On the implementation of a recently proposed dosimetric formalism to a robotic radiosurgery system

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1 On the implementation o a recently proposed dosimetric ormalism to a robotic radiosurgery system Evaggelos Pantelis 1, 2, Warren Kilby 3, Argyris Moutsatsos 1, K. Zourari 1, P. Karaiskos 1, P. Papagiannis 1, C. Antypas 2, C. Hourdakis 4 1 Medical Physics Laboratory, Medical School, University o Athens, 75 Mikras Asias, , Athens, Greece 2 CyberKnie Center, Iatropolis-Magnitiki Tomograia, Ethnikis-Antistaseos, , Athens, Greece 3 Accuray Inc., Sunnyvale, Caliornia Ionizing Radiation Calibration Laboratory, Greek Atomic Energy Commission (GAEC), Agia Paraskevi, Athens Greece

2 Proposed ormalism by the IAEA/AAPM or the dosimetry o small and non-standard ields Reerence dosimetry D w, = M N K K, re D,w,o, o, Relative dosimetry Where : D w D = Ω, w, Ω,, = M M,, k,,

3 Scope o the work To implement the static small ield dosimetry route o the proposed dosimetric ormalism to a CyberKnie robotic radiosurgery system in order to : 1. Measure the K, re, correction actor o a Farmer chamber or reerence dosimetry in the ield. 2. Experimentally validate the K,, correction actors calculated ollowing a published Monte Carlo based methodology or small ields OF measurements.

4 CyberKnie reerence dosimetry measurements ield : 60 mm ixed 800 mm SDD Reerence dosimetry measurements are commonly perormed using Farmer chamber re Alonso et al. (Med. Phys., 35, , 2008) : The or a Farmer type chamber and the 60 mm ield o a CyberKnie system is unity within 0.1% * and 0.4% ** ~ 1.4% dose gradient across the Farmer ion chamber active volume due to the absence o lattening ilter * P. Francescon et al., in (theoretical Robotic Radiosurgery assessment based (CyberKnie on dose-proile society, Sunnyvale, 2005), Vol. 1, pp data, Kawachi et al., Med. Phys., 35, , ** F. Araki, Med.Phys. 33, 2008) (2006) K,,

5 Alanine reerence dosimetry Cylindrical pellets o 5 mm diameter and 2.5 mm height Provided and measured by the corresponding service o National Physical Laboratory (NPL) Calibrated in terms o absorbed dose to water in a 60 Co beam and multi-ion-chamber reerence dosimetry Ion Chamber Cavity Length (mm) Cavity Volume (mm 3 ) Farmer Semilex M Semilex Scdx CC PinPoint Exradin A

6 CyberKnie reerence dosimetry measurements 60 mm collimator 800mm SDD ( ) Measurement set-up: d = 50 mm (Geom. centre o detector active volume) Alanine dosimetry 5 pellets (5 mm diam., 2.5 mm height) 1106 MUs (=10 Gy) / pellet Measurements perormed using a water phantom (watertight sleeve ~0.1 mm thick) RW3 solid water cylindrical holder or accurate positioning o each pellet at the reerence depth. PTW Farmer type chamber measurements with the same MU setting to exclude output linearity eects. The experiment was repeated using dierent alanine batch provided by NPL

7 CyberKnie dosimetry measurements Measurement set-up: 60 mm collimator 800mm SDD ( ) d = 50 mm (Geom. centre o detector active volume) Ion chamber dosimetry TPR 20/10 ( re ) = re = 100x100 mm 2

8 Farmer vs. alanine reerence dosimetry results Parameter First batch Second batch Alanine Farmer Alanine Farmer Measured Dose (cgy) k, re re k,

9 Farmer vs. shorter cavity length ion chamber reerence dosimetry results Farmer re k, shorter cav. IC Farmer Alanine Farmer Combined estimation % K,0 is overestimated by ~0.5% in 6 MV lattening ree beams when TPR 20/10 is used as an energy speciier * * G. Xiong and D. W. O. Rogers, Med. Phys, 35, , (2008)

10 5mm, 7.5 mm, 10 mm and 15 mm ixed collimator ields Detectors: Small ield output actor measurements Two microchambers (situated with their stem parallel to beam axis) Exradin A16 (2.4mm diameter, 2.7mm length) PTW PinPoint (2 mm diameter, 5 mm length) Three diode detectors PTW shielded p-type diode PTW unshielded p-type diode SUN Nuclear n-type EDGE detector Alanine pellets TLD type 100 1x1x1 mm 3 microcubes EBT Gachromic ilms All the detectors were positioned with their eective point o measurement at the beam d = 15 mm

11 Correction actors or small ield OF measurements Microchamber and diode detectors Detector measurements or the 5, 7.5 and 10 mm ields were corrected applying K,, correction actors calculated ollowing a published machine speciic Monte Carlo based methodology* Alanine detectors The 5 mm diameter o the alanine dosimeters introduce signiicant volume averaging eects in CK small ield OF measurements K,, calculated using published polymer gel measured dose distribution data or the CK ields** *P. Francescon,et al., Med. Phys., 35, , (2008) *P. Francescon,et al., J. Appl. Clin., 10, , **E. (2008) Pantelis et al., Med. Phys., 35, , (2008)

12 Correction actors or small ield OF measurements Detector M M 5 mm 7.5 mm 10 mm k,, M M k,, M M k,, A (15) (3) (10) (3) (6) (3) PinPoint (17) (3) (7) (3) (5) (3) Diode (1) (3) (1) (3) (1) (3) Diode (1) (3) (2) (3) (1) (3) EDGE (1) (3) (1) (3) (1) (3) Alanine (8) (8) (12) (4) (13) (3) TLD (4) (6) (8) - EBT ilms (17) (16) (18) - Polymer gels (21) (27) (29) -

13 Measured vs. corrected output actor results w. mean ± w. mean ± w. mean ± w. mean ± Signiicant variations which are increased with decreasing ield size (up to 33% or the 5 mm ield) 5 mm ield: all microchamber and diode corrected output actors agree within 1.5% and 3% compared to alanine and TLD results, respectively

14 Conclusions Reerence dosimetry Alanine vs. Farmer reerence dosimetry : = ± Shorter cavity length ion chamber reerence dosimetry results combined with K, re, indings o the literature : 1.01 K, re, Both results agree within experimental uncertainty and urther work is being K, re, perormed to calculate the correction actor o the Farmer chamber with lower uncertainty using Monte Carlo simulations. Small ield output actor measurements Substantial improvements in measurement accuracy can be obtained when using microchambers and diodes by applying appropriately calculated correction actors to the detector measurements.

15 Thank you or your attention An extensive description o this ongoing study can be ound in : Pantelis et al., Med. Phys., 37, (2010)