ESTRO 2020 Abstract book

S393 ESTRO 2020

A retrospective study on 2526 previously irradiated PCa patients was performed. Patients were treated with 3D- CRT (21.3%), IMRT (68.1%), or VMAT (10.6%). Second tumors incidence was analysed in 3 categories: pelvic, pelvic and abdominal, and “any site”. The correlation with RT technique was analysed using log-rank test and Cox’s proportional hazard method. Results With a median follow-up of 72 months (range: 9-185), 92 (3.6%) cases of second tumors were recorded with 48 months (range: 9-152) median interval from RT. Actuarial 10-year second tumor free survival (STFS) was 87.3%. Ten- year STFS in patients treated with 3D-CRT and IMRT/VMAT was 85.5% and 84.5%, respectively (p: .627). A significantly higher 10-year cumulative incidence of second tumors in the pelvis was registered in patients treated with IMRT/VMAT compared to 3D-CRT (10.7% vs 6.0%; p: .033). This lower incidence of second pelvic cancers in patients treated with 3D-CRT was confirmed at multivariable analysis (HR: 2.42, 95%CI: 1.07-5.47, p: .034). Conclusion The incidence of second pelvic tumors after RT of PCa showed a significant correlation with treatment technique. Further analyses in larger series with prolonged follow-up are needed to confirm these results. PH-0649 Can we dose escalate in anal cancer without an increase in dose to OARs? N. Abbott 1 , M. Robinson 2 , J. Copeland 3 , M. Harrison 4 , M. Hawkins 5 , R. Muirhead 2 , R. Adams 6 , D. Sebag-Montefiore 7 1 RTTQA group- Velindre Cancer Centre, Medical Physics, Cardiff, United Kingdom ; 2 Oxford University Hospitals NHS Trust, Oxford, United Kingdom ; 3 Leeds University, Leeds Clinical Trials Research Unit, Leeds, United Kingdom ; 4 Mount Vernon Hospital, Mount Vernon Centre for Cancer, Northwood, United Kingdom ; 5 University College London, UcL, London, United Kingdom ; 6 Cardiff University Department of Cancer & Genetics, and Velindre Cancer Centre, Cardiff, United Kingdom ; 7 St James University Hospital, Leeds Cancer Centre, Leeds, United Kingdom Purpose or Objective PersonaLising Anal cancer radiotherapy dOse (PLATO) is an integrated protocol, including anal cancer trial 5 (ACT5) for high risk patients. ACT5 is a randomised trial comparing UK standard and dose escalated concurrent chemo-radiotherapy. Dose escalation above UK standard dose [1] is to a PTV_Boost encompassing primary disease and positive nodes plus a 5mm margin. A pilot study of 60 patients was carried out across 12 UK sites. This was to allow for planned interim analysis of acute toxicity [2], protocol compliance, and to determine if there is a significant increase to OAR dose metrics associated with dose escalation.

We present statistical analysis of trial OAR dose metrics, comparing the dose escalation arms with standard dose treatments to determine if a statistically significant increase is seen in OAR dose metrics. Material and Methods 58 patients were evaluable in the ACT5 pilot phase from 12 UK sites. Patients were randomised as below. * Standard arm: 18 patients All patients had a prospective RTQA of contouring and planning to ensure compliance with protocol and review of OAR dose constraints. Trial OAR dose metrics were compared in both dose escalation arms to those in the standard dose arm using the Mann-Whitney U test. Results There was no statistically significant increase in any OAR dose metric apart from genitalia D5% and Femoral Heads D5%. A statistically significant difference was seen for D5% genitalia in both dose escalation arms, see Table 1. * Dose escalation arm1: 21 patients * Dose escalation arm2: 19 patients

For the genitalia, one arm1 and one arm2 patient did not meet the mandatory trial dose constraints, with a total of 22 patients meeting mandatory but not optimal constraints. This was due to overlap with PTV_Boost in both cases, with both plans considered optimal and acceptable for clinical use. For the femoral heads, although there is a statistically significant increase in dose for D5% for arm2, all values were within the optimal constraint, D5%<50Gy, with a larger standard deviation seen for the standard arm, 2.3 vs 6.3 (arm2 vs standard: min 29.1Gy-7.7Gy, max 38.0Gy- 42.7Gy, mean 34.3Gy-34.0Gy). Conclusion Dose escalation to primary disease and node positive disease is achievable without a statistically significant increase in the vast majority of OAR dose constraints.

Dose escalation was as below * Standard arm: PTV_Boost n/a

* Dose escalation arm1: PTV_Boost 58.8 Gy in 28# * Dose escalation arm2: PTV_Boost 61.6 Gy in 28#