Mania Aspradakis John Byrne Hugo Palmans John Conway Jim Warrington Karen Rosser Simon Duane

Transcription

1 Mania Aspradakis John Byrne Hugo Palmans John Conway Jim Warrington Karen Rosser Simon Duane

2 Why are we concerned with small MV photon fields? SRS dosimetry Total scatter factor with various detectors Cone Factor (St) % Diamond CEA (Film) Kodak(Film) TLD Pinpoint(par) Pinpoint(per) 0.125ion(par) 0.125ion(per) MC(1mm) MC(5mm) Which detector and measurement methodology? Das et al, J Radiosurgery, 3, , Cone Diameter (mm)

3 Why are we concerned with small MV photon fields? Incidents due to errors in dosimetry Is there enough education and training to carry out dosimetric measurements in small fields? 3

4 Why are we concerned with small MV photon fields? Small MV photon fields on equipment originally designed and/or configured for treatments using broad photon fields E.g. source size modelling on TPS Aspradakis et al Med Dos 30, 233, 2005 Appropriate fluence and dose models on TPSs? 4

5 Why are we concerned with small MV photon fields? Use of specialised equipment and techniques How to calibrate specialised equipment? 5

6 With decreasing field size: 6

7 With decreasing field size: The dose at the inner part of the field is influenced by the lateral range of the electrons compared to the field size r max 5 x 4 mm x 200 mm 2 7

8 Definition of small MV photon field For the selected energy and medium, is the field size large enough to ensure lateral CPE? Is the entire source in the detector s-eyeview? Is the detector small enough not to perturb fluence significantly? 8

9 IPEM report 103: contents Chapter 1: Introduction Chapter 2: Physics and challenges in small field dosimetry Chapter 3: Detectors Chapter 4: Machine QA Chapter 5: General considerations Chapter 6: Reference dose measurement Chapters 7, 8, 9: Relative dose measurement Chapter 10: Monte Carlo Chapter 11: Verification Chapter 12: Summary and conclusions 9

10 Reference dose measurement with air-filled ionisation chambers o p,q Q water air p,q Q water air p,q Q water air Q air p,q Q water air Q air Q,Q k S k S k S e W k S e W k = ρ ρ ρ ρ Conclusion: Existing water to air ratios of Spencer-Attix restricted mass collision stopping powers published for broad (10cm x 10cm) fields can be used for dosimetry in small and composite fields Challenge: derivation of perturbation factors for available small field (mini- and micro-) ionisation chambers 10

11 Reference dose measurement with air-filled ionisation chambers - volume effect OAR(x,y) is the off axis distribution of field A in orthogonal directions x and y Kawachi el al (2008), Med Phys 35 (10) A chamber of cavity length of 24mm underestimates dose by 1.5% in the 6cm field on Cyberknife Reference dose measurements in a 6cm diameter radiation field need to be carried out with an ionisation chamber of length not greater than 10mm at a source-to-chamber distance of 80cm 11 (Cyberknife).

12 Reference dosimetry 1. Use of mini- or micro- air-filled ionisation chambers (BUT signal to noise ratio?) 2. The lack of perturbation factors increases uncertainty in the measurement 3. Need to consider: Chamber fully covered by the radiation field Leakage Cable effects Polarity effects 4. Liquid-filled ion-chambers, diamonds and diodes not yet sufficiently characterised and commissioned for use in erence dosimetry 5. Specialised systems: alternative erence conditions or adopt the proposed IAEA/AAPM formalism (Alfonso el al (2008), Med Phys 35 (11)) alternative procedures to determine beam quality (Sauer, (2009). Med Phys 36(9): ). 12

13 Relative dosimetry: measurement of penumbra To detect the penumbra correctly use a small diode (consider directional dependence) Check the detector response outside the geometrical field Correct for over/under-response or use an appropriate detector. [e.g. (shielded) diode or radiochromic film] Ø7 mm Ø23 mm Heydarian et al PMB (1996)

14 Relative dosimetry Determination of depth functions (RDD/PDD/TPR) - 1 Conversion between RDD/PDD and TPR/TMR not recommended How valid are existing conversion formulas at small fields and nonstandard SSDs? BJR25 (1996), NCS 1998, Bjarngard et al, (1996), Med Phys 23(5): Functional representation of TPR not recommended Xiao et al, (1998), Phys Med Biol 43(8): , Sauer et al (2009) Med Phys 36 (12) 14

15 Relative dosimetry Determination of depth functions (RDD/PDD/TPR) - 2 Measurement Micro ionisation chambers (volume < 0.01cm 3 ) Small Diode; for smallest field: SFD Radiochromic film (e.g Gafchromic EBT, MD-55) Caul alignment of detector with CAX Estimation of the volume effect with changing depth Direct measurement of TPR/TMRs (sourcedetector-distance constant!) 15

16 Relative dosimetry: Field size factor, S cp S D cp w = D D w ( A, z) ( A, z) w ( A) = k( A) Ddet = ke ( A) kp( A) Ddet relative Dose MV meas IC PiP DiGre DiYe MOS1 MOS2 DIA SES / cm Field size dependence of energy correction factor k E for different detectors? perturbation correction factor k p? volume effect Dose / Gy/100MU MV corr IC PiP DiGre DiYe MOS1 MOS2 DIA Sauer & Wilbert MP 34, 2007, SES / cm

17 Relative dosimetry: Field size factor, S cp measurement with an ionisation chamber high perturbation? S cp ( A) = D D w ( A,z ) ( A z ) w ( A, z ) ( A, z ) [ f ] A water,air A M S =, M ρ air w A A conversion factor [ k ] A p,det A Challenge: perturbation factors A: field size (aperture) z : erence depth 17

18 Relative dosimetry: Field size factor, S cp measurement with a diode high energy dependence S diode cp ( A) = D D ( A,z ) ( A, z ) = M M ( A,z ) k ( ) [ ] [ det A p,det ]A A A, z E, A k Characterise the sensitivity of the diode ( A) = a A b k + E 1 for detector Ø <A 18

19 Relative dosimetry: Field size factor, S cp Cross-calibrate a Si-diode at medium field sizes with a small IC ('daisychain') Evaluate energy/field size dependence Avoid volume effects (use a small detector) Corroboration of data Li et al MP 22, 1995, LEE Signal ratios IC PiP DiGre 6 MV DiYe MOS1 MOS2 DIA Eklund & Ahnesjö PMB SES / cm Sauer & Wilbert MP 34, 2007,

20 Relative dosimetry: Field size factor, S c in-air output ratio Mini-ionisation chamber or diode Mini-phantom design - high density material AAPM TG 74, Zhu et al 2009, MP 36(11) Caul alignment of detector with CAX Measuring at extended SSD problematic 20

21 IPEM report 103: supporting chapters Chapter 4: Machine acceptance and quality assurance machine alignment - better than 1mm/1 calibration and QA of collimating jaw - better than 0.5mm Chapter 5: General considerations with measurements detector construction, leakage, cable effects etc Chapter 10: Monte Carlo use of the method for small field applications Chapter 11: Verification on methods for verifying plans comprising of small fields 21

22 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 22

23 Small field MV photon dosimetry Future requirements TPS and equipment manufacturers to share information on the definition of field size on their systems Machine QA procedures and tolerances to be extended to include checks at narrow collimated beams Validate (or develop) appropriate detector systems and detector response correction methods (Computer-Aided-Dosimetry?) for erence and relative dosimetry Extend current typical datasets (e.g BJR Suppl25, NCS12) to small fields Better fluence and dose engines on TPSs and MU calculators to reduce systematic errors in treatment planning calculations 23

24 Thank you for your attention! Acknowledgement Dr Anders Ahnesjö Prof Otto Sauer Mr Geoff Budgell Prof Frank Verhaegen Ms Marie Goodall, IPEM Office