In Vivo Radiobiological Investigations with the TOP-IMPLART Proton Beam on a Medulloblastoma Mouse Model

Protons are now increasingly used to treat pediatric medulloblastoma (MB) patients. We designed and characterized a setup to deliver proton beams for in vivo radiobiology experiments at a TOP-IMPLART facility, a prototype of a proton-therapy linear accelerator developed at the ENEA Frascati Research Center, with the goal of assessing the feasibility of TOP-IMPLART for small animal proton therapy research. Mice bearing Sonic-Hedgehog (Shh)-dependent MB in the flank were irradiated with protons to test whether irradiation could be restricted to a specific depth in the tumor tissue and to compare apoptosis induced by the same dose of protons or photons. In addition, the brains of neonatal mice at postnatal day 5 (P5), representing a very small target, were irradiated with 6 Gy of protons with two different collimated Spread-Out Bragg Peaks (SOBPs). Apoptosis was visualized by immunohistochemistry for the apoptotic marker caspase-3-activated, and quantified by Western blot. Our findings proved that protons could be delivered to the upper part while sparing the deepest part of MB. In addition, a comparison of the effectiveness of protons and photons revealed a very similar increase in the expression of cleaved caspase-3. Finally, by using a very small target, the brain of P5-neonatal mice, we demonstrated that the proton irradiation field reached the desired depth in brain tissue. Using the TOP-IMPLART accelerator we established setup and procedures for proton irradiation, suitable for translational preclinical studies. This is the first example of in vivo experiments performed with a "full-linac" proton-therapy accelerator.