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61. Dose-area-product determination with large planes-parallel ionization chambers: New approach to reference dosimetry in active scanning protontherapy
Abstracts / Physica Medica 56 (2018) 59–132
using the EGSnrc C++ geometry package and absorbed doses to the air cavity and to water were calculated using the egs_chamber user code with a target statistical uncertainty below 0.1%. Simulations were run in parallel using the ENEA CRESCO4 cluster. Results. NE 2571 simulations for Co-60 and 6 MV to 25 MV photon beams showed that, while adoption of renormalized photoelectric cross-sections did not affect calculation results, variations in absorbed dose to the air cavity and absorbed dose to water due to the update of I and d values were up to 0.5% and 0.2%, respectively. Nevertheless, changes partially offset each other and the overall effect on kQ values was below 0.2%. Work is in progress for other widely used ionization chambers. Conclusions. In the range 6–25 MV (TPR20;10 of photon beams from 0.663 to 0.80), the calculated values of kQ for the NE 2571 ionization chamber tend to be lower than the current values in the IAEA TRS 398 protocol with differences up to about 0.5% at the highest TPR20;10 values. References 1. ICRU. Key Data for Ionizing-Radiation Dosimetry: Measurement Standards and Applications, ICRU Report 90. J ICRU 2016;14(1). https://doi.org/10.1016/j.ejmp.2018.04.070
61. Dose-area-product determination with large planes-parallel ionization chambers: New approach to reference dosimetry in active scanning protontherapy S. Lorentini a, M. Schwarz a,b a Azienda Provinciale per i Servizi Sanitari, Proton Therapy Department, Trento, Italy b TIFPA-INFN, Trento, Italy
Purpose. Reference dosimetry in pencil beam scanning (PBS) proton therapy is not still well established. Recently a few experiences dealing with the use of large area planes-parallel ionization chambers (LPPICs) in determining integrated-dose-to-water (Dose-
101
Area-Product, DAPw) were reported [1]. Aim of this work was to determine DAPw;Q (equal to MQ NDAPw;Q0 KQ ;Q0 Þ with a LPPIC using a cross-calibrated chamber factor (NDAPw) obtained directly under a proton beam and not via Co60 calibration. Methods. Absorbed point dose-to-water (Dw) determined with a small detector in the middle of a broad homogeneous field obtained as the superposition of proton pencil beams regularlyspaced by Dx and Dy is equivalent to a determination of DAPw using a LPPIC and a single pencil beam at the same depth. Thus, as a valid approximation, DAPw = Dw*Dx*Dy. A LPPIC (Bragg peak chamber PTW-37040, radius = 4,1 cm) was cross-calibrated in water vs. a reference Markus type IC in the entrance region of a 16 16 cm2 homogeneous 190 MeV scanned proton pseudomonoenergetic layer with a spot spacing of 2 mm. Then, determinations of DAPw for single pencil beams (energy from 70 to 226 MeV) at 2 cm depth in water were performed. The values of DAPw scored with LLPIC were compared to those obtained with a PPC05 (Iba-Dosimetry) in the middle of 10 10 cm2 unmodulated layers of the same energies at the same depth. Results. Chamber response homogeneity has been assessed with good results. The cross-calibrated NDAPw was equal to 1,512E08 Gy/C. KQ ;Q0 was set to 1 as the same beam quality was used both for cross-calibration and measurements. Figure and table show the results of the comparison in terms of DAPw between the LLPIC and PPC05. Conclusions. Reference dosimetry based on DAPw and LPPICs is feasible. DAPw values got with Bragg peak chamber in single pencil beams and with PPC05 in large scanned layers are in good agreement. The determination of NDAPw via cross-calibration in proton beams allows to get rid of chamber specific KQ ;Q0 factors reducing uncertainties. References 1. C. Gomà et al.. Reference dosimetry of proton pencil beams based on dose-area product: a proof of concept. PMB 2017;62(12):4991–5005. https://doi.org/10.1016/j.ejmp.2018.04.071
Differences in terms of DAPw between a Bragg peak chamber (BPC PTW 34070) and a Markus IC (Iba PPC05). On the left figure a plot of the comparison is displayed (combined standard uncertainties are of the order of 2% for both series), while on the right table the numerical differences are reported.
using the EGSnrc C++ geometry package and absorbed doses to the air cavity and to water were calculated using the egs_chamber user code with a target statistical uncertainty below 0.1%. Simulations were run in parallel using the ENEA CRESCO4 cluster. Results. NE 2571 simulations for Co-60 and 6 MV to 25 MV photon beams showed that, while adoption of renormalized photoelectric cross-sections did not affect calculation results, variations in absorbed dose to the air cavity and absorbed dose to water due to the update of I and d values were up to 0.5% and 0.2%, respectively. Nevertheless, changes partially offset each other and the overall effect on kQ values was below 0.2%. Work is in progress for other widely used ionization chambers. Conclusions. In the range 6–25 MV (TPR20;10 of photon beams from 0.663 to 0.80), the calculated values of kQ for the NE 2571 ionization chamber tend to be lower than the current values in the IAEA TRS 398 protocol with differences up to about 0.5% at the highest TPR20;10 values. References 1. ICRU. Key Data for Ionizing-Radiation Dosimetry: Measurement Standards and Applications, ICRU Report 90. J ICRU 2016;14(1). https://doi.org/10.1016/j.ejmp.2018.04.070
61. Dose-area-product determination with large planes-parallel ionization chambers: New approach to reference dosimetry in active scanning protontherapy S. Lorentini a, M. Schwarz a,b a Azienda Provinciale per i Servizi Sanitari, Proton Therapy Department, Trento, Italy b TIFPA-INFN, Trento, Italy
Purpose. Reference dosimetry in pencil beam scanning (PBS) proton therapy is not still well established. Recently a few experiences dealing with the use of large area planes-parallel ionization chambers (LPPICs) in determining integrated-dose-to-water (Dose-
101
Area-Product, DAPw) were reported [1]. Aim of this work was to determine DAPw;Q (equal to MQ NDAPw;Q0 KQ ;Q0 Þ with a LPPIC using a cross-calibrated chamber factor (NDAPw) obtained directly under a proton beam and not via Co60 calibration. Methods. Absorbed point dose-to-water (Dw) determined with a small detector in the middle of a broad homogeneous field obtained as the superposition of proton pencil beams regularlyspaced by Dx and Dy is equivalent to a determination of DAPw using a LPPIC and a single pencil beam at the same depth. Thus, as a valid approximation, DAPw = Dw*Dx*Dy. A LPPIC (Bragg peak chamber PTW-37040, radius = 4,1 cm) was cross-calibrated in water vs. a reference Markus type IC in the entrance region of a 16 16 cm2 homogeneous 190 MeV scanned proton pseudomonoenergetic layer with a spot spacing of 2 mm. Then, determinations of DAPw for single pencil beams (energy from 70 to 226 MeV) at 2 cm depth in water were performed. The values of DAPw scored with LLPIC were compared to those obtained with a PPC05 (Iba-Dosimetry) in the middle of 10 10 cm2 unmodulated layers of the same energies at the same depth. Results. Chamber response homogeneity has been assessed with good results. The cross-calibrated NDAPw was equal to 1,512E08 Gy/C. KQ ;Q0 was set to 1 as the same beam quality was used both for cross-calibration and measurements. Figure and table show the results of the comparison in terms of DAPw between the LLPIC and PPC05. Conclusions. Reference dosimetry based on DAPw and LPPICs is feasible. DAPw values got with Bragg peak chamber in single pencil beams and with PPC05 in large scanned layers are in good agreement. The determination of NDAPw via cross-calibration in proton beams allows to get rid of chamber specific KQ ;Q0 factors reducing uncertainties. References 1. C. Gomà et al.. Reference dosimetry of proton pencil beams based on dose-area product: a proof of concept. PMB 2017;62(12):4991–5005. https://doi.org/10.1016/j.ejmp.2018.04.071
Differences in terms of DAPw between a Bragg peak chamber (BPC PTW 34070) and a Markus IC (Iba PPC05). On the left figure a plot of the comparison is displayed (combined standard uncertainties are of the order of 2% for both series), while on the right table the numerical differences are reported.