Study of the secondary neutral radiation in proton therapy: Toward an indirect in vivo dosimetry

• Published in: Medical physics (Lancaster) 2012-12, Vol.39 (12), p.7303-7316
• Main Authors: Carnicer, A., Letellier, V., Rucka, G., Angellier, G., Sauerwein, W., Hérault, J.
• Format: Article
• Language: English
• Subjects: 60 APPLIED LIFE SCIENCES; Alloys; Applications; biomedical equipment; BRAGG CURVE; BRASS; Collimators; Computer Simulation; DOSE EQUIVALENTS; DOSE RATES; DOSIMETRY; Dosimetry/exposure assessment; IN VIVO; MCNPX; Models, Statistical; Monte Carlo; MONTE CARLO METHOD; Monte Carlo methods; NEUTRONS; PHOTONS; Position sensitive detectors; proton; PROTON BEAMS; Proton therapy; PROTONS; Protons - therapeutic use; Quality assurance equipment; QUALITY CONTROL; RADIATION DOSES; RADIATION PROTECTION AND DOSIMETRY; radiation therapy; Radiation treatment; RADIOLOGY AND NUCLEAR MEDICINE; Radiometry - methods; RADIOTHERAPY; Radiotherapy Dosage; Radiotherapy Planning, Computer-Assisted - methods; Radiotherapy, Conformal - methods; secondary emission; secondary radiation; SIMULATION; SOBP; Therapeutic applications, including brachytherapy; VALIDATION
• ISSN: 0094-2405; 2473-4209
• DOI: 10.1118/1.4765049

Purpose: Secondary particles produced in the collision of protons with beam modifiers are of concern in proton therapy. Nevertheless, secondary radiation can provide information on the dosimetric parameters through its dependency on the modulating accessories (range shifter and range modulating wheel). Relatively little data have been reported in the literature for low-energy proton beams. The present study aims at characterizing the neutron and photon secondary radiation at the low-energy proton therapy facility of the Centre Antoine Lacassagne (CAL), and studying their correlation to the dosimetric parameters to explore possible practical uses of secondary radiation in the treatment quality for proton therapy. Methods: The Monte Carlo code MCNPX was used to simulate the proton therapy facility at CAL. Neutron and photon fluence, Φ, and ambient dose equivalent per proton dose, H*(10)/D, were determined across the horizontal main plane spanning the whole treatment room. H*(10)/D was also calculated at two positions of the treatment room where dosimetric measurements were performed for validation of the Monte Carlo calculations. Calculations and measurements were extended to 100 clinical spread-out Bragg Peaks (SOBPs) covering the whole range of therapeutic dose rates (D/MU) employed at CAL. In addition, the values of D and MU were also calculated for each SOBP and the results analyzed to study the relationship between secondary radiation and dosimetric parameters. Results: The largest production of the secondary particles takes place at the modulating devices and the brass collimators located along the optical bench. Along the beam line and off the beam axis to 2.5 m away, H*(10)/D values ranged from 5.4μSv/Gy to 5.3 mSv/Gy for neutrons, and were 1 order of magnitude lower for photons. H*(10)/D varied greatly with the distance and angle to the beam axis. A variation of a factor of 5 was found for the different range of modulations (SOBPs). The ratios between calculations and measurements were 2.3 and 0.5 for neutrons and photons, respectively, and remained constant for all the range of SOBPs studied, which provided validation for the Monte Carlo calculations. H*(10)/D values were found to correlate to the proton dose rate D/MU with a power fit, both for neutrons and photons. This result was exploited to implement a system to obtain D/MU values from the measurement of the integrated photon ambient dose equivalent H*(10) during treatment, which provides a me