Out-of-field dose measurements in a water phantom using different radiotherapy modalities

Navigating Complexities: Leadless Pacemaker Management in Proton Therapy for a Pacemaker-Dependent Bilateral Breast Cancer Patient

This case study explores the strategic decision-making and safety considerations in managing a unique scenario where a pacemaker dependent patient, requiring adjuvant radiotherapy for bilateral breast cancer. The conventional pacemaker was located entirely within the treatment target, without the option for transposition because of the bilateral chest treatment, resulting in significant risk of malfunction caused by exposing it to the full prescribed dose. Consequently, the decision was made to replace the conventional pacemaker with a leadless device Micra implanted directly into the heart to mitigate direct device radiation and potential adverse effects of proton therapy on the cardiac device. Following Micra implantation, the patient underwent the proton treatment without complications or serious device malfunctions. This study explores solutions to address the challenges posed by within-the-field cardiac devices and highlights the use of pencil beam proton therapy for individuals with leadless cardiac devices while acknowledging the potential for neutron production and the associated risk of single-event upsets (SEU) in cardiac implantable electronic devices (CIEDs). The findings underscore the significance of strategic decision-making, risk assessment, and continuous monitoring for successful outcomes, particularly in the context of proton therapy for patients with advanced cardiac considerations.

Comparison of the accuracy of Monte Carlo and Ray Tracing dose calculation algorithms for multiple target brain treatments on CyberKnife

Single plan multiple brain targets (MBT) stereotactic radiosurgery dose difference between Monte Carlo (MC) and Ray Tracing (RT) algorithms has not been studied. A retrospective study and dose measurements were performed to access factors influencing dose differences. Fifty-three RT treatment plans with a total of 209 brain metastases were extracted from Precision Treatment Planning System (TPS). These plans were generated using fixed cones and were delivered using the CyberKnife M6 system. The same treatment plans were recalculated using MC algorithm and keeping the beam parameters unchanged. MC calculated plan parameters were extracted and dose differences were normalised to MC calculated dose. Correlations were investigated. RT and MC calculated off-centre-ratio (OCR) and tissue-phantom-ratio (TPRs) were exported from the TPS and compared with measured. Plans with 5 gross tumour volumes (GTVs) were created on a phantom and dose measured using a CC04 ionisation chamber and microdiamond detector for comparison with calculated doses. Calculated and measured TPR agreed within ± 1% beyond depth of maximum dose. The OCR showed differences up to 4.3% in the penumbra and out-of-field (OOF) regions. Largest RT and MC calculated GTV mean dose difference was - 5.7%. An increase in the number of GTVs and reduction in the geometric separation of metastases were associated with increased differences between RT and MC calculated doses. In conclusion, calculated dose disagreement in MBT depends on the number of GTVs per plan, number of GTVs within a certain separation distance and plan complexity. MC dose calculation is recommended for complex CyberKnife SRS of MBT.

Pacemaker Malfunction During Passive Proton Beam Therapy for Localized Prostate Cancer: Case Reports and a Literature Review

We report two cases of pacemaker malfunction occurring during proton beam therapy (PBT) for localized prostate cancer treatment. The first case involved mode changes in the pacemaker, while the second exhibited prolongation of the RR interval. Remarkably, both cases did not manifest significant clinical changes. Our findings indicate that careful consideration should be given to passive PBT in patients with localized prostate cancer who have pacemakers, like the considerations in patients with thoracic and abdominal cancers. Moreover, our report highlights the importance of recognizing potential cardiac implantable electronic devices malfunction in various PBT scenarios.

Analytical models for external photon beam radiotherapy out-of-field dose calculation: a scoping review

A growing body of scientific evidence indicates that exposure to low dose ionizing radiation (< 2 Gy) is associated with a higher risk of developing radio-induced cancer. Additionally, it has been shown to have significant impacts on both innate and adaptive immune responses. As a result, the evaluation of the low doses inevitably delivered outside the treatment fields (out-of-field dose) in photon radiotherapy is a topic that is regaining interest at a pivotal moment in radiotherapy. In this work, we proposed a scoping review in order to identify evidence of strengths and limitations of available analytical models for out-of-field dose calculation in external photon beam radiotherapy for the purpose of implementation in clinical routine. Papers published between 1988 and 2022 proposing a novel analytical model that estimated at least one component of the out-of-field dose for photon external radiotherapy were included. Models focusing on electrons, protons and Monte-Carlo methods were excluded. The methodological quality and potential limitations of each model were analyzed to assess their generalizability. Twenty-one published papers were selected for analysis, of which 14 proposed multi-compartment models, demonstrating that research efforts are directed towards an increasingly detailed description of the underlying physical phenomena. Our synthesis revealed great inhomogeneities in practices, in particular in the acquisition of experimental data and the standardization of measurements, in the choice of metrics used for the evaluation of model performance and even in the definition of regions considered out-of-the-field, which makes quantitative comparisons impossible. We therefore propose to clarify some key concepts. The analytical methods do not seem to be easily suitable for massive use in clinical routine, due to the inevitable cumbersome nature of their implementation. Currently, there is no consensus on a mathematical formalism that comprehensively describes the out-of-field dose in external photon radiotherapy, partly due to the complex interactions between a large number of influencing factors. Out-of-field dose calculation models based on neural networks could be promising tools to overcome these limitations and thus favor a transfer to the clinic, but the lack of sufficiently large and heterogeneous data sets is the main obstacle.

A simple analytical model for a fast 3D assessment of peripheral photon dose during coplanar isocentric photon radiotherapy

Considering that cancer survival rates have been growing and that nearly two-thirds of those survivors were exposed to clinical radiation during its treatment, the study of long-term radiation effects, especially secondary cancer induction, has become increasingly important. To correctly assess this risk, knowing the dose to out-of-field organs is essential. As it has been reported, commercial treatment planning systems do not accurately calculate the dose far away from the border of the field; analytical dose estimation models may help this purpose. In this work, the development and validation of a new three-dimensional (3D) analytical model to assess the photon peripheral dose during radiotherapy is presented. It needs only two treatment-specific input parameter values, plus information about the linac-specific leakage, when available. It is easy to use and generates 3D whole-body dose distributions and, particularly, the dose to out-of-field organs (as dose–volume histograms) outside the 5% isodose for any isocentric treatment using coplanar beams [including intensity modulated radiotherapy and volumetric modulated arc therapy (VMAT)]. The model was configured with the corresponding Monte Carlo simulation of the peripheral absorbed dose for a 6 MV abdomen treatment on the International Comission on Radiological Protection (ICRP) 110 computational phantom. It was then validated with experimental measurements using thermoluminescent dosimeters in the male ATOM anthropomorphic phantom irradiated with a VMAT treatment for prostate cancer. Additionally, its performance was challenged by applying it to a lung radiotherapy treatment very different from the one used for training. The model agreed well with measurements and simulated dose values. A graphical user interface was developed as a first step to making this work more approachable to a daily clinical application.

MONTE CARLO SIMULATIONS OF OUT-OF-FIELD LET SPECTRA IN WATER PHANTOM IRRADIATED BY SCANNING PENCIL PROTON BEAM

LET spectra can be measured by track-etched detectors. However, these detectors are not able to identify the type of interacting particles. Monte Carlo simulations can provide this missing information. In this work, Monte Carlo simulations based on the EURADOS Work Group 9 experiment consisting of systematic 3D mapping of out-of-field doses and LET spectra in a prototype water phantom were performed. The simulations aimed to identify the types of particles contributing to the out-of-field LET spectra. The total absorbed dose, LET and energy spectra were calculated. The calculated dose distributions and LET spectra were compared with the ones measured by radiophotoluminiscence and track-etched detectors. The out-of-field particles and their LET values were identified. No statistically significant differences between the measured and simulated spectra were revealed in the LET range of 100-2000 keV μm-1.

