Type of publication:
Conference abstract
Author(s):
Iddenden J.; Howard D.; Hudson E.; Teo M.; Diez P.; Miles E.; Turtle L.; Patel R.; Appelt A.; *Gollins S.
Citation:
Radiotherapy and Oncology. Conference: ESTRO 2024. Glasgow United Kingdom. 194(Supplement 1) (pp S5977-S5980), 2024. Date of Publication: 01 May 2024.
Abstract:
Purpose/Objective: APHRODITE (ISRCTN16158514), funded by Yorkshire Cancer Research, is a phase II randomised controlled trial comparing radical (chemo)radiotherapy (CRT) alone versus dose-escalated CRT with a simultaneous integrated boost (SIB). Patients with early stage rectal cancer, who are considered by their multidisciplinary team as unsuitable for radical total mesorectum excision or have a strong preference for organ preservation, will all receive 50.4Gy in 28 fractions to a small mesorectal-only planning target volume (PTV). Those in the experimental dose-escalation arm also receive up to 62Gy SIB to the primary tumour volume (PTVp). The trial is currently in active recruitment with a target sample size of 104 patients. Few studies exist detailing dose-volume constraints applied in this setting and none which examine the frequency to which they are achieved1. Anonymised trial plans were retrospectively reviewed to determine if the optimal organ at risk (OAR) dose-volume constraints as set out in the trial protocol are achievable. The conformity of the target volumes coverage was also assessed. Material/Methods: All centres completed the pre-trial radiotherapy quality assurance programme prior to recruiting. Radiotherapy planning data was requested for all patients. To date, 8 centres have recruited patients, with plan data for 46 out of 73 trial patients available at the time of analysis. Radiotherapy was planned according to their randomisation following APHRODITE dose-volume constraints. All plans were retrospectively reviewed on Velocity version 4.1 (Varian Medical Systems, Inc.) and dose-volume constraints extracted from DVH data. Conformity indices, as defined by RTOG (95% isodose volume/volume of PTV), were calculated for all PTVs. The standard deviation was calculated for optimal OAR dose-volume constraints and target volume conformity. Mann-Whitney U tests (two-tailed) were performed to test differences between the standard and dose-escalated arms. Result(s): 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 dosevolume 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(s): 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.
DOI: 10.1016/S0167-8140%2824%2902083-8
