Calibration of helical tomotherapy machine using EPR/alanine dosimetry

Purpose: Current codes of practice for clinical reference dosimetry of high‐energy photon beams in conventional radiotherapy recommend using a 10 × 10   cm 2square field, with the detector at a reference depth of 10 cm in water and 100 cm source to surface distance (SSD) (AAPM TG‐51) or 100 cm source‐to‐axis distance (SAD) (IAEA TRS‐398). However, the maximum field size of a helical tomotherapy (HT) machine is 40 × 5   cm 2defined at 85 cm SAD. These nonstandard conditions prevent a direct implementation of these protocols. The purpose of this study is twofold: To check the absorbed dose in water and dose rate calibration of a tomotherapy unit as well as the accuracy of the tomotherapy treatment planning system (TPS) calculations for a specific test case. Method: Both topics are based on the use of electron paramagnetic resonance (EPR) using alanine as transfer dosimeter between the Laboratoire National Henri Becquerel (LNHB) C60 o‐ γ ‐ray reference beam and the Institut Curie's HT beam. Irradiations performed in the LNHB referenceC60 o‐ γ ‐ray beam allowed setting up the calibration method, which was then implemented and tested at the LNHB 6 MV linac x‐ray beam, resulting in a deviation of 1.6% (at a 1% standard uncertainty) relative to the reference value determined with the standard IAEA TRS‐398 protocol. Results: HT beam dose rate estimation shows a difference of 2% with the value stated by the manufacturer at a 2% standard uncertainty. A 4% deviation between measured dose and the calculation from the tomotherapy TPS was found. The latter was originated by an inadequate representation of the phantom CT‐scan values and, consequently, mass densities within the phantom. This difference has been explained by the mass density values given by the CT‐scan and used by the TPS which were not the true ones. Once corrected using Monte Carlo N‐Particle simulations to validate the accuracy of this process, the difference between corrected TPS calculations and alanine measured dose values was then found to be around 2% (with 2% standard uncertainty on TPS doses and 1.5% standard uncertainty on EPR measurements). Conclusion: Beam dose rate estimation results were found to be in good agreement with the reference value given by the manufacturer at 2% standard uncertainty. Moreover, the dose determination method was set up with a deviation around 2% (at a 2% standard uncertainty).