An evaluation on the use of alanine/ESR dosimetry in moving targets: The case of lung treatment with photons and proton beams

Purpose: The combination of target and beam motions is a concern in radiotherapy, as it results in the so-called interplay effects. These effects may lead to an inappropriate dose distribution delivered to patients, potentially compromising treatment outcomes, being particularly common in the case of thoracic tumours. An excellent external radiotherapy treatment option – compared to conventional photon modality – is proton beams delivered with pencil beam scanning (PBS), which might offer better sparing of healthy tissues with comparable dose tumour coverage. This study, therefore, aimed to evaluate whether alanine, based on electron spin resonance (ESR), a well-established dosimeter, can be applied in the context of moving targets of clinical photon and proton beams and assessing interplay effects. Materials and methods: Alanine/ESR dosimeters (Harwell, UK) inserted in an anthropomorphic moving phantom (CIRS, USA) were irradiated with photon and proton beams, at the San Matteo Hospital and Fondazione CNAO, respectively, both in Pavia (Italy). Different moving conditions, simulating or not abdominal compression (20-mm maximum respiratory excursion), were evaluated. ESR measurements were performed in an ELEXSYS spectrometer (Bruker, Germany), and the peak-to-peak amplitude was used to reconstruct the dose. Results: Good agreement between planned and measured doses was obtained with deviations within 3.3 % and 2.2 % for photons (flattening-filter) and protons, respectively, in all moving conditions. No significant differences were observed as a function of the moving regimen. Conclusions: This study showed that alanine/ESR can be a suitable dosimetric system for use in end-to-end tests for moving targets, including the evaluation of interplay effects.