Small photon field dosimetry: present status

1. Small photonfield dosimetry: presentstatus Maria ManiaAspradakis Cantonal Hospital of Lucerne Switzerland

2. IPEM report 103: main scope • Educate on the physics and challenges in the dosimetry of small MV photon fields. • Review commercially available detectors and measurement methodologies suitable for implementation in the clinic. • Give recommendations of good practice in order to reduce uncertainty in the determination of dosimetric parameters • Explain the need to commission TPSs specifically for small fields. • To point out directions along which future work and research efforts are required in this challenging field of dosimetry. Aspradakis M, IDOS, IAEA, Vienna, Nov 2011

3. Other reviews on small field dosimetry • R. Alfonso, P. Andreo, R. Capote, M. S. Huq, W. Kilby, P. Kjäll, T. R. Mackie, H. Palmans, K. Rosser, J. Seuntjens, W. Ullrich, and S. Vatnitsky, “A new formalism for reference dosimetry of small and nonstandard fields,” Med. Phys. 35, 5179–5187 (2008). • I. J. Das, G. X. Ding, and A. Ahnesjö, “Small fields: Nonequilibriumradiation dosimetry,” Med. Phys. 35, 206–215 (2008). • H Palmans (2011) CN-182-INV006, Small and composite field dosimetry: the problems and recent progress. IDOS Conference, Vienna. Note: There has been an explosion in the literature since 2008 on the topic of small field dosimetry!

4. Outline of presentation • Small MV photon field conditions • Present status on: • reference dosimetry • output factor determination Discussion in this presentation restricted to dose determination in static MV photon fields

5. Small MV photon field conditions • For the selected energy and medium, the field size is not large enough to ensure lateral CPE (lack of LEE). • The entire source is not in the detector’s-eye-view (source occlusion). • The detector is not small enough and perturbs fluence significantly (detector issues)

6. Detector size relative to field size Small field conditions exist when one of the edges of the sensitive volume of a detector is less then a lateral charged particle equilibrium range (rLEE) away from the edge of the field Slide courtesy: H. Palmans Li et. al., Med. Phys. 22, (1995), 1167-1170

7. In narrow fields: source occlusion source occlusion by the collimators point source used in calculation Very narrow dose profile finite source size used in calculation Partial view of extended direct beam source from the point of measurement Radiation detector measures in non-uniform dose region Treuer et al PMB 38 (12), 1992

8. Overlapping penumbras  apparent field widening definition of field size? Das et al., Med. Phys. 35: 2008, 206-15

9. Uncertainty in output factor correction introduced due to field size definition Cranmer-Sargison et al, Med. Phys. 38, 2011. 6592-6602 Benhmakhlouf et al, Med. Phys. 41,2014, 041711 Slide courtesy: H. Palmans

10. Detector issues in small field dosimetry • Energy dependence of detector response • Perturbation effects • Volume averaging • Ionization chambers: wall, central electrode, air cavity different from water • Solid state detectors (e.g. diodes): housing, shielding, coating of silicon chip

11. Detector issues in small field dosimetry Perturbation effects: volume averaging 2.5 mm 5 mm 5.8 mm 0.3% 6.6 mm 1.4% 16.25 mm 23 mm Pantelisel al, Med Phys 37 (5), (2010)

12. Correction for volume averaging Azangwe et al, Med. Phys. 41, 072103 (2014) A correction for volume averaging can be derived from the ratio of the detector response in its central part to the detector response over its whole volume Georg et al, 2nd ESTRO Forum, Pre-meeting workshop 2013

13. Correction for volume averaging Morin et al MP, 40(1), 2013 Ralston et al PMB, 57, 2012

14. Detector issues in small field dosimetry Perturbation effects: detector construction Crop et al., Phys Med Biol 54:2951 (2009) C. McKerracher& D.I. Thwaites Radiotherapy and Oncology 79 (2006) 348–351

15. Detector perturbationthe influence of detector density at small field sizes 15MV Scott et al PMB, 57 (2012) 4461-4476

16. Small field reference dosimetry

17. IAEA/AAPM formalism for reference dosimetry Small static MV fields: reference dose determination beam quality of machine specific reference field machine specific reference field Alfonso el al (2008), Med Phys 35 (11)

18. IAEA/AAPM formalism for reference dosimetry field instrument correction factors ref detector • Determined through: • Experiment: by a primary standard • Experiment: using dosimeters that can measure reference dose traceable to a primary standard and which have sufficiently low uncertainty (alanine, radiochromic film, diamond, liquid ion-chambers ...) • Calculation: Monte Carlo simulations (MC) field instrument

19. IAEA/AAPM formalism for reference dosimetrysmall static MV photon fields Reference dosimetry on Cyberknife: chamber factors calculated with MC Franscesconet al (2012), PMB 57 3741-3758 Gago-Arias el al (2013), Med Phys 40 (1), 011721-1, & Erratum Med Phys 40, 011721-1-10, 2013 Exradin A12 NE 2571 PTW 30006 PTW 31014 PTW 31014 IBA CC13

20. Specification of a reference-class ionisation chamber Not all micro-chamberdesignsareconsideredsuitableforreference dosimetry McEwen , Med Phys 37, 2010, 2179-93

21. Small field relative dosimetry - output factor , Scp Sauer & Wilbert Med. Phys. 34, 2007, 1983-1988

22. IAEA/AAPM formalism for relative dosimetryoutput factor determination in small static fields Small field detector specific correction factors Alfonso el al (2008), Med Phys 35 (11), new CoP: IAEA TECDOC###

23. Small fields: relative dosimetry – output factor Scp Detector specific correction factors 6MV Siemens Elekta Francesconet al Med. Phys. 38(12), 2011

24. IAEA/AAPM formalism for relative dosimetrySmall field detector correction factors Accounts for three main detector perturbation effects: Ratio of correction factors to account for the spectral dependence of photon energy absorption in the detector medium Ratio of volume averaging correction factors Ratio of charged particle fluence perturbation correction factors

25. Only different degree in CPE & spectral effects considered These result confirm previous conclusions that unshielded diodes a better choice of detector than shielded diodes. The corrections for mini-ionization chambers used in this study (active volume between 0.015 cm3 and 0.05 cm3) were generally lower than 10% and for micro-chambers (active volume<0.015 cm3) lower than 3%. active volume > 0.1 cm3 corrections of 20%-30% ! Azangweet al Med. Phys. 41 (7), 2014

26. IAEA/AAPM formalism for relative dosimetrySmall field detector correction factors Benmakhloufet al Med. Phys. 41 (4), 2014

27. IAEA/AAPM formalism for relative dosimetrySmall field detector correction factors Benmakhloufet al Med. Phys. 41 (4), 2014

28. Summary & conclusions • Research in small field dosimetry, after the publication of IPEM report 103, has lead to improved understanding of detector response in such fields. • Current research efforts in small field dosimetry focus on the determination of detector-specific output correction factors. • Detectors requiring output corrections greater that 5% are not recommended for dose determination in small fields. • The new IAEA TECDOC will include a consistent set of such data and will be an international code of practice for small static field dosimetry.

29. Thank you for your attention! ‘Sunset over the Libyan sea’, Ierapetra, Crete Maria.Aspradakis@luks.ch