ESTRO 2020 Abstract book

S85 ESTRO 2020

PH-0161 Elective clinical target volumes for rectal IMRT delivery – moving towards a UK wide consensus S. O'Cathail 1 , R. Muirhead 1 , D. Sebag-Montefiore 2 , M. Hawkins 3 1 CRUK/MRC Institute for Radiation Oncology University of Oxford, Oncology, Oxford, United Kingdom ; 2 Leeds Cancer Centre, University of Leeds, Leeds, United Kingdom ; 3 University College London, Oncology, London, United Kingdom Purpose or Objective There is variation in definition of clinical target volume (CTV) in rectal cancer radiotherapy. In preparation for future UK rectal IMRT based trials, a survey was carried out to identify key areas of divergence, relative to international CTV guidance, and inform consensus. Material and Methods A survey of UK clinical oncologists who specialise in the treatment of anorectal cancer was carried out. Six clinical case scenarios were supplied: cT1-3a N0 [A]; cT3 N0/1 CRM clear [B]; cT3 N0/1 CRM threatened [C]; cTx N2 [D]; extra mesorectal nodes [E]; T4 direct anterior organ involvement [F]. Anonymously, clinicians indicated which of 6 nodal compartments they would include: mesorectal [M]; pre-sacral [PS]; lateral (internal iliac and obturators) [LLN]; external iliac [EIN]; ischiorectal fossa [IRF]; sphincter complex [SC]. Survey grids were completed for upper, mid and lower rectal location for each of the 6 cases (18 total). The International CTV consensus guidelines appendix of nodal compartments was circulated for reference of nodal compartments. A traffic light scoring system was used to indicate inclusion; green = include, orange = unsure; red = exclude. Collated scores were expressed as percentage agreement amongst respondents. Analysis was carried out in R.

Results From 11 UK centres, 30 clinical oncologists completed the survey. In the upper rectum, areas of divergence included case A where only 60% and 63% would include LLN and PS, and case F where 21% would include EIN. In the mid rectum, areas of divergence were mainly the LLN where 38%, 68% and 71% would include for cases A, B, C respectively. 75% would include LLN for cases D-F. Low rectal cases had the most heterogeneity. LLN were variably included for cases A-C (52%, 72%, 74% respectively). 24% and 15% included EIN and IRF for case F. 55-67% opted to include SC for all cases. Conclusion Elective nodal irradiation of lateral pelvic compartments are a key area of divergence among oncologists, varying according to clinical information and tumour anatomical position. A national rectal IMRT working group will address these issues as part of future consensus guidance.