A torus source and its application for non-primary radiation evaluation

Objective. Non-primary radiation doses to normal tissues from proton therapy may be associated with an increased risk of secondary malignancies, particularly in long-term survivors. Thus, a systematic method to evaluate if the dose level of non-primary radiation meets the IEC standard requirements is needed.Approach. Different from the traditional photon radiation therapy system, proton therapy systems are composed of several subsystems in a thick bunker. These subsystems are all possible sources of non-primary radiation threatening the patient. As a case study, 7 sources in the P-Cure synchrotron-based proton therapy system are modeled in Monte Carlo (MC) code: tandem injector, injection, synchrotron ring, extraction, beam transport line, scanning nozzle and concrete reflection/scattering. To accurately evaluate the synchrotron beam loss and non-primary dose, a new model called the torus source model is developed. Its parametric equations define the position and direction of the off-orbit particle bombardment on the torus pipe shell in the Cartesian coordinate system. Non-primary doses are finally calculated by several FLUKA simulations.Main results. The ratios of summarized non-primary doses from different sources to the planned dose of 2 Gy are all much smaller than the IEC requirements in both the 15-50 cm and 50-200 cm regions. Thus, the P-Cure synchrotron-based proton therapy system is clean and patient-friendly, and there is no need an inner shielding concrete between the accelerator and patient.Significance. Non-primary radiation dose level is a very important indicator to evaluate the quality of a PT system. This manuscript provides a feasible MC procedure for synchrotron-based proton therapy with new beam loss model. Which could help people figure out precisely whether this level complies with the IEC standard before the system put into clinical treatment. What' more, the torus source model could be widely used for bending magnets in gantries and synchrotrons to evaluate non-primary doses or other radiation doses.

Risk of cardiac implantable device malfunction in cancer patients receiving proton therapy: an overview

Age is a risk factor for both cardiovascular disease and cancer, and as such radiation oncologists frequently see a number of patients with cardiac implantable electronic devices (CIEDs) receiving proton therapy (PT). CIED malfunctions induced by PT are nonnegligible and can occur in both passive scattering and pencil beam scanning modes. In the absence of an evidence-based protocol, the authors emphasise that this patient cohort should be managed differently to electron- and photon- external beam radiation therapy (EBRT) patients due to distinct properties of proton beams. Given the lack of a PT-specific guideline for managing this cohort and limited studies on this important topic; the process was initiated by evaluating all PT-related CIED malfunctions to provide a baseline for future reporting and research. In this review, different modes of PT and their interactions with a variety of CIEDs and pacing leads are discussed. Effects of PT on CIEDs were classified into a variety of hardware and software malfunctions. Apart from secondary neutrons, cumulative radiation dose, dose rate, CIED model/manufacturer, distance from CIED to proton field, and materials used in CIEDs/pacing leads were all evaluated to determine the probability of malfunctions. The importance of proton beam arrangements is highlighted in this study. Manufacturers should specify recommended dose limits for patients undergoing PT. The establishment of an international multidisciplinary team dedicated to CIED-bearing patients receiving PT may be beneficial.

Joint EURADOS WG9-WG11 rem-counter intercomparison in a Mevion S250i proton therapy facility with Hyperscan pulsed synchrocyclotron

Objective Proton therapy is gaining popularity because of the improved dose delivery over conventional radiation therapy. The secondary dose to healthy tissues is dominated by secondary neutrons. Commercial rem-counters are valuable instruments for the on-line assessment of neutron ambient dose equivalent (H*(10)). In general, however, a priori knowledge of the type of facility and of the radiation field is required for the proper choice of any survey meter. The novel Mevion S250i Hyperscan synchrocyclotron mounts the accelerator directly on the gantry. It provides a scanned 227 MeV proton beam, delivered in pulses with a pulse width of 10 µs at 750 Hz frequency, which is afterwards degraded in energy by a range shifter modulator system. This environment is particularly challenging for commercial rem-counters; therefore, we tested the reliability of some of the most widespread rem-counters to understand their limits in the Mevion S250i stray neutron field. Approach This work, promoted by the European Radiation Dosimetry Group (EURADOS), describes a rem-counter intercomparison at the Maastro Proton Therapy centre in the Netherlands, which houses the novel Mevion S250i Hyperscan system. Several rem-counters were employed in the intercomparison (LUPIN, LINUS, WENDI-II, LB6411, NM2B-458, NM2B-495Pb), which included simulation of a patient treatment protocol employing a water tank phantom. The outcomes of the experiment were compared with models and data from the literature. Main results We found that only the LUPIN allowed for a correct assessment of H*(10) within a 20% uncertainty. All other rem-counters underestimated the reference H*(10) by factors from 2 to more than 10, depending on the detector model and on the neutron dose per pulse. In pulsed fields, the neutron dose per pulse is a fundamental parameter, while the average neutron dose rate is a secondary quantity. An average 150-200 µSv/GyRBE neutron H*(10) at various positions around the phantom and at distances between 186 cm and 300 cm from it was measured per unit therapeutic dose delivered to the target. Significance Our results are partially in line with results obtained at similar Mevion facilities employing passive energy modulation. Comparisons with facilities employing active energy modulation confirmed that the neutron H*(10) can increase up to more than a factor of 10 when passive energy modulation is employed. The challenging environment of the Mevion stray neutron field requires the use of specific rem-counters sensitive to high-energy neutrons (up to a few hundred MeV) and specifically designed to withstand pulsed neutron fields.

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.

