Activation of Collimators Irradiated With Clinical Proton Beams and Development of a Semiempirical Model for Activity Calculation.

Patient-specific collimators used in proton therapy are activated after use. The aim of this work is to assess the residual activity in brass collimators considering clinical beams, so far studied only for monoenergetic beams, and to develop a model to calculate the activity. Eight brass collimators irradiated with different clinical and monoenergetic beams were included in the study. The collimators were analyzed with gamma spectrometry in the framework of three independent studies carried out at the two French proton therapy sites. Using FLUKA (a fully integrated particle physics Monte Carlo simulation package), simulations were performed to determine radionuclides and activities for all the collimators. The semiempirical model was built using data calculated with FLUKA for a range of clinical beams (different maximum proton energies, modulations, and doses). It was found that there was global coherence in experimental results from different studies. The relevant radionuclides at 1 mo postirradiation were Co, Co, and Zn, and additionally, Mn, Co, and Co for high-energy beams. For nondegraded monoenergetic beams, differences between FLUKA and spectrometry were within those reported in reference benchmark studies (±30%). Due to the use of perfect monochromatic sources in the FLUKA model, FLUKA results systematically underestimated experimental activities for clinical beams, especially for Zn, depending on the beam energy spread (modulation, degradation, beam line characteristics). To account for the energy spread, correction factors were derived for the semiempirical model. The model is applicable to the most relevant radionuclides and total amounts. Secondary neutrons have a negligible contribution to the activity during treatment with respect to proton activation.

Study of the responses and calibration procedures of neutron and gamma area and environmental detectors for use in proton therapy.

Ambient dose equivalent measurements with radiation protection instruments are associated to large uncertainties, mostly due to the energy dependence of the instrument response and to the dissimilarity between the spectra of the standard calibration source and the workplace field. The purpose of this work is to evaluate its impact on the performance of area and environmental detectors in the proton therapy environment, and to provide practical solutions whenever needed and possible. The study was carried out at the Centre Antoine Lacassagne (CAL) proton therapy site, and included a number of commercially available area detectors and a home-made environmental thermoluminescent dosimeter based on a polyethylene moderator loaded with TLD600H/TLD700H pairs. Monte Carlo simulations were performed with MCNP to calculate, first, missing or partially lacking instrument responses, covering the range of energies involved in proton therapy. Second, neutron and gamma spectra were computed at selected positions in and outside the CAL proton therapy bunkers. Appropriate correction factors were then derived for each detector, workplace location and calibration radionuclide source, which amounts to up to 1.9 and 1.5 for neutron and photon area detectors, respectively, and suggest that common ambient dose equivalent instruments might not meet IEC requirements. The TLD environmental system was calibrated in situ and appropriate correction factors were applied to account for the cosmic spectra. Measurements performed with this system from 2014 to 2017 around the installation were consistent with reference natural background dose data and with pre-operational levels registered at the site before the construction of the building in 1988, showing thus no contribution from the site clinical activities. An in situ verification procedure for the radiation protection instruments was also implemented in 2016 at the low energy treatment room using the QA beam reference conditions. The method presents main methodological, practical and economic advantages over external verifications.

Cataract Avoidance With Proton Therapy in Ocular Melanomas.

Purpose: The lens is a radiosensitive organ. Any dose of cephalic irradiation can give rise to radiation-induced cataracts. Contrary to other forms of radiotherapy, proton therapy (PT) can spare all or part of the lens due to accurate dose deposition. We investigated whether a lens-sparing approach was relevant to avoid cataracts in uveal melanoma patients. Methods: Patients were referred for PT from onco-ophthalmologists of private and academic institutions. Patients without preexisting cataracts or implants were entered in a prospective database. Dose thresholds responsible for cataracts were investigated in volumes of lens or lens periphery. Lens opacifications and de novo vision-impairing cataracts (VICs) had biannual follow up by ophthalmologists blinded to lens dose. Correlations between dose-volume relationships and VICs were assessed using univariate/multivariate regressions. Results: Between 1991 and 2015, 1696 uveal melanoma patients were consecutively treated with PT. After a median follow up of 48 months, 14.4% and 8.7% of patients had cataracts and VIC within median times of 19 and 28 months, respectively. Median values of mean lens and lens periphery doses were 1.1 (radiobiologically effective [RBE] dose in photon-equivalent grays [GyRBE]) and 6.5 GyRBE, respectively. The lens received no dose in 25% of the patients. At an irradiated lens volume of ≤5%, there was no significantly increased risk for VIC below a dose of 10 GyRBE. Conclusions: A lens-sparing approach is feasible and results not only in reduced need for cataract surgery but also in better fundus-based tumor control. Reassessment of radioprotection rules for lens dose thresholds may follow.

InterCardioRisk: a novel online tool for estimating doses of ionising radiation to occupationally-exposed medical staff and their associated health risks.

Those working in interventional cardiology and related medical procedures are potentially subject to considerable exposure to x-rays. Two types of tissue of particular concern that may receive considerable doses during such procedures are the lens of the eye and the brain. Ocular radiation exposure results in lens changes that, with time, may progress to partial or total lens opacification (cataracts). In the early stages, such opacities do not result in visual disability; the severity of such changes tends to increase progressively with dose and time until vision is impaired and cataract surgery is required. Scattered radiation doses to the eye lens of an interventional cardiologist in typical working conditions can exceed 34 µGy min-1 in high-dose fluoroscopy modes and 3 µGy per image during image acquisition (instantaneous rate values) when radiation protection tools are not used. A causal relation between exposure to ionising radiation and increased risk of brain and central nervous system tumours has been shown in a number of studies. Although absorbed doses to the brain in interventional cardiology procedures are lower than those to the eye lens by a factor between 3.40 and 8.08 according to our simulations, doses to both tissues are among the highest occupational radiation doses documented for medical staff whose work involves exposures to x-rays. We present InterCardioRisk, a tool featuring an easy-to-use web interface that provides a general estimation of both cumulated absorbed doses experienced by medical staff exposed in the interventional cardiology setting and their estimated associated health risks. The tool is available at http://intercardiorisk.creal.cat.