Evaluation of photoneutron dose equivalent in 10 MV and 15 MV beams for wedge and open fields in the Elekta Versa HD linac

In a high-energy medical linear accelerator (linac), if the interaction of photon energy is higher than the neutron binding energy of high atomic material, it emits a neutron field through a photonuclear reaction. The objective of this current study is to measure the photoneutron dose equivalent produces in a motorized wedge field and open field of 10 MV and 15 MV photon beams in Elekta Versa HD™ linac. The PNDE values were recorded at various positions along the patient plane using the Bubble Detector-Personal Neutron Dosimeter (BD-PND). The results revealed that the PNDE values are higher in 20 × 20 cm² than 10 × 10 cm² field sizes for both the 60° wedge and open fields of 10 MV and 15 MV beams. In addition, the 60° wedge fields generate higher photoneutron contamination when compared with the 45°, 30° wedge fields and open field sizes. Hence, on average PNDE values produced by the 15 MV beam were higher by a factor of 1.98 and 2.11 times for open and 60° wedge fields than the 10 MV beam, respectively.

Dose Estimation Using Optically Stimulated Luminescence Dosimeter and EBT3 Films for Various Treatment Techniques in Alderson Rando Phantom and Estimation of Secondary Cancer Incidence for Carcinoma of Left Breast

Aim

The aim of this study was to measure the dose to planning target and organ at risk (OAR) using Alderson Rando phantom for various treatment techniques in left breast radiotherapy and to estimate the secondary cancer incidence.

Materials and Methods

Eleven different combinations of plans containing four techniques (three dimensional conformal radiotherapy, intensity-modulated radiation therapy [IMRT], volumetric modulated arc therapy [VMAT], and combination of 3DCRT and VMAT plans (HYBRID)) were created with 6 MV FF and 6 MV FFF (flattening filter and flattening filter-free) photon energies in phantom. Planned target volume and OAR doses in 23 different locations were measured using optically stimulated luminescence dosimeter (OSLD) and EBT3 films. Assuming the age of exposure as 30 years, lifetime attributable risk (LAR) was estimated based on excess absolute risk (EAR) models outlined in the Biological Effects of Ionizing Radiation VII report.

Results

Film showed maximum deviations of 6.15% with IMRT_C_FF plan when compared with treatment planning system (TPS). The maximum percentage difference of 1.7% was found with OSLD measurement when compared with TPS for VMAT_T_FFF plan. EAR estimation was done for all the OARs including target. The LARs for left lung, right lung, and right breast were evaluated. The maximum LAR values of 2.92 ± 0.14 were found for left lung with VMAT_C_FFF plans.

Conclusion

This study shows that both OSLD and EBT3 films are suitable for dose measurements using Rando phantom. OSLD shows superior results when compared with films, especially with relatively larger distances. Maximum LAR values were found with VMAT_C_FFF plans. Considering the secondary cancer risk associated with the patients treated in the younger age group, it is suggested that in vivo dose estimation should be a part of treatment quality audit whenever possible.

Nontarget and Out-of-Field Doses from Electron Beam Radiotherapy

In clinical radiotherapy, the most important aspects are the dose distribution in the target volume and healthy organs, including out-of-field doses in the body. Compared to photon beam radiation, dose distribution in electron beam radiotherapy has received much less attention, mainly due to the limited range of electrons in tissues. However, given the growing use of electron intraoperative radiotherapy and FLASH, further study is needed. Therefore, in this study, we determined out-of-field doses from an electron beam in a phantom model using two dosimetric detectors (diode E and cylindrical Farmer-type ionizing chamber) for electron energies of 6 MeV, 9 MeV and 12 MeV. We found a clear decrease in out-of-field doses as the distance from the field edge and depth increased. The out-of-field doses measured with the diode E were lower than those measured with the Farmer-type ionization chamber at each depth and for each electron energy level. The out-of-field doses increased when higher energy megavoltage electron beams were used (except for 9 MeV). The out-of-field doses at shallow depths (1 or 2 cm) declined rapidly up to a distance of 3 cm from the field edge. This study provides valuable data on the deposition of radiation energy from electron beams outside the irradiation field.

Personalized 3D-printed anthropomorphic whole-body phantom irradiated by protons, photons, and neutrons

The objective of this study was to confirm the feasibility of three-dimensionally-printed (3D-printed), personalized whole-body anthropomorphic phantoms for radiation dose measurements in a variety of charged and uncