Session Item

The addition of a focal boost to the intraprostatic lesion(s) improves outcome for patients with intermediate and high risk prostate cancer. Furthermore, this can be achieved with no additional toxicity or impact on quality of life, as was demonstrated in the multicenter phase III randomized FLAME trial. The purpose of the present analysis was to evaluate the technical and clinical feasibility of a novel dose-adaptive workflow on an 1.5T MR-linac for an ultra-hypofractionated focal boost scheme.

Until October 2021, 20 patients with intermediate and high risk localized prostate cancer were treated at the Radboudumc on the MR-linac in 5 fractions of 7 Gy with an iso-toxic focal boost up to 50Gy within a multicenter phase II study. The focal boost dose was reduced if necessary to adhere to the constraints of the organs at risk. In a fully adaptive Adapt-To-Shape (ATS) workflow the contours of the reference MRI were propagated to the daily MRI and manually adapted when required within a ring of 1cm around the target volume. Based on these contours the reference plan was adapted using a full re-optimisation. The optimisation goals (iso-constraints) of the GTV, CTV and OARs for the daily online plan were manually adjusted to acquire an optimal balance of a GTV dose as high as possible, within the OAR constraints. A maximum of three optimisation rounds with iso-constraints adaptation was allowed to prevent prolonged intrafraction time. After completion of plan optimisation, the ATS workflow was followed by a position verification scan. When intrafraction motion was ≥2 mm, an additional translation (Adapt-To-Position) was performed. During beam-on, a 3D T2 weighted image was acquired. After each fraction, all contours were adapted by an experienced RTT on the beam-on scan and the delivered dose per fraction for all structures was accumulated. The actual delivered doses of all previous fractions were taken into account for adjustment of the iso-constraints in the remaining fractions when deemed necessary or the creation of a new reference plan, see Figure 1 for a visual of the workflow.

All fractions were delivered at the MR-linac and it was feasible to obtain the actual delivered dose of the previous fractions before the subsequent fractions and take this into account in the online plan optimization process. Based on the accumulated dose, in 14 out of the 20 patients the online optimization goals were manually adjusted. In 3 out of the 20 patients a new reference plan was created for the remaining fractions. This was performed when the OAR constraints during beam-on were largely exceeded due to intrafraction motion. 

A novel dose-adaptive workflow based on the actual delivered dose of the target volume and OARs in previous fractions is feasible for ultra-hypofractionated radiotherapy with an iso-toxic focal boost. Future studies should focus on automatization and artificial intelligence solutions to allow for rapid online dose-adaptive workflows.