J.A. Terrón

A new online detector for estimation of peripheral neutron equivalent dose in organ.

PURPOSEnPeripheral dose in radiotherapy treatments represents a potential source of secondary neoplasic processes. As in the last few years, there has been a fast-growing concern on neutron collateral effects, this work focuses on this component. A previous established methodology to estimate peripheral neutron equivalent doses relied on passive (TLD, CR39) neutron detectors exposed in-phantom, in parallel to an active [static random access memory (SRAMnd)] thermal neutron detector exposed ex-phantom. A newly miniaturized, quick, and reliable active thermal neutron detector (TNRD, Thermal Neutron Rate Detector) was validated for both procedures. This first miniaturized active system eliminates the long postprocessing, required for passive detectors, giving thermal neutron fluences in real time.nnnMETHODSnTo validate TNRD for the established methodology, intrinsic characteristics, characterization of 4 facilities [to correlate monitor value (MU) with risk], and a cohort of 200 real patients (for second cancer risk estimates) were evaluated and compared with the well-established SRAMnd device. Finally, TNRD was compared to TLD pairs for 3 generic radiotherapy treatments through 16 strategic points inside an anthropomorphic phantom.nnnRESULTSnThe performed tests indicate similar linear dependence with dose for both detectors, TNRD and SRAMnd, while a slightly better reproducibility has been obtained for TNRD (1.7% vs 2.2%). Risk estimates when delivering 1000 MU are in good agreement between both detectors (mean deviation of TNRD measurements with respect to the ones of SRAMnd is 0.07 cases per 1000, with differences always smaller than 0.08 cases per 1000). As far as the in-phantom measurements are concerned, a mean deviation smaller than 1.7% was obtained.nnnCONCLUSIONSnThe results obtained indicate that direct evaluation of equivalent dose estimation in organs, both in phantom and patients, is perfectly feasible with this new detector. This will open the door to an easy implementation of specific peripheral neutron dose models for any type of treatment and facility.

Analytical model for photon peripheral dose estimation in radiotherapy treatments

This work aims to generate a simple analytical model that allows estimation of peripheral photon equivalent dose to organs of individual patients, valid for any isocentric technique. Photon radiation scattered in the LINAC head has been simulated as a virtual source of radiation emitting isotropically so that, before reaching a point inside the patient, it decreases with the square law and with attenuation due to air and tissue. Leakage has been simulated as a constant background dose along the patient. Firstly, a dose-to-points basic model was proposed and parameterized by fitting it to absorbed doses measured with TLD-700 in a humanoid phantom. Secondly, this model was generalized to any other situation involving intensity-modulated beams of any size and shape. Validation of this general model, usable beyond 10 cm from the field edge, was carried out by comparing estimation with TLD-100 doses for VMAT and IMRT treatments as well as with experimental data and models existing in the bibliography. Finally, an equivalent dose-to-organs model has been proposed by rescaling individual anatomical dimensions onto a mathematical phantom in order to make an estimation of organ length for dose calculation. The parameterized extended model, accounting for intensity-modulated beams of any shape, predicts measurements with a maximum relative uncertainty of ±25%. This general model, easy to apply in a clinical routine thanks to the ready availability of input parameters, has been proposed and validated for estimation of photon equivalent doses to peripheral organs. Finally, as a first step, it has been implemented into a piece of software termed PERIPHOCAL (PERIpheral PHOton CALculation), which is easily transferred to a commercial treatment planning system (TPS).

Improving the neutron-to-photon discrimination capability of detectors used for neutron dosimetry in high energy photon beam radiotherapy.

The increasing interest of the medical community to radioinduced second malignancies due to photoneutrons in patients undergoing high-energy radiotherapy, has stimulated in recent years the study of peripheral doses, including the development of some dedicated active detectors. Although these devices are designed to respond to neutrons only, their parasitic photon response is usually not identically zero and anisotropic. The impact of these facts on measurement accuracy can be important, especially in points close to the photon field-edge. A simple method to estimate the photon contribution to detector readings is to cover it with a thermal neutron absorber with reduced secondary photon emission, such as a borated rubber. This technique was applied to the TNRD (Thermal Neutron Rate Detector), recently validated for thermal neutron measurements in high-energy photon radiotherapy. The positive results, together with the accessibility of the method, encourage its application to other detectors and different clinical scenarios.

