Dose Accumulation for Pelvic Stereotactic Ablative Radiotherapy Reirradiation

Aims Despite the increasing use of reirradiation, our understanding of appropriate normal tissue dose constraints remains limited. This is intrinsically tied to major uncertainties concerning evaluation of cumulative doses from multiple treatment courses. This study aimed to: i) retrospectively evaluate cumulative normal tissue doses in patients treated with pelvic stereotactic ablative radiotherapy (SABR) reirradiation, taking account of anatomical change and fraction size effects, and ii) produce preliminary data regarding safe cumulative normal tissue doses. Materials and methods Fifty-six patients treated with pelvic SABR reirradiation for locoregional recurrence after prior radical or (neo)adjuvant radiotherapy in the pelvis were included. Original-treatment computed tomography (CT) scans were deformably registered to the reirradiation CTs; and target volumes, organs at risk (OARs), and dose distributions were transferred from the original anatomy to the reirradiation scan. Original and reirradiation dose distributions were converted into equivalent dose in 2-Gy fractions (EQD2). Cumulative doses were calculated using deformable image registration (DIR)–based dose summation and/or summed maximum doses (D0.5 cc) for each OAR. Severe toxicity events up to 2 years post reirradiation were evaluated. Results Most patients had prostate cancer (85.7%) and were treated for pelvic nodal recurrence (75%) with a single target volume (91.1%) using a prescription dose of 30 Gy in 5 fractions (90.3%). The median time between original and reirradiation was 53 months (interquartile range [IQR]: 36-79). Based on DIR, cumulative doses in EQD2 of up to 82.8 Gy for the rectum, 110.2 Gy for the bladder, 69.8 Gy for the colon, 101.4 Gy for the sacral plexus, and 108.1 Gy for the vessels were observed. Based on summed D0.5 cc, cumulative doses of up to 111.9 Gy were delivered to the small bowel. No severe toxicity events which could be attributed to reirradiation were observed. Conclusions This study has demonstrated feasibility of per-voxel anatomically and radiobiologically appropriate 3-dimensional evaluation of cumulative normal tissue doses in patients previously treated with pelvic SABR reirradiation. No toxicity events could be attributed to the cumulative or reirradiation doses delivered.