ESTRO 38 Abstract book

S616 ESTRO 38

0.01) (Fig.1). Also in the comparison of the plans of each patients, better CI and HI were observed in VMAT plans (CI: 0.582 ± 0.193 vs. 0.810 ± 0.087, p< 0.01, HI: 1.237 ± 0.150 vs 1.121 ± 0.045 p < 0.05). V10 was 14.57% ± 8.02% and 17.24% ± 9.32 respectively (p = 0.35) for DCA and VMAT, respectively. The mean treatment time was 4 minutes 53 seconds (± 1.4 seconds) and 3 minutes 56 seconds (± 2.8 seconds) for DCA and VMAT, respectively (p < 0.01) (Fig.2).

respectively. Every condition was applied identically to both plans except optimization algorithms for fair comparison. The dose distributions were calculated using the AcurosXB (ver.13.7) with the calculation grid of 0.2 cm. All VMAT plans were normalized that the target volume irradiated by 18Gy the prescription dose (V 100% ) would be 80%. For organs at risk (OARs), D 10% , of spinal cord, dose received at 0.03 cc of spinal cord (D 0.03cc ) and mean dose to the spinal cord were calculated. The homogeneity indices (HIs) of target volumes and total monitor units (MUs) for each VMAT plan were calculated. High-definition MLC TM (HD MLC) and 10 MV flattening filter free (FFF) photon beams of TrueBeam STx TM were used for VMAT plans. Results The average values of D 10% , D 0.03cc and mean dose to spinal cord of PRO plans were 896.62 ± 59.66 cGy, 1004.67 ± 65.70 cGy, 619.83 ± 67.23 cGy and PO were 981.2 ± 76.89 cGy, 1116.3 ± 80.94 cGy, 655.01 ± 64.87 cGy, respectively. (p < .001) The average value of HIs of PRO plans were 1.13 ± 0.09 while that was 1.17 ± 0.1 for PO plans, showing better homogeneity of target volumes. (p < .001) The average values of total MU of PRO and PO plans were 6146 ± 576.79 and 5156 ± 627.29, respectively. (p < .001) Conclusion Using PRO with higher MU for spine SABR VMAT plans are showing better results for the spinal cord doses while maintaining better target volume coverage as PO. PO-1109 Plan quality and treatment time comparison between DCA and VMAT for cranial SRT Y. Kato 1 , D. Watanabe 1 , R. Wakabayashi 1 , M. Inohira 1 , S. Itoya 1 , J. Okubo 1 , K. Miyamoto 1 , S. Shimizu 2 , Y. Oshiro 2 1 Tsukuba Medical Center Hospital, Radiological Technology, Tsukuba, Japan ; 2 Tsukuba Medical Center Hospital, Radiation Oncology, Tsukuba, Japan Purpose or Objective Dynamic conformal arc therapy (DCA) and intensity modulated radiotherapy (IMRT) are major techniques of cranial stereotactic radiotherapy (SRT). Some reports have suggested that IMRT using step and shoot and dynamic techniques improved dose concentration and homogeneity, but increases low dose area and treatment time compared to DCA. In this study, we compare DCA and volumetric modulated arc therapy (VMAT) techniques for cranial IMRT, and compare dosimetry parameters and treatment time. Material and Methods Each 10 clinically-used treatment plans with DCA and VMAT for a single brain metastatic legion were randomly selected for this study. The DCA treatment was re-planned with VMAT techniques, therefore total of 30 treatment plans were evaluated. In the DCA plans, 6 non-coplanar arcs with couch rotations ranged from 285° to 75° generally spaced at 30° interval were used. In the VMAT plans, 4-arcs with couch rotation ranged from 270° to 45° spaced at 45° interval were used. Conformity index (CI) and homogeneity indexes (HI) were calculated to compare dosimetry for target lesion, and V10 was calculated to evaluate low dose extent. The objective for each plan was to mean dose of the PTV covered by 100% of prescribed dose while minimizing the dose to the normal brain. All plans were generated using a single isocenter. A dose of 42 Gy in 7 fraction was prescribed and normalized to the isocenter. CI and HI were compared between clinically used DCA and VMAT. Also between clinically used and re- planned plan for each patient, CI, HI, and V10 were compared. Actual value was evaluated for treatment time. Results In comparison of the clinically-used plans, the mean CI was 0.479 ± 0.177 and 0.773 ± 0.095 for DCA and VMAT, respectively (p < 0.01), and the mean HI was 1.585 ± 0.321 and 1.172 ± 0.046 for DCA and VMAT, respectively (p <

Conclusion VMAT significantly improved dose concentration and homogeneity to the target compared to DCA and reduced treatment time. Also, there was no significant difference in low dose spreading. PO-1110 CT-based HDR brachytherapy for salvage prostate cancer: the way to avoid or delay hormonal treatment N. Dunnewold 1 , E. Kouwenhoven 1 , P. Koper 1 , S. Van den Berg 1 , J. Van der Klein 1 , J. Noordermeer 1 , N. Tak-Nobel 1 , K. Van Wingerden 1 , M. Mast 1 , H. De Jager 1 1 Haaglanden Medical Center, Radiotherapy, The Hage, The Netherlands Purpose or Objective Despite ongoing technical improvements, about 40% of the patients will experience a PSA relapse after external radiotherapy [1]. Many recurrences are confined to the prostate and burdensome androgen deprivation therapy is prescribed frequently. In the era of mpMRI and PSMA-PET, early local recurrences can be diagnosed and treated locally using HDR salvage brachytherapy. This treatment has been described using MRI in the operating room (OR) [2]. However, many departments do not have access to such imaging equipment, making it challenging to adopt this procedure. We describe an adapted CT-based workflow in our department. Material and Methods Up until now we treated eight patients, 4 - 11 years after their primary radiotherapy treatment, with mpMRI and PSMA-PET proven local recurrence. Mean PSA by diagnosis of recurrence was 3.3 ug/l (range 1.5-5.8). For GTV delineation the PSMA-PET, T2 weighted SPACE MR and T2W TSE MR were co-registered. The CTV was obtained from the GTV, applying a margin of 5mm, avoiding the urethra and removing the parts outside the prostate. Defining the