Dosimetric commissioning and quality assurance of scanned ion beams at the Italian National Center for Oncological Hadrontherapy

Abstract

Purpose: To describe the dosimetric commissioning and quality assurance (QA) of the actively scanned proton and carbon ion beams at the Italian National Center for Oncological Hadrontherapy. Methods: The laterally integrated depth-dose-distributions (IDDs) were acquired with the PTW Peakfinder, a variable depth water column, equipped with two Bragg peak ionization chambers. FLUKA MC code was used to generate the energy libraries, the IDDs in water, and the fragment spectra for carbon beams. EBT3 films were used for spot size measurements, beam position over the scan field, and homogeneity in 2D-fields. Beam monitor calibration was performed in terms of number of particles per MU using both a Farmer-type and an Advanced Markus ionization chamber. The beam position at the isocenter, beam monitor calibration curve, dose constancy in the center of the spread-out-Bragg-peak, dose homogeneity in 2D-fields, beam energy, spot size, and spot position over the scan field are all checked on a daily basis for both protons and carbon ions and on all beam lines. Results: The simulated IDDs showed an excellent agreement with the measured experimental curves. The measured FWHM of the pencil beam in air at the isocenter was from 0.7 to 2.2 cm for protons. For carbon ions, two sets of spot size are available: from 0.4 to 0.8 cm (for the smaller) and from 0.8 to 1.1 cm (for the larger one). The spot position was within +/-1 mm over the whole 20x20 cm2 scan field; homogeneity in a uniform squared field was within +/-5% for both particle types at any energy. QA results exceeding tolerance levels were rarely found. In the reporting period, the machine downtime was around 6%, of which 4.5% was due to planned maintenance shutdowns. Conclusions: After successful dosimetric beam commissioning, quality assurance measurements performed during a 24-month period show very stable beam characteristics.