Investigation of changes in planning target volume and regression probability of rectal boost using in-silico cone-beam computed tomography-guided online-adaptive radiotherapy

Abstract

Background and purpose

Radiotherapy boost to the primary tumour may enable organ preservation in locally advanced rectal cancer (LARC). This study evaluated cone-beam computed tomography (CBCT)-guided online-adaptive radiotherapy (ART) to reduce rectal boost planning target volume (PTV_Boost) margins and allow dose escalation.

Materials and methods

Eleven LARC patients were included in this in silico study. Population-based PTV_Boost margins were computed for non-adaptive and online-ART using van Herk’s formalism. Dose/volume results were compared between: non-adaptive RT with a 25 x 2.16 Gy boost (Non-ART_54Gy), ART with a 25 x 2.16 Gy boost (ART_54Gy), and ART with an escalated boost of 25 x 2.4 Gy (ART_60Gy). Tumour regression probability was compared between each plan using a dose–response model.

Results

PTV_Boost margins for non-adaptive vs. online-ART were 14.2 vs. 3.3 mm in the antero-posterior, 5.0 vs. 3.2 mm in the left–right, and 12.3 vs. 8.7 mm in the supero-inferior axes. PTV_Boost and pelvic lymph node PTV coverage (V95%) were significantly improved with ART_54Gy and ART_60Gy compared to Non-ART_54Gy (p < 0.001). High-priority organ-at-risk constraints (priority 1&2) were violated in 26.8 % of cases for Non-ART_54Gy, 21.2 % of cases for ART_54Gy, and 20.8 % of cases for ART_60Gy. Tumour regression probability was superior for ART_60Gy (20.8 %) compared to ART_54Gy (17.0 %, p < 0.001) and Non-ART_54Gy (16.9 %, p < 0.001).

Conclusions

Online-ART significantly reduce rectal boost PTV margin. It allows better target volume coverage with a similar risk of radiation-induced toxicities, even when escalating the dose. Therefore, online-ART should be considered to perform dose-escalation in LARC patients with the objective of organ preservation.