Small field dosimetry poses challenges due to the lack of charged particle equilibrium and penumbra effects. Monte Carlo modeling and specialized dosimeters are used to measure small fields between 0.3-4 cm used in techniques like IMRT, stereotactic radiosurgery, and cyberknife. Film, pinpoint chambers, and MOSFET dosimeters provide high spatial resolution but require corrections. Gel dosimeters allow 3D dose mapping but are time-consuming. Future work aims to minimize measurement perturbations in small fields.

1. R. Ahmad Razali, P. Hadjitheodorou, L. Jambi,
M. Marshall, J. Wardle
MSc Medical Physics
May 2013

2.  What are small fields?
 Uses of Small Fields.
 Challenges:
◦ CPE & Penumbra.
◦ Monte Carlo Modelling.
 Dosimeters.
 Conclusions.

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

4. 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

5.  Intensity Modulated Radiotherapy
 Helical Tomotherapy
 GammaKnife
 CyberKnife
 SRS & SRT
http://cdn.medgadget.com/img/45ndftgq.jpg

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

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

8. http://www.universityneurosurgery.com/index.php?src=gendocs&ref=TheGammaKnife&categ
ory=SpecialtiesServices&submenu=TheGammaKnife
http://www.gamma-knife-surgery.com/

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

10. http://www.plymouthhospitals.nhs.uk/ourservices/healthcareprofessionalsguide/Pages/Ste
reotacticRadiosurgery.aspx
http://www.cancerjournal.net/article.asp?issn=0973-
1482;year=2012;volume=8;issue=2;spage=215;epage=221;aulast=Bernard

11.  “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.

12.  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.

13.  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.

14.  Pinpoint Air Ion Chamber.
 Radiochromic Film.
 Scintillator Fibre Optic Dosimeter (SFOD).
 Metal-oxide silicon semiconductor field-
effect transistor (MOSFET) dosimeters.
 Gel Dosimeter.

15.  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

16.  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.

17.  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.

18.  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.

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

20. 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

21.  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.

22.  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.
http://www.techpowerup.com/articles/overclocking/voltmods/21

23. 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.

24.  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

25.  Advantages:
◦ Radiation dose distribution in 3D
Disadvantages:
◦ Long processing time to acquire a 3D distribution
image.
◦ High cost.

26.  What does the future hold for gel dosimeters?
http://koenigshof-lippstadt.de/fotos/polymer-gel

27.  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.