challenges of small field dosimetry
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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.
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