhttp://www.european-hospital.com/de/article/3433-Helical_tomotherapy.html

Gamma Knife

http://www.universityneurosurgery.com/index.php?src=gendocs&ref=TheGammaKnife&category=SpecialtiesServices&submenu=TheGammaK

nifehttp://www.gamma-knife-surgery.com/

Cyber Knife

http://cdn.medgadget.com/img/45ndftgq.jpghttp://coloradocyberknife.com/treatment/spine-tumors/

SRS / SRT

http://www.plymouthhospitals.nhs.uk/ourservices/healthcareprofessionalsguide/Pages/StereotacticRadiosurgery.aspx

http://www.cancerjournal.net/article.asp?issn=0973-1482;year=2012;volume=8;issue=2;spage=215;epage=221;aulast=Bernard

“CPE exists at point P in volume V, if each charged particle carrying certain energy out of the volume is replaced by another identical particle carrying the same energy into the volume.”

In small fields CPE does not exist, therefore the dose is not equal to the KERMA.

Cavity theory is used for dosimetry and is directly related to CPE.

Charged Particle Equilibrium (CPE)

Bragg-Gray Principle “The amount of ionization produced in a small gas filled

volume cavity surrounded by a larger, solid absorbing medium is directly proportional to the radiation energy absorbed by the solid”

Spencer-Attix Made a correction to consider the creation of secondary

(delta) electrons, generated by hard collisions in the slowing down of the primary electrons in the sensitive volume of the dosimeter.

Burlin Extended both the Spencer-Attix and Bragg-Gray cavities

theories to cavities of intermediate dimensions by introducing a large cavity limit with the use of a weighting technique.

Cavity Theory

2 main approaches: Making simulations and obtaining the corresponding

corrections from the equivalent detector measurements. Calculating the dosimeter quantities equivalent to

performing a measurement under ideal conditions.

Advantages Flexibility. Simplicity of the system which can be extended and

developed as needed. Easily understood by non-programmers.

Disadvantages Length of time needed for complex simulations. Estimated output.

Monte Carlo Modelling

Pinpoint Air Ion Chamber. Radiochromic Film. Scintillator Fibre Optic Dosimeter (SFOD). Metal-oxide silicon semiconductor field-

effect transistor (MOSFET) dosimeters. Gel Dosimeter.

Dosimeters

Pinpoint Air Ion Chamber It is a cylindrical air

chamber with a steel electrode located centrally.

Its main properties are waterproof and fully restrained.

The wall material is tissue equivalent made of a mixture of PMMA covered with a graphite layer.

http://www.standardimaging.com/print_product.php?id=78

Advantages Small sensitive volume.

Disadvantages Overestimating the penumbra width due to its

confined size. Altering the electron transport due to the presence of

air inside the detector.

Pinpoint Air Ion Chamber

Thin plastic film impregnated with radiation-sensitive chemicals or radiochromic dyes.

Acts through a polymerization process in which energy is transferred from an energetic photon or particle to the receptive part of the leuko-dye or colorlessphotomonor molecule, initiating color formation through chemical changes.

Radiochromic Film

Advantages◦ Self-developing, daylight handling and near tissue-

equivalence.◦ High intrinsic spatial resolution which makes it

particularly useful for analysing small fields.◦ Measures large range of doses, 0.2Gy to 100Gy.◦ No energy dependence.

Disadvantages◦ Cannot perform 3D measurements, single

use(cost), complex geometries.

Radiochromic Film

Radiochromic Film

http://www.maestro-research.org/pageprincipale1.htmhttp://www.jacmp.org/index.php/jacmp/article/view/3165/1880

Scintillating Fibre Optic Dosimeter (SFOD)

Yoo W.J., et al. 2013. Development of a scintillating fiber-optic dosimeter for measuring the entrance surface dose in diagnostic radiology. Radiation

Measurements, 48, 29-34

Organic Scintillator. No dosimetric perturbations due to small size. Linear energy response as low Z material. High resolution. Real time measurements.

Plastic Optical Fibres. Flexible, precise positioning. No interference from EM fields. Minimal perturbation.

CMOS (Complementary Metal Oxide Semiconductor)

Multichannel Analysis High spatial resolution, sensitivity, and quantum efficiency.

Scintillating Fibre Optic Dosimeter (SFOD)

When a MOSFET device is irradiated, three things happen within the sensitive region of silicon dioxide layer;• Build-up of trapped charge in the oxide. • Increase in the number of interface traps and increase in

the number of bulk oxide traps. • These results in a negative threshold voltage shift (VTH),

which can persist for years.

The dose is proportional to the voltage shift difference before and after exposure which can be measurable.

Metal-oxide silicon semiconductor field-effect transistor

http://www.techpowerup.com/articles/overclocking/voltmods/21

Metal-oxide silicon semiconductor field-effect transistor

Advantages of MOSFET devices are:• It reads directly with a very thin active area (less than 2

mm2).• Its physical size is less than 4 mm2 when packaged.• The post radiation signal is permanently stored and is dose

rate independent

Disadvantages: • Has a short life.• Requires repeated calibration for accurate dose

measurements.

1st suggested by Day & Stein in 1950 to use radiation sensitive materials (gel compounds) for the purpose of radiation dosimetry.

Clinical applications include to date:◦ IMRT◦ Stereotactic radiosurgery◦ Brachytherapy

Gel Dosimeters

Advantages:◦ Radiation dose distribution in 3D

Disadvantages:◦ Long processing time to acquire a 3D distribution

image.◦ High cost.

Gel Dosimeters

What does the future hold for gel dosimeters?

Gel Dosimeters

http://koenigshof-lippstadt.de/fotos/polymer-gel

The limitations include: Non-existence of CPE result from the secondary

electrons. Size of collimation causing a penumbra and non-

homogeneity of the field. Dosimeters perturb the level of disequilibrium

causing a cavity. Advantages outweigh the complexities.

Conclusion.

Thank You

Any Questions?