ESTRO 2024 - Abstract Book

S5979

RTT - Treatment planning, OAR and target definitions

ESTRO 2024

Dose-volume constraints for the APHRODITE trial were developed from a retrospective mesorectum only planning study for a cohort of early-stage rectal cancer patients². All constraints were considered optimal, rather than mandatory, due to the paucity of data on normal tissue dose limits in the setting of rectal cancer organ preservation. In all cases, the V95% ≥ 99% for both PTV and PTVp (dose-escalated arm only) were achieved. Table 1 demonstrates that centres were able to meet the optimal OAR dose-volume constraints in both trial arms in the majority of cases. Randomisation to receive a 62Gy boost did not have a statistically significant impact on achieving optimal OAR dose volume constraints when compared to the standard arm dose. Evaluation of conformity indices in Table 2 suggested that there was a negligible difference in the conformity of PTV coverage between standard and dose-escalated patients. Mean conformity index for the mesorectal PTV was 1.15 for standard arm patients and 1.16 for patients in the escalated arm (p=0.67). For comparison, mean conformity index for the boost PTVp in the escalated arm was 1.18. The analysis of the target volume conformity test showed that 95% dose conformity is widely achievable across both trial arms in this multi-centre study.

Table 2: Conformity indices of target volume and standard deviation Target Volume Mean Conformity Index Standard Deviation Standard (PTV) 1.15 0.06 Escalated (PTV) 1.16 0.05 Escalated (PTVp) 1.18 0.11

Conclusion:

Delivering a SIB dose of up to 62.0Gy to the primary tumour volume does not have a statistically significant impact on the achievability of optimal OAR dose-volume constraints set out in the APHRODITE trial. Retrospective analysis of available plan data has shown that highly conformal SIB plans can be produced in a multi-centre setting, with resulting dose distributions being comparable to those in the standard arm.

Keywords: Novel, Dose-escalated, Planning

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