Neutron measurements in radiotherapy: A method to correct neutron sensitive devices for parasitic photon response

One of the major causes of secondary malignancies after radiotherapy treatments are peripheral doses, known to increase for some newer techniques (such as IMRT or VMAT). For accelerators operating above 10MV, neutrons can represent important contribution to peripheral doses. This neutron contamination can be measured using different passive or active techniques, available in the literature. As far as active (or direct-reading) procedures are concerned, a major issue is represented by their parasitic photon sensitivity, which can significantly affect the measurement when the point of test is located near to the field-edge. This work proposes a simple method to estimate the unwanted photon contribution to these neutrons. As a relevant case study, the use of a recently neutron sensor for in-phantom measurements in high-energy machines was considered. The method, called Dual Energy Photon Subtraction (DEPS), requires pairs of measurements performed for the same treatment, in low-energy (6MV) and high energy (e.g. 15MV) fields. It assumes that the peripheral photon dose (PPD) at a fixed point in a phantom, normalized to the unit photon dose at the isocenter, does not depend on the treatment energy. Measurements with ionization chamber and Monte Carlo simulations were used to evaluate the validity of this hypothesis. DEPS method was compared to already published correction methods, such as the use of neutron absorber materials. In addition to its simplicity, an advantage of DEPs procedure is that it can be applied to any radiotherapy machine.

Peripheral neutron dose estimation: comparison between experimental measurements and TPS estimation

A newly TPS algorithm, implemented in Pinnacle3, has been developed for peripheral neutron dose estimation for radiotherapy patients. The script gives doses in several organs according to gender, treatment location and delivered high energy monitor units. In order to validate these estimations, doses for a total of 119 patients were calculated with Pinnacle3 TPS script, and compared to the experimental measurements with the new Thermal Neutron Digital Detector (TNRD). The studied patients, cover a wide range of pathologies for three different linacs. The comparison shows that with the implemented script we obtain a good correlation between measurement and theoretical values. On the other hand, periodic neutron characterization of the facility should be considered for better estimations.

Effects of cable extension and photon irradiation on TNRD neutron detector in radiotherapy

A new thermal neutron detector (TNRD), developed for nuclear research, has shown to be effective for clinical use in peripheral neutron dose estimation, either in patient and ‘in-phantom’ measurements. This work shows some TNRD difficulties when adapting it to radiotherapy environments, mainly due to the fact that it has shown structural limitations. Two problems have been studied: (1) the influence of cable lengthening, necessary to be operative in a radiotherapy environment and (2) cable irradiation during the measurements. As we are measuring very small signals, we have to take into account not only these two facts but also the quality of the materials and connectors used. Thus, we studied cable elongation and irradiation influences in conventional and extreme situations once the setup was improved, in order to avoid uncertainties which could be of the order of the signal. Mean deviations of -0.15% from the original TNRD cable extension have been noticed. For the wide variety of conditions tested, in terms of both dose delivered and setup of the radiotherapy exposure, uncertainties smaller than 1.2% have been estimated.

Peripheral equivalent neutron dose model implementation for radiotherapy patients

PURPOSEnNeutron peripheral contamination in high-energy radiotherapy implies an increase of secondary radiation-induced cancer risk. Although peripheral neutron dose (PND) has been evaluated in organs, few studies have been performed regarding patient size. This work aims to improve an existing methodology for adult patient PND estimations to generalize it to young and children, for its implementation in treatment planning systems (TPS).nnnMETHODSnAs a first step, we aimed to generalize the previous model to be usable with any thermal neutron detector. Then, taking into account total neutron spectra and dose-to-point thermal neutron fluence measurements for three phantom sizes (adult, teen and child) and two common treatment locations (H&N and abdomen), the new model was proposed. It represents an upgraded parameterization and extension of the existing one, including patient anatomy. Finally, comparison between estimations and measurements, as well as validation against the original model, was carried out for 510 measured patients.nnnRESULTSnConcordance found between experimental and theoretical estimations makes us confident about later implementation in treatment planning systems. Comparison among the previous and upgraded models shows no significant differences for the adult case. However, an important underestimation (34.1% on average) can be observed regarding child case for the original one.nnnCONCLUSIONSnAn improved generalization of an existing PND model, considering patient anatomy has been validated and used in real patients. The final methodology is easily implementable in clinical routine and TPS thanks to the ready availability of input parameters (patient height and weight, high-energy MU and facility characterization).

