Abstract Book

S804

ESTRO 37

3-5 Recurrence

2(4) 19(39)

11(23) 16(33)

0.01

Yes

No

Conclusion In patients of locally advanced rectal cancer presenting with LPLN undergoing NACRT if there is a radiological response at the LPLN doing LPLND may be an over treatment. But the patients having persistent LPLN post NACRT have poor outcomes, and more aggressive treatment like complete LPLND is indicated which may result in improved outcomes EP-1483 Early anal cancer: traditional large field vs modified small field - a volumetric comparison S. Sampaio 1 , S. Carroll 1 , S. Amos 1 , L. Delaney 1 , M. Butson 1 1 Chris O'Brien Lifehouse, Radiation Oncology, Sydney, Australia Purpose or Objective In early stage anal cancer (T1-2 N0) recurrence above the sacroiliac joints is rare and as suggested by pattern of failure studies. Recent UK ACT ( United Kingdom Anal Cancer Trial) study protocols now recommend less extensive elective nodal volumes for these patients.We aimed to quantify the volume of small bowel and bone marrow that is spared by treating only the first/second echelon at risk nodal groups in this low risk population, treating a small field (SF) in which the superior PTV border is at the bottom of the sacroiliac joints (SIJ’s) compared with contemporary large field (LF) design in which the superior border is at the L5/S1 interspace. Material and Methods Patients with T1-2 N0 biopsy proven anal SCC treated with definitive chemoradiation using traditional large fields between 2010 and 2017 were identified. For each patient, duplicate study sets were created and the PTV volume adapted to treat only the first echelon nodes so that the field border was at the bottom of the SIJ’s (Figure 1. and 2.). Once plans for each dataset were created, DVH parameters for small bowel and iliac crest (bone marrow) were compared.

Results Plans for 22 patients were obtained. Preliminary data shows the mean difference in field height between LF and SF plans was 3.6cm. SF plans resulted in 32% reduction in the volume of small bowel receiving dose. There was a mean reduction in the mean dose to small bowel of 30%. SF plans were associated with a 58% reduction in the volume of iliac crests (bone marrow) receiving dose. Mean dose to iliac crests was 53% lower for SF vs LF plans. We plan to present more extensive DVH data comparisons. Conclusion There is substantial improvement in small bowel and bone marrow dose with smaller radiation fields compared to contemporary large fields. Adoption of small fields for these low risk patients is likely to improve the rates of acute toxicity, namely diarrhoea and myelosupression without compromising outcomes. EP-1484 Lateral pelvic lymph node metastasis in rectal cancer receiving NACRT – a time for serious relook R. Engineer 1 , M. Karthik 1 , S. Lewis 1 , A. D'souza 2 , S. K 3 , S. Arya 3 , V. Ostwal 4 , A. Ramaswamy 4 , P. Patil 5 , S. Mehta 5 , S. Chopra 1 , M. Ramadwar 6 , A. Saklani 2 1 Tata Memorial Hospital, Radiation Oncology, Mumbai, India 2 Tata Memorial Hospital, Surgical Oncology, Mumbai, India 3 Tata Memorial Hospital, Radiology, Mumbai, India 4 Tata Memorial Hospital, Medical Oncology, Mumbai, India 5 Tata Memorial Hospital, Gastroenterology, Mumbai, India 6 Tata Memorial Hospital, Pathology, Mumbai, India