ESTRO 2021 Abstract Book

S1613

ESTRO 2021

Conclusion Photon TP is superior to proton TP regarding robustness towards seroma formation. Photon TP could encompass an

elevation up to 20mm without influencing . This complies with the clinical guideline used at our institution (recalculation of TP, if the seroma height is above 20mm). Proton TPs were highly sensitive to small seromas and to the location. Seromas located laterally entailed the largest dose loss. The study suggests that an individual assessment is necessary, even for small seromas, when treating mastectomized BC patients with proton therapy, complying with the guideline at our proton therapy center. PO-1894 Comparison of Intra Operative and Post Implant Plans for Low Dose Rate Prostate Brachytherapy Ö. Şenkesen 1 , H. Küçücük 2 , E.O. Göksel 1 , E. Tezcanli 2 , Z. Özen 2 , S. Küçücük 3 , G. Kemikler 4 , R. Dişçi 5 , I. Aslay 2 1 Acıbadem Mehmet Ali Aydınlar University, Radiation Oncology Department, Istanbul, Turkey; 2 Acıbadem Altunizade Hospital, Radiation Oncology Department, Istanbul, Turkey; 3 Yeni Yüzyıl University, , , Turkey; 4 Istanbul University, Institute of Oncology, Istanbul, Turkey; 5 Istinye University, Biostatistics, Istanbul, Turkey Purpose or Objective We aimed to evaluate the post-implant dosimetry (PID) for low-dose rate (LDR) seed implantation for prostate cancer patients by comparing the dose volume histograms (DVH) between intraoperative (IOP) and post-implant plans. Materials and Methods Hundred and thirty-four patients who underwent LDR prostate brachytherapy between 2000-2019 were included in this study. Applications for IOP were performed under general anesthesia, in the high lithotomy position with the guidance of Transrectal Ultrasonography. Computed Tomography and Magnetic Resonance images for PID were obtained in supine position. Target volume was the prostate and the organs at risk (OAR) included rectum and urethra. Prostate volumes receiving 90% (pV 90 ), 100% (pV 100 ), 150% (pV 150 ) of the prescribed dose (PD), the doses received by 90% of the prostate volume (pD 90 ) were evaluated as well as urethral doses received by 10% (uD 10 ), 30% (uD 30 ) and 50% (uD 50 ) of the urethra volume and the urethra volumes receiving 100% (uV 100 ), 150% (uV 150 ) of the PD, Rectal volumes receiving 50% (rV 50 ), 100% (rV 100 ) and 150% (rV 150 ) of the PDs were also evaluated. The IOP prostate dose constraints were pD 90 to receive the PD, pV 100