SU-E-T-195: Commissioning the Neutron Production of a Varian TrueBeam Linac

Purpose: The purpose of this work is the characterization of a new Varian TrueBeam™ facility in terms of neutron production, in order to estimate neutron equivalent dose in organs during radiotherapy treatments. Methods: The existing methodology [1] was used with the reference SRAMnd detector, calibrated in terms of thermal neutron fluence at the reference field operated by PTB (Physikalisch-Technische-Bundesanstalt) at the GeNF (Geesthacht-Neutron-Facility) with the GKSS reactor FRG-1 [2]. Thermal neutron fluence for the 5 available possibilities was evaluated: 15 MV and 10&6 MV with and without Flattening Filter (FF and FFF, respectively). Irradiation conditions are as described in [3]. In addition, three different collimator-MLC configurations were studied for 15 MV: (a) collimator of 10×10 cm2 and MLC fully retracted (reference), (b) field sizes of 20×20 cm2 and 10×10 cm2 for collimator and MLC respectively, and (c) collimator and MLC aperture of 10×10 cm2. Results: Thermal fluence rate at the “reference point” [3], as a consequence of the neutron production, obtained for (a) conformation in 15 MV is (1.45±0.11) x10⁴ n•cm2/MU. Configurations (b) and (c) gave fluences of 96.6% and 97.8% of the reference (a). Neutron production decreases up to 8.6% and 5.7% for the 10 MV FF and FFF beams, respectively. Finally, it decreases up to 2.8% and 0.1% for the 6 MV FF and FFF modes, respectively. Conclusion: This work evaluates thermal neutron production of Varian TrueBeam™ system for organ equivalent dose estimation. The small difference in collimator-MLC configuration shows the universality of the methodology [3]. A decrease in this production is shown when decreasing energy from 15 to 10 MV and an almost negligible production was found for 6 MV. Moreover, a lower neutron contribution is observed for the FFF modes.[1]Phys Med Biol,2012;57:6167–6191.[2]Radiat Meas,2010;45:1513–1517.[3]Med Phys,2015;42:276–281.

SU‐E‐T‐365: Estimation of Neutron Ambient Dose Equivalents for Radioprotection Exposed Workers in Radiotherapy Facilities Based On Characterization Patient Risk Estimation

Purpose: Previous measurements with Bonner spheres1 showed that normalized neutron spectra are equal for the majority of the existing linacs 2. This information, in addition to thermal neutron fluences obtained in the characterization procedure33, would allow to estimate neutron doses accidentally received by exposed workers, without the need of an extra experimental measurement. Methods: Monte Carlo (MC) simulations demonstrated that the thermal neutron fluence distribution inside the bunker is quite uniform, as a consequence of multiple scatter in the walls⁴. Although inverse square law is approximately valid for the fast component, a more precise calculation could be obtained with a generic fast fluence distribution map around the linac, from MC simulations⁴. Thus, measurements of thermal neutron fluences performed during the characterization procedure3, together with a generic unitary spectra2, would allow to estimate the total neutron fluences and H*(10) at any point⁵. As an example, we compared estimations with Bonner sphere measurements1, for two points in five facilities: 3 Siemens (15–23 MV), Elekta (15 MV) and Varian (15 MV). Results: Thermal neutron fluences obtained from characterization, are within (0.2–1.6×10⁶) cm−2•Gy−1 for the five studied facilities. This implies ambient equivalent doses ranging from (0.27–2.01) mSv/Gy 50 cm far from the isocenter and (0.03–0.26) mSv/Gy at detector location with an average deviation of ±12.1% respect to Bonner measurements. Conclusion: The good results obtained demonstrate that neutron fluence and H*(10) can be estimated based on: (a) characterization procedure established for patient risk estimation in each facility, (b) generic unitary neutron spectrum and (c) generic MC map distribution of the fast component. [1] Radiat. Meas (2010) 45: 1391 – 1397; [2] Phys. Med. Biol (2012) 5 7:6167–6191; [3] Med. Phys (2015) 42:276 - 281. [4] IFMBE (2012) 39: 1245–1248. [5] ICRU Report 57 (1998)

SU-E-T-249: Neutron Model Upgrade for Radiotherapy Patients Monitoring Using a New Online Detector

PURPOSEnThe purpose of this work is to improve the existing methodology to estimate neutron equivalent dose in organs during radiotherapy treatments, based on a Static Random Access Memory neutron detector (SRAMnd) [1]. This is possible thanks to the introduction of a new digital detector with improved characteristics, which is able to measure online the neutron fluence rate in the presence of an intense photon background [2]. Its reduced size, allows the direct estimation of doses in specific points inside an anthropomorphic phantom (NORMA) without using passive detectors as TLD or CR-39. This versatility will allow not only to improve the existing models (generic abdomen and H&N [1]) but to generate more specific ones for any technique.nnnMETHODSnThe new Thermal Neutron Rate Detector (TNRD), based on a diode device sensitized to thermal neutrons, have been inserted in 16 points of the phantom. These points are distributed to infer doses to specific organs. Simultaneous measurements of these devices and a reference one, located in front of the gantry, have been performed for the mentioned generic treatments, in order to improve the existing model.nnnRESULTSnThese new devices have shown more precise since they agree better with Monte Carlo simulations. The comparison of the thermal neutron fluence, measured with TNRD, and the existing models, converted from events to fluence, shows an average improvement of (3.90±3.37) % for H&N and (12.61±9.43) % for abdomen, normalized to the maximum value.nnnCONCLUSIONnThis work indicates the potential of these new devices for more precise neutron equivalent dose estimation in organs, as a consequence of radiotherapy treatments. The simplicity of the process makes possible to establish more specific models that will provide a better dose estimation. References[1] Phys Med Biol 2012; 57:6167-6191.[2] A new active thermal neutron detector. Radiat. Prot. Dosim. (in press).