ESTRO 2024 - Abstract Book

S4670

Physics - Optimisation, algorithms and applications for ion beam treatment planning

ESTR0 2024

within the primary[4]. With photon SBRT, large cases can be more challenging to deliver the biologically equivalent doses (BED) required to tackle these particularly challenging disease clones, due to limitations from nearby luminal structures e.g. bowel. Particle therapy may be able to increase BED delivery to tumour whilst minimising dose to surrounding luminal structures. There is also a radiobiological rationale for particle therapy in overcoming further radioresistance due to its higher-linear energy transfer (LET) in comparison to photons. To date, multiple small clinical case-series have demonstrated comparative local control rates and low toxicity of particles in RCC, supporting the rationale for prospective clinical studies[5, 6].

A retrospective observational analysis of large primary RCCs comparing pencil-beam scanning (PBS) proton beam therapy (PBT) and carbon ion therapy (CIRT) was conducted.

Material/Methods:

This work was carried out between 3 centres and involved international collaboration. Sequential large cases of primary RCC were identified on CT planning scans from one institution. Full institutional approvals were obtained to plan these anonymised cases with gantry pencil beam scanning proton beam therapy (PBS PBT) and fixed-beam carbon ion therapy (CIRT) at 2 respective specialist centres. The prescription dose was 57.6Gy(RBE) in 12 fractions; 4.8Gy(RBE) per fraction. A 4.8Gy(RBE) regime was chosen as this would be a standard hypofractionated CIRT regime in routine use for radioresistant cancers[7]. BED is between 97.7Gy-163.9Gy (alpha/beta ratio for RCC ranges between 2.6-6.9); hence this dose regime would be acceptable for likely tumour control[8]. These were also challenging cases by definition, due to size and proximity to organs at risk (OARs). Dose constraints were as per standard 12 fractions regimes within a CIRT institution[9]. Cases were planned to achieve homogenous coverage of the target: D95%≥95% in the worst case scenario. In cases where the target volume overlapped with serial OARs/luminal structures, coverage was for the ITV-OAR/PRV volume i.e. 5mm bowel PRV. Plans underwent robust optimisation for uncertainties of 5mm (geometric) and 3.5% (range) using ITV. Static serial organs and target volume were assessed under robust analysis. Lowest achievable OAR doses were then reported. Doses to skin, spinal cord, cauda equina, small bowel, duodenum, large bowel, stomach, liver, pancreas, spleen and contralateral kidney were recorded. Means, medians, and ranges were calculated.

Results: