ESTRO 2024 - Abstract Book

S4323

Physics - Intra-fraction motion management and real-time adaptive radiotherapy

ESTRO 2024

Conclusion:

Our analysis resulted in lower values than the currently applied protocol based 1 and 3 mm safety margins for cranial and prostate treatments, respectively, so the currently used margins are safe to perform the treatments. For prostate treatments, possibility of further reduction of the safety margins could be considered. It can be concluded that, despite long treatment times, small safety margins can be ensured by frequent imaging and corrections.

Keywords: Cyberknife, IGRT, safety margin

2325

Digital Poster

Reduction of PTV margins in online adaptive radiotherapy for prostate and cervix cancer patients

John Alfred Brennsæter 1 , Tordis J Dahle 1 , Jannicke N Moi 1 , Ingvild F Svanberg 1 , Gry S Haaland 1 , Sara Pilskog 1,2

1 Haukeland University Hospital, Department of Oncology and Medical Physics, Bergen, Norway. 2 University of Bergen, Department of Physics and Technology, Bergen, Norway

Purpose/Objective:

Cone beam CT (CBCT) based online adaptive radiotherapy (oART) is a new development in radiotherapy. With oART, the requirements for planning target volume (PTV) margins differ from standard therapy because motion occurs during a session. In this study, we aim to evaluate a margin reduction for locally advanced prostate and cervix patients treated with oART.

Material/Methods:

Intrafraction motion of the elective pelvic lymph nodes was evaluated by two radiation therapists (RTTs) for 150 fractions from 10 prostate patients and 200 fractions from 15 cervix patients treated with oART. PTV margins of 3, 4 and 5 mm were added to these lymph nodes for all patients. For each oART session, an initial CBCT was first taken. Here, important organs for the co-registration (prostate, seminal vesicles, uterus, rectum and bladder) were delineated, and CTVs were subsequently generated based on a structure-guided deformable registration of the planning CT and the CBCT. The CTVs were reviewed and edited if necessary by the RTTs. An adaptive plan was then generated, and before treatment delivery, a second CBCT (verification CBCT) was acquired to verify patient anatomy and correct for possible intrafraction motion occurring when constructing the oART plan. The seven first prostate patients were treated with 5 mm PTV margin, while the last three patients were treated with 4 mm margin. All cervix patients were treated with 5 mm margin. After treatment, the RTTs reviewed the verification CBCTs and evaluated whether the various PTV margins would have covered the adapted clinical target volume, scoring each fraction as approved, inconclusive or rejected (Table 1). Couch shifts corresponding to the rigid prostate match between the CBCTs were analysed with respect to the RTT evaluation.