ESTRO 2021 Abstract Book

S674

ESTRO 2021

Conclusion DWI resulted in smaller volumes delineation compared to all the other three modalities, CT, T2-weighted MRI and PET-CT imaging. Furthermore, PET-CT obtained smaller volumes respect to CT. Almost perfect agreements, as reported through Dice index, were reported for each imaging modality between the two observers, both radiation oncologists. As functional imaging, MRI seems to remain the optimal strategy for boost volume delineation in case of dose escalation, obtaining smaller volumes compared to PET-CT. Despite this aspect, PET-CT could be considered for boost volume definition when MRI is not available. PD-0839 Outcomes for 193 rectal cancer patients receiving contact brachytherapy for organ preservation A. Stewart 1 , H. Minnaar 1 , C. Jordan 1 , S. Rossides 1 , L. Wang 1 , R. Ragnakumaran 1 , N. Patel 2 , A. Franklin 1 1 Royal Surrey County Hospital, St Luke's Cancer Centre, Guildford, United Kingdom; 2 University of Cambridge, Department of Sociology, Cambridge, United Kingdom Purpose or Objective Contact X-Ray brachytherapy (CXB) is a useful treatment modality when aiming to achieve organ preservation for rectal cancer. We present the outcomes of patients treated at a single institution with CXB as all or part of their treatment. Materials and Methods Patient data was prospectively collected using a customised online database. All patients who received CXB from the inception of the CXB service were included in the series. Data was analysed using Excel. Patient characteristics are presented in table 1. Results 193 patients were treated with CXB between April 2014 and December 2019. The median follow up was 950 days (range 41-2982 days). The median age at first treatment was 72 years (range 33-103 years). 73% of patients were male (140:53 M:F). Of the operable or high surgical risk patients 105 were treated radically with radiotherapy (RT), 28 were treated radically with RT after local resection and 38 had RT following local recurrence, had previous pelvic RT or refused surgery. 22 patients were medically unfit for surgery. 47 patients received CXB as sole treatment (24/47 had previous radiotherapy). EBRT doses ranged from 25Gy/5F to 54Gy/30F with the majority receiving 45Gy/25F (87/146) or 25Gy in 5F (35/146). 67% of all patients achieved a clinical complete response (cCR) or a near cCR. 4% had partial response and 13% had persistent disease. Response assessment was not made or was missing in the remaining 16% of patients.

74% (78/105) of radically treated patients achieved cCr or near cCR. 16% (13/78) of these subsequently relapsed.