Session Item

Re-irradiation (reRT) treatment planning is challenging: anatomical changes may occur between radiotherapy courses and fraction size effects should be considered in evaluating cumulative doses. The STRIDeR (Support Tool for Re-Irradiation Decisions guided by Radiobiology) project aims to integrate compensation for anatomical change, per-voxel dose summation, and fraction size correction into reRT treatment planning within a commercial treatment planning system (TPS). This will allow original radiotherapy dose distributions to be meaningfully used as background dose for reRT plan optimisation.

The STRIDeR project employs the scripting capabilities in RayStation (RaySearch Laboratories, Stockholm, Sweden) to create a clinically viable reRT planning pathway. The original dose distribution was deformably registered to the reRT dataset with dose mapping, per-voxel summation, and fraction size correction. This allowed optimisation of the reRT plan using the original dose distribution as anatomically appropriate background dose, transformed to the reRT fraction schedule. Normal tissue repair was incorporated as an option, as well as evaluation of cumulative EQD2 doses.

Data from a cohort of 21 patients who previously received radical pelvic radiotherapy and later received 5-fraction pelvic Stereotactic Ablative Radiotherapy reRT were used to evaluate the STRIDeR tool. For the purposes of the evaluation, the aim was to achieve 95% coverage of the PTV (D95%) with the prescription dose (PD). Organ at risk (OAR) constraints could be relaxed, or increasing degrees of OAR repair included, in a stepwise manner to achieve coverage. Steps 1a-1c used a PD of 30Gy, with each step permitting increasing constraint relaxation/ OAR repair. Steps 2a-c used 25Gy, with the same constraints. 

The STRIDeR planning tool was successfully implemented in RayStation and employed in all 21 test cases. Clinically acceptable plans were produced in 18/21 cases (Figure 1). 30Gy coverage and optimal constraints (Step 1a) were achieved in 7/21 cases. 30Gy PD was maintained with relaxed constraints (Step 1b) in 10/21 cases, while in one case PD had to be reduced to 25Gy with relaxed constraints (Step 2b). Median PTV D95% was: 30.8Gy (range: 25.5-31.8Gy). In three cases, despite the planning pathway working, clinically acceptable plans could not be achieved while respecting OAR constraints. All three cases had overlap of at least one OAR with the reRT PTV, with very limited OAR tolerance remaining after the original radiotherapy course. This was not fully appreciated during clinical reRT delivery.

The STRIDeR tool integrates anatomical change and per-voxel optimisation on fraction size corrected background dose for reRT treatment planning within a commercial TPS. Normal tissue repair can be optionally included. The pathway was successfully tested in a cohort of reRT patient cases. This provides scope for more informed reRT, improved evaluation of cumulative OAR doses and NTCP modelling.