ESTRO 2024 - Abstract Book

S5303 ESTRO 2024 parameters by which its efficacy can be characterized and quantified e.g. Grid Factor 6 (GF). We here report on the commissioning of minibeam multi-slit collimator (MSC) for the experimental proton beam line, and the first in vivo assessment of pMBRT with homogenous target dose performed at DCPT. Radiobiology - Normal tissue radiobiology

Material/Methods:

We optimize the configuration of a static MSC to deliver a homogeneous dose in the Planning Target Volume (PTV) through Monte Carlo simulation. The optimized design was experimentally validated. Due to reduced nuclear activation and neutron production brass was preferred over tungsten as a collimator material of choice. The thickness of the collimator was 5.0 cm which demonstrated sufficient to stop the beam (83 to 107 MeV) used for radiobiological experiment. The center-to-center distance (CTC) of the collimator was 2.25 mm with 44 % geometrical throughput6 equivalent to 1 mm aperture. The PTV is extended between 5.5 cm to 8.5 cm depth in water. The treatment plan was generated using ‘Eclipse’. For pMBRT irradiation, the original plan was rescaled to deliver a homogeneous dose at the PTV which was identical to the PTV dose of the corresponding non-collimated configuration. A lateral dose gradient between peaks and valleys was introduced before the PTV. Retaining the PTV homogeneity and dose is essential in this study, as this provides us a point of equipoise. At a position immediately after the minibeam collimator the beam was most heterogeneous, we investigated radiation-induced acute skin damage in 42 unanesthetized female C3H/HeNRj mice (6 mice for each data point). The target volume (mouse leg) was placed in air 2.0 cm away from the exit side of the MSC. Biological damage was quantified by assessing acute toxicity (hair loss, moist desquamation, and toe visibility) up to 22 days post treatment. As shown in Fig 1, we have established the dose response relation in air using acute skin toxicity scoring method 7 for a conventional proton beam configuration. With increasing dose, the probability of skin toxicity increases following a similar relation observed for previous reference experiments in water. We investigated the effects of pMBRT at two specific dose points. For an identical dose and target volume coverage, we observed a notable reduction in normal tissue complications with pMBRT compared to the conventional irradiation approach. Specifically, at a 30 Gy dose point, pMBRT demonstrated no discernible biological effects for both mild and more severe skin toxicity (Score 1.5 and 2.0), in stark contrast to the significant toxicity observed with conventional irradiation. At 40 Gy, we did observe visible skin toxicity with pMBRT; however, the effect was markedly less pronounced, and required a 60% higher dose for 50% of the mice to display mild skin toxicity. For conventionally irradiated mice we also observed higher skin toxicity levels (Score 2.5, 3.0 and 3.5), but none of these occurred for the pMBRT irradiated mice.. These findings indicate a GF of 0.63 (or in its inverse, means a potential dose escalation factor of 1.6). Results: