factors for NE2561 ionization chambers in 3 cm x 3 cm beams of 6 MV and 10 MV photons

Transcription

1 Calorimetric determination of k Q factors for NE2561 ionization chambers in 3 cm x 3 cm beams of 6 MV and 10 MV photons PTB s water calorimeter in front of Elekta Precise medical linac

2 Calorimetric determination of k Q factors for NE2561 ionization chambers in 3 cm x 3 cm beams of 6 MV and 10 MV photons 1.motivation: dosimetry in small radiation fields 2.calorimetric determination of D W 3.details: heat conduction effects in small fields 4 results and summary

3 motivation: dosimetry in small radiation fields Dosimetry in small fields? (static or composite) for example TomoTherapy CyberKnife GammaKnife IMRT Reference dosimetry (e.g.: IAEA-TRS 398) D W,Q = M Q N D,W,Q 0 k Q,Q0 k Q,Q 0 field size: 10 cm x 10 cm! water phantom ionization chamber k Q,Q 0 still valid in small fields? Primary standard for D W in small fields u(d W ) < 0.5%

4 calorimetric determination of D W D W = ΔT c p Πk i ΔT : 0.24 mk/gy operated at 4 C k i : corrections for heat conduction, perturbation, dose distribution, heat defect,

5 ..details of the water calorimeter (operated at 4 C) 95 mm 30 cm 0.5 mm

6 calorimetric determination of D W in 3 cm x 3 cm radiation field D W = ΔT c p Πk i ΔT : 0.24 mk/gy operated at 4 C k i : corrections for heat conduction, perturbation, dose distribution, heat defect, static radiation field: 3 cm x 3 cm (external collimator) strong heat conduction effects dose distribution in water for 6 MV photons

7 details: heat conduction effects in small fields (3cm x 3cm) beam OFF calculated temperature distribution (2D) ΔR / Ohm without heat conduction ΔT thermistor #1 beam ON t / s measurement signal with heat conduction 6 MV photons 120 s irradiation 120 s drift ΔR / Ω 0,5 0,4 0,3 0,2 0,1 0,0 thermistor # t / s

8 details: heat conduction effects in small fields (3cm x 3cm) measurement signal #1 1,4 measurements #1- #4 0,5 1,2 0,4 1,0 #4 0,3 0,8 #3 ΔR / Ω 0,2 ΔR / Ω 0,6 #2 0,4 6 MV photons 120 s irradiation 120 s drift 0,1 0,0 thermistor # t / s 0,2 0,0 #1 thermistor # t / s heat conduction effect depends on: dose distribution irradiation time # of irradiation position of the thermistors

9 details: heat conduction effects in small fields (3cm x 3cm) Heat transport calculations (FEM) dose distribution, irradiation time, irradiation sequence #1-4, position of the thermistors 1,4 6 MV 120s thermistor #1 irradiations #1-4 ΔR / Ω 1,2 1,0 0,8 0,6 0,4 #2 #3 #4 0,2 #1 0, t / s Correction factors k C

10 details: heat conduction effects in small fields (3cm x 3cm) Calculated heat conduction corrections k C MV 120s ,4 1,2 1,0 6 MV 120s thermistor #1 irradiations #1-4 #4 correction factor k C kc(#1) kc(#2) kc(#3) kc(#4) correction k C 1% - 4% ΔR / Ω 0,8 0,6 #2 # ,4 0,2 # ,0 position / mm t / s ratio k C (#1) / k C (#2)

11 details: heat conduction effects in small fields (3cm x 3cm) Experimental verification of calculated heat transport calculations MV 120s Calorimetric measurements MV and 10 MV 60 s and 120 s different detectors irradiation#1 / irradiation# ΔT < 1 µk calculation calculated ratio k C (#2) / k C (#1) position / mm good agreement between experimental data and calculation

12 Calorimetric determination of k Q factors for NE2561 ionization chambers in 3 cm x 3 cm beams of 6 MV and 10 MV photons Results: k Q (3x3) / k Q (10x10) k Q (3x3) / k Q (10x10) 1,008 1,004 1,000 0,996 0,992 6 MV 10 MV 0, # measurement k Q (3x3) / k Q (10x10) 1,012 1,008 1,004 1,000 0,996 0, # measurement ± ±

13 Calorimetric determination of k Q factors for NE2561 ionization chambers in 3 cm x 3 cm beams of 6 MV and 10 MV photons Summary: 1 water calorimeter is applicable in small radiation fields; u(d W ) < 0.4% 2 k Q for NE2561 chamber in 3 cm x 3 cm beams of 6 MV and 10 MV photons 3 no variation of k Q with field size (10 cm x 10 cm 3 cm x 3cm)