The Lens Opacities Classification System III Grading in Irradiated Uveal Melanomas to Characterize Proton Therapy-Induced Cataracts

PURPOSE

To evaluate the use of the Lens Opacities Classification System III grading (LOCS III) for the characterization of radiation-induced cataract, and to correlate the proton beam projection onto the lens with cataract location and grade as defined by the LOCS III.

DESIGN

Prospective, interventional case series.

METHODS

Fifty-two consecutive patients with cataract following proton therapy were included. All cataracts were graded using LOCS III. Relationships between proton beam and cataract subtypes, as well as between dose, proportion of lens irradiated, and extent of cataracts, were assessed.

RESULTS

Tumor diameter, volume, stage, and equatorial tumor location were associated with extent of posterior subcapsular cataracts (PSC) that were diagnosed at a median (interquartile range) 36 months (22;83) after treatment. In multivariate analysis, the tumor volume (P < .01) and an equatorial tumor location (P = .01) were risk factors for extensive PSC. Lens irradiation was avoided in 10 patients. In the remaining 42 patients (81%), the extent of PSC significantly correlated with the dose to the lens receiving 10, 26, and 47 Gy (P = .03, P = .03, and P = .04, respectively), the dose to the lens periphery receiving 10 and 26 Gy (P = .02 and P = .02, respectively), and the dose to the ciliary body receiving 10 and 26 Gy (P = .03 and P = .02, respectively). Nuclear color significantly correlated with the dose to the ciliary body receiving 10 Gy (P = .03) and 26 Gy (P = .02). After adjustment of the results on tumor volume and tumor location, the volume of lens receiving 10 Gy (P = .04) and 26 Gy (P = .03) remained significantly associated with the extent of PSC.

CONCLUSIONS

Proton dose correlated with the occurrence of PSC and nuclear color cataracts as defined by LOCS III grading. Better characterization of cataracts with the LOCS III after irradiation may help to further fill gaps in the current understanding of the mechanisms of radiation-induced cataracts.

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.

Occurrence of Phosphenes in Patients Undergoing Proton Beam Therapy for Ocular Tumor

PURPOSE

Phosphenes are frequently reported by patients irradiated in the head and neck area. The aim of the present study was to characterize and investigate potential mechanisms of proton beam therapy (PBT)-induced phosphenes in a large population of patients undergoing PBT for ocular tumors.

DESIGN

Prospective cohort study.

METHODS

Consecutive patients who underwent PBT in a single center were included. Immediately after the first session, all patients completed a questionnaire collecting information about the presence of phosphenes as well as their color, shape, and duration. Patient, tumor and treatment characteristics (dose volume histograms) were also collected.

RESULTS

Among the 474 patients included, 62.8% reported phosphenes during the first session of PBT. Reported colors were mainly blue-violet (70.5%) and white (14.1%). The prevalence of phosphenes was higher in younger patients (P = .003); other patient or ocular characteristics were not associated with the occurrence of phosphenes. Irradiation of the macula (P < .001) and/or optic disc (P < .001) were significantly associated with the presence of phosphenes, whereas blue-violet color was only associated with young age and irradiation of macular area (P = .04). Pupillary constriction was reported for 57.1% of patients with phosphenes vs 18.5% of patients without (P < .001). Blue-violet phosphenes (P < .001) and irradiation of macula (P = .001) were statistically associated with pupillary constriction.

CONCLUSIONS

The present study reported a high rate of phosphenes in patients irradiated by PBT for ocular tumor. Their blue-violet color and their association with a pupillary constriction probably indicates the stimulation of S-cones and retinal ganglion cells that reflects the activation of the afferent visual pathway.

Imaging of monochromatic beams by measuring secondary electron bremsstrahlung for carbon-ion therapy using a pinhole x-ray camera

A feasibility study on the imaging of monochromatic carbon-ion beams for carbon-ion therapy was performed. The evaluation was based on Monte Carlo simulations and beam-irradiation experiments, using a pinhole x-ray camera, which measured secondary electron bremsstrahlung (SEB). The simulation results indicated that the trajectories of the carbon-ion beams with injection energies of 278, 249 and 218 MeV/u in a water phantom, were clearly imaged by measuring the SEB with energies from 30 to 60 keV, using a pinhole camera. The Bragg-peak positions for these three injection energies were located at the positions where the ratios of the counts of SEB acquisitions to the maximum counts were approximately 0.23, 0.26 and 0.29, respectively. Moreover, we experimentally demonstrated that it was possible to identify the Bragg-peak positons, at the positions where the ratios coincided with the simulation results. However, the estimated Bragg-peak positions for the injection energies of 278 and 249 MeV/u were slightly deeper than the expected positions. In conclusion, for both the simulations and experiments, we found that the 25 mm shifts in the Bragg-peak positions can be observed by this method.

An indirect in vivo dosimetry system for ocular proton therapy

Secondary radiation, particularly neutron radiation, is a cause of concern in proton therapy. However, one can take advantage of its presence by using it to retrieve useful information on the primary proton beam. At the Centre Antoine Lacassagne the secondary radiation in the treatment room has been studied in function of the beam modulation. A strong correlation was found between the secondary ambient dose equivalent per proton dose H*(10)/D and proton dose rate D/MU. A large volume ionisation chamber fixed on the wall at 2.5 m from the nozzle was used with an in-house computer interface to retrieve the value of D/MU derived from the measurement of photon H*(10) integrated over treatment time, using the correlation curve. This system enables the verification of D and D/MU to be made independently of the monitoring of the primary beam and represents a first step towards an alternative in vivo dosimetry in proton therapy.