ESTRO 2024 - Abstract Book

S3474

Physics - Dose prediction, optimisation and applications of photon and electron planning

ESTRO 2024

1 Netherlands Cancer Institute, Radiation Oncology, Amsterdam, Netherlands. 2 Royal Marsden NHS Foundation Trust, Radiation Oncology, London, United Kingdom. 3 Sunnybrook Health Sciences Centre, Radiation Oncology, Toronto, Canada

Purpose/Objective:

Genitourinary toxicity and erectile dysfunction are the most commonly observed side effects in patients following external beam radiation therapy for localized prostate cancer (PCa). Dose de-escalation to the urethra and neurovascular tissue surrounding the prostate may reduce these toxicities. The DESTINATION trial aims to establish whether escalating the dose to the gross tumor volume (GTV) whilst de-escalating the dose to the prostate clinical target volume (CTV) and its surrounding organs at risk (OARs) using a 0 mm margin for the prostate planning target volume (PTV) can improve quality of life without compromising biochemical control in patients with favorable intermediate-risk PCa. The MRI-invisible tumor load in the CTV consists of tumor cells that are not homogeneously distributed but clumped together in a median of 2 foci, constituting 2-5% of the total tumor load (Figure 1) [1, 2]. Limited underdosage of a small part of the CTV in case of using a 0 mm PTV margin may therefore be justified by the distribution of subclinical disease in the prostate. The aim of this study was to simulate the impact of prostate intra-fraction motion on the dose proposed to be delivered to the CTV and GTV in DESTINATION treatment plans. In addition, we explored the potential benefit of beam-gated treatment by analyzing treatment fractions showing limited prostate translations of < 2 mm in all directions.

Material/Methods:

Data from 23 patients with intermediate risk PCa previously treated on the 1.5T Unity MR-Linac was used for this planning study. For each patient, 5 daily T2-weighted MRIs acquired pre-treatment and during the last 2 minutes of irradiation were collected. Target structures and OARs were delineated on the daily pre-treatment MRIs. GTV was defined as tumor(s) visible on multi-parametric MRI. The CTV was defined as the whole prostate including the proximal 1 cm of seminal vesicles plus a margin of 4 mm around the GTV, excluding OARs. No PTV margin was applied for both CTV and GTV. For each pre-treatment MRI an automated treatment plan was generated using automated prioritized treatment planning software (mCycle, Elekta AB), simulating an online adaptive workflow. Patients were planned to receive 30 Gy to the CTV, with a focal boost to the GTV up to 45 Gy while strictly adhering to OARs constraints. Subsequently, CTV and GTV delineations were rigidly propagated from the pre-treatment to the post-treatment MRI using translations and rotations. Intra-fraction motion of the CTV was determined by using