Proton minibeam radiation therapy for treating metastases: A treatment plan study

Background: Proton minibeam radiation therapy (pMBRT) is a new radiotherapy approach that has shown a significant increase in the therapeutic window in glioma-bearing rats compared to conventional proton therapy. Such preclinical results encourage the preparation of clinical trials.

Purpose: In this study, the potential of pMBRT for treating clinical indications candidates for the first clinical trials (i.e., brain, lung, and liver metastases) was evaluated.

Methods: Four clinical cases, initially treated with stereotactic radiotherapy (SRT), were selected for this study. pMBRT, SRT, and conventional proton therapy (PT) dose distributions were compared by using three main criteria: (i) the tumor coverage, (ii) the mean dose to organs-at-risk, and (iii) the possible adverse effects in normal tissues by considering valley doses as the responsible for tissue sparing. pMBRT plans consisted of one fraction and one-two fields. Dose calculations were computed by means of Monte Carlo simulations.

Results: pMBRT treatments provide a similar or superior target coverage than SRT, even using fewer fields. pMBRT also significantly reduces the biologically effective dose (BED) to organs-at-risk. In addition, valley and mean doses to normal tissues remain below tolerance limits when treatments are delivered in a single fraction, contrary to PT treatments.

Conclusions: This work provides a first insight into the possibility of treating metastases with pMBRT. More favorable dose distributions and treatment delivery regimes may be expected from this new approach than SRT. The advantages of pMBRT would need to be confirmed by means of Phase I clinical trials.