ESTRO 36 Abstract Book

S400 ESTRO 36 _______________________________________________________________________________________________

1 Samsung Medical Center- Samsung Biomedical Research Institute- Sungkyunkwan University School of Medicine, Radiation Oncology, Seoul, Korea Republic of Purpose or Objective A new technique for manufacturing a patient-specific dosimetric phantom (PSDP) using three-dimensional printing (PSDP_3DP) was developed, and its geometrical and dosimetric accuracy was analyzed. Material and Methods External body contours and structures of the spine and metallic fixation screws (MFS) were delineated from CT images of a patient with MFS who underwent stereotactic body radiation therapy for recurrent thoracic spine metastasis. Contours were converted into a STereoLithography file format using in-house program. A hollow, four-section PSDP was designed and manufactured using three types of 3DP to allow filling with a muscle- equivalent liquid and insertion of dosimetric film and grass dosimeters in the axial and coronal planes, respectively. To evaluate the geometrical and dosimetric accuracy of PSDP_3DP, CT images were obtained and compared with patient CT data for volume, mean density, and Dice similarity coefficient (DSC) for contours. The dose distribution in the PSDP_3DP was calculated by applying the same beam parameters as for the patient, and the dosimetric characteristics of the PSDP_3DP were compared with the patient plan.

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 sensitivity degradation of radiation and dose per pulse 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. dependence of M512. Material and Methods

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.