ESTRO 38 Abstract book

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volume parameters obtained using these rules to that obtained using current guidelines for advanced cancers. The proposed rules led to a mean 18% relative reduction of planning target volume, which resulted in better sparing of at-risk organs. Conclusion This meta-analysis suggests a smaller CTV for cT2 tumours than the current guidelines designed for advanced cancers.

days) on D1. Stereotactic ablative radiotherapy (SABR; 6.6 Gy/fraction) was delivered every other day x 5 fractions beginning D8. Durvalumab continued as maintenance Q14 days until resection or progression. The run-in phase I utilized a standard 3+3 design prior to phase 2 expansion. Dose limiting toxicities (DLTs), adverse events (AEs) and serious adverse events (SAEs) were assessed during the first 10 weeks of study treatment. CT scans were obtained every 2 months for response assessment. Endoscopic research biopsies were obtained pre- and 6-8 weeks post- SABR, and weekly blood samples were obtained on D1, weekly for 10 weeks, and then every 2 months until resection or progression to assess for immune correlates of response. Results Since 8/2017, 15 of 30 planned patients were enrolled of which 9 (60%) are LA and 6 (40%) were BR. Median age was 70 and 8 (53%) were female. No DLTs were identified in the phase 1 run in and enrollment in the phase 2 study continues. Grade 3 toxicities have been identified in 2 patients (nausea, anorexia). Ten patients have discontinued study treatment due to surgery (n=6) and disease progression (n=4; local = 1, distant = 2, local+distant = 1). All resections have been margin- negative. Objective response rates (RECIST 1.1) included SD (n=9) and PR (n=6). Two patients have died. Median PFS has not been reached. Conclusion To our knowledge, this is the first report of a PD-L1 inhibitor-RT combination in LA PC. The regimen was safe, well tolerated and appears to be clinically active with high rates of margin-negative resection. PO-0814 Clinical target volume in radiation therapy for organ preservation in T2 rectal cancer J. Socha 1 , L. Pietrzak 2 , A. Zawadzka 3 , A. Paciorkiewicz 3 , A. Krupa 2 , K. Bujko 2 1 Military Institute of Medicine, Department of Radiotherapy, Warsaw, Poland ; 2 Maria Skłodowska-Curie Memorial Cancer Centre, Department of Radiotherapy I, Warsaw, Poland ; 3 Maria Skłodowska-Curie Memorial Cancer Centre, Medical Physics Department, Warsaw, Poland Purpose or Objective There are no guidelines on clinical target volume (CTV) delineation for cT2 rectal cancer treated with organ preservation. Material and Methods A systematic review and meta-analysis were performed to determine the extent of distal mesorectal (DMS) and distal intramural spread (DIS), the risk of lateral lymph node (LLN) metastases in pT2 tumours, and regional recurrence pattern after organ preservation. Results The rate of DMS >1 cm was 1.9% (95% CI: 0.4%–5.4%), maximum extent: 1.3 cm. The rate of DIS >0.5 cm was 4.7% (95% CI: 1.3%–11.5%), maximum extent: 0.8 cm. The rate of LLN metastases was 8.2% (95% CI: 6.7%–9.9%) for tumours below or at peritoneal reflexion and 0% for higher tumours. Regional nodal recurrences alone (i.e. without concomitant intraluminal recurrences) were recorded in 1.1% (95% CI: 0.5%–1.7%) of patients after watch-and-wait and in 2.1% (95% CI: 1.2%–3.4%) after preoperative radiotherapy and local excision. Thus, the following rules for CTV delineation are proposed: caudal border 1.5 cm from the tumour to account for DMS or 1 cm to account for DIS, whichever is more caudal; cranial border at S2/S3 interspace; inclusion of LLN for tumours at or below peritoneal reflexion (see Table 1, Figure 1). A planning study was performed in eight patients to compare dose– Poster: Clinical track: Lower GI (colon, rectum, anus)

PO-0815 Stereotactic radiation therapy in colorectal cancer brain metastasis: a multicentric cohort A. Paix 1 , F. Thillays 2 , J. Biau 3 , N. Vulquin 4 , I. Pop 5 , K. Debbi 6 , A.L. Grosu 5 , G. Noël 1 1 Centre Paul Strauss, Radiothérapie, Strasbourg, France ; 2 Institut Cancérologique de l'ouest, Radiothérapie, Saint Herblain, France ; 3 Centre Jean Perrin, Radiothérapie, Clermont-Ferrand, France ; 4 Centre Georges-François Baclesse, Radiothérapie, Dijon, France ; 5 Universitätsklinikums, Klinik für Strahlenheilkunde, Freiburg, Germany ; 6 Centre Henry Kaplan - CHU Tours, Radiothérapie, Tours, France Purpose or Objective Colorectal cancer (CRC) is the third most common cancer in western countries, but brain metastases only occur in 1% of CRC patients. Overall survival in CRC patients rises as new systemic drugs became available. Thus the incidence of brain metastases in CRC patients will likely increase. We conducted a multicentric analysis to evaluate and compare the outcomes of stereotactic radiosurgery (SRS) and hypofractionated stereotactic radiotherapy (HFSRT) in CRC brain metastasis management. Material and Methods On behalf of the association of French-speaking neuro- oncologist (ANOCEF), we retrospectively collected individual data of patients treated with SRS or HFSRT for CRC brain metastases in 6 hospitals in France and Germany. The primary endpoint of the study was the radiological response rate define as a complete response, partial response or stability of the metastasis according to RANO