ESTRO 36 Abstract Book

S396 ESTRO 36 2017 _______________________________________________________________________________________________

dosimetric characteristics of the PSDP_3DP were compared with the patient plan.

sensitivity degradation of radiation and dose per pulse dependence of M512. Material and Methods The M512 is a monolithic 2D 512 diode array detector fabricated on p-type Si substrate. The detector active area is 52x52 cm 2 with 2 mm pixel pitch. M512 was developed at the Center for Medical Radiation Physics (CMRP) for quality assurance in SRS and SBRT. In this study, two types of Si substrate including M512-Bulk and M512-Epi were investigated. The M512-Bulk has been fabricated on low resistivity bulk silicon with thickness 470 µm while the M512-Epi has been manufactured on an epitaxial high resistivity p-silicon with 38 µm thick grown on a low resistivity of 370 µm thick substrate. Both detectors were irradiated on the 60 Co source in the total dose ranging from 0 to 40 kGy for M512-Bulk and 0 to 60 kGy for M512-Epi detectors. The 6 MV photon beam was used to investigated the sensitivity degradation and dose per pulse dependence. To evaluate the sensitivity degradation, the detector response was measured after irradiation with dose increments of 10 kGy. The dose per pulse dependence was determined by varying the SSD from 100 to 370 cm corresponding to the dose per pulse ranging from 0.278 to 0.021 mGy/pulse. The PDD was measured using a square field size 10x10 cm 2 by fixing the SSD at 100 cm and varying detectors depth in a phantom from 0.5 to 30 cm and comparing with the CC13 chamber. Results M512-Epi demonstrates excellent radiation stability with the sensitivity degradation of 0.3 %/10 kGy while M512- Bulk shows the degradation of 1%/10 kGy. The detector response decreases with the dose per pulse decrease. M512-Bulk shows less dose per pulse dependence compare with the M512-Epi with the sensitivity response (pC/Gy) decreasing about 2% while the sensitivity of M512-Epi decreased by 8% at a dose per pulse change of 10 times. For depth dose measurement, both substrates show a great agreement within ±2% when compared to the IC response. Conclusion The difference detector Si substrates show the difference in degradation of the detector sensitivity. The M512-Epi demonstrate 3.5 times better radiation hardness in comparison with M512-Bulk while show more the dose per pulse dependence. However, for typical treatment when SSD <150 cm for all beam angles the sensitivity of the detector decreases within 2% for both substrates making M512 -Epi more preferable choice as QA detector for dosimetry in SRS and SBRT. PO-0760 Investigation of PRESAGE formulation on signal quenching in a proton beam M. Carroll 1,2 , M. Alqathami 2 , G. Ibbott 2 1 University of Texas at Houston, Graduate School of Biomedical Sciences, Houston, USA 2 The University of Texas MD Anderson Cancer Center, Radiation Physics, Houston, USA Purpose or Objective PRESAGE®, a radiochromic polyurethane dosimeter, has shown potential as a 3D dosimetry system for conventional radiotherapy systems. When irradiated by protons, however, signal quenching is observed in high-LET regions. This quenching may result from either (or both) the local saturation of the Leucomalachite green (LMG) or recombination of the radical initiator (RI) along proton tracks. This work studied the magnitude of these quenching mechanisms and the effects of changes to formulaic concentrations of these components to further minimize or eliminate the quenching effect. Material and Methods Ten formulations of PRESAGE® were manufactured under standardized conditions but with RI concentrations ranging from 3-30 (wt%) and low LMG concentration (2 wt%). Six more formulations were then manufactured with high LMG

Results In comparing between the patient and PSDP_3DP, the percent differences in volume for the external body, spine, and MFS were -4.1%, 6.4%, and 10.0%, while the DSCs were 0.98, 0.91, and 0.89, respectively. The differences in density between the external body and spine were 7.5% and 15.5%, respectively. In the axial plane at the target center, Large dose differences were observed at the border of the external body contour (low- dose region), while most of the center region (high-dose region) was in good agreement, with a dose difference within 5%. The DHVs of both plans were well matched. Specifically, the mean differences in dose for GTV, CTV, spinal cord, and external body were -0.5%, -0.5%, 4.0%, and -2.8%, respectively.

Conclusion The physical accuracy and dosimetric characteristics of the PSDP were comparable with patient data. The ability to manufacture a PSDP representing an extreme patient condition was demonstrated. PO-0759 Validation of the influence of M512 substrate resistivity on sensitivity degradation of radiation N. Stansook 1 , M. Petasecca 1 , K. Utitsarn 1 , M. Carolan 2 , P. Metcalfe 1 , M.L.F. Lerch 1 , A.B. Rosenfeld 1 1 Wollongong University, Center for Medical Radiation Physics CMRP, wollongong, Australia 2 Wollongong hospital, Illawarra Cancer Care Centre, Wollongong, Australia Purpose or Objective The diode detector has been wildly used as a quality assurance (QA) tools in radiotherapy. However, the detector is affected by accumulative radiation damage leading to degradation of the sensitivity and dose per pulse dependence. The objective of this study is to investigate the influence of the substrate resistivity on