EPR/alanine dosimetry in Stereotactic Radiosurgery treatments through Helical Tomotherapy

Intracranial stereotactic radiosurgery (SRS) is a technique to deliver an ablative radiation dose with an extremely sharp dose gradient to small brain tumors. This tecnique allows to deliver high doses of radiation to the tumor sparing the surrounding healthy tissue. In this study the accuracy of the dose delivered in a SRS session by a non conventional radiotherapy machine, the TomoTherapy Hi-Art System, was investigated using an "end-to-end" test. This is perfomed by means of alanine Electron Paramagnetic Resonance (EPR) pellets. The response of these dosimeters is compared to that of gafchromic films which are particularly suitable for twodimensional dose verification providing accurate estimate of the distribution and of the gradients of the absorbed dose. Dose verifications were made using alanine dosimeters placed in an antropomorphic head phantom (Alderson Rando Phantom) under different treatment conditions in case of both single and multiple brain tumors. 1.25mm slice kVCT scan of the phantom was used to generate SRS plans on the TomoTherapy Planning Station platform, prescribing dose at the 95% isodose level of the "PTV" using different combinations of pitch and Modulation Factor. Before each session a MVCT was performed for setup verification. Commercial alanine dosimeters (Synergy Health, Germany) were irradiated in various positions of the phantom. EPR measurements were carried out through Bruker ECS106 spectrometer working at about 9.7 GHz. The dose values measured through alanine dosimeters and gafchromic films show a good agreement with the dose values calculated by the TomoTherapy Treatment Planning System, for both organs at risk and tumors. Alanine absolute dose measurements showed to be useful for the dosimetric validation of HT SRS treatments.