A randomised trial of supine versus prone breast radiotherapy (SuPr study): comparing set-up errors and respiratory motion.

PURPOSE To test a prone position against the international-standard supine position in women undergoing whole-breast-radiotherapy (WBRT) after wide-local-excision (WLE) of early breast cancer (BC) in terms of feasibility, set-up errors, and respiratory motion. METHODS Following WLE of BC with insertion of tumour-bed clips, patients underwent 4D-CT for WBRT-planning in supine and prone positions (the latter using an in-house-designed platform). Patients were randomised to undergo WBRT fractions 1-7 in one position, switching to the alternate position for fractions 8-15 (40Gy/15-fractions total). Cone-beam CT-images (CBCT) were acquired prior to fractions 1, 4, 7, 8, 11 and 14. CBCT data were matched to planning-CT data using (i) chest-wall and (ii) clips. Systematic and random errors were calculated. Maximal displacement of chest-wall and clips with respiration was measured on 4D-CT. Clinical- to planning-target-volume (CTV-PTV) margins were calculated. Patient-comfort-scores and treatment-times were evaluated. RESULTS Twenty-five patients were randomized. 192/192 (100%) planned supine fractions and 173/192 (90%) prone fractions were completed. 3D population systematic errors were 1.3-1.9mm (supine) and 3.1-4.3mm (prone) (p=0.02) and random errors 2.6-3.2mm (supine) and 3.8-5.4mm (prone) (p=0.02). Prone positioning reduced chest-wall and clip motion (0.5±0.2mm (prone) versus 2.7±0.5mm (supine) (p<0.001)) with respiration. Calculated CTV-PTV margins were greater for prone (12-16mm) than for supine treatment (10mm). Patient-comfort-scores and treatment times were comparable (p=0.06). CONCLUSIONS Set-up errors were greater using our prone technique than for our standard supine technique, resulting in the need for larger CTV-PTV margins in the prone position. Further work is required to optimize the prone treatment-platform and technique before it can become a standard treatment option at our institution.