Volumetric analysis of anal cancer relapses following radical chemoradiation.

576 Background: Definitive chemoradiotherapy is standard of care in anal squamous cell carcinoma. The ACT II trial set the standard achieving three year overall survival rates of 73%. However patients with locally advanced disease have a ~50% local relapse rate. Studies have failed to demonstrate an improvement in local relapse rate by altering the systemic therapy. Although 2/3 of relapses are local as opposed to regional or distant, there is limited knowledge on the 3D position of these relapses. We aim to retrospectively review the patterns of local failure in three dimensions, relative to the radiotherapy plan. Methods: Between February 2007 and April 2012, 77 patients were treated for squamous cell carcinoma of the anus with radical chemoradiotherapy. Early stage (T0/2) and locally advanced (T3/4) tumours constituted 53% and 44% respectively. 37% of patients had nodal disease. As per ACT II protocol we used a gross tumour volume (GTV) to PTV margin of 3cm and prescribed to a dose of 50.4Gy. In patien...