Online Adaptive Five-Fraction Ablative Radiotherapy for Prostate Cancer Using a Conventional Linear Accelerator

To assess the dosimetric results of an online adaptive stereotactic body radiation therapy (SBRT) program to treat patients with prostate cancer using a conventional linear accelerator.

Prostate SBRT with 5 fractions is performed at the Hospital Quirónsalud Barcelona using an online adaptive method previously described (Pract Radiat Oncol. 2022 Mar-Apr;12(2):e144-e152). A CBCT-based adaptive "plan-of-the-day" (POD) is generated for each treatment fraction, which consists of a single volumetric modulated arc therapy. A dosimetric evaluation of the PODs was performed for the first 20 patients and included analysis of the target (prostate), organs-at-risks (OARs), and patient-specific quality assurance (PSQA). Each of the PODs was also compared with the corresponding conventional image guided radiation therapy (IGRT) method. Finally, the adaptive treatment timing is analyzed for the 100 PODs in this study.

The online adaptive treatment method ensured optimal target coverage in all treatment fractions for all patients. However, the simulated IGRT plans did not result in adequate prostate coverage (V40 Gy ≥ 95%) in 70% of fractions. Small average differences between PODs and IGRT plans were found in the OARs. The dose sparing in the rectum and bladder provided by some simulated IGRT plans, was associated with a compromised prostate coverage. The PSQA resulted in an excellent agreement between the online-calculated plans and the independent dose checks performed for all 100 PODs. The average duration of the plan adaptation was 20.1 ± 6.1 minutes and the average overall session time including adaptation and treatment delivery was 26.0 ± 6.3 minutes.

The online adaptive program using a conventional linac to treat prostate cancer described in this study is clinically feasible and in adherence with the acceptance criteria set by the PACE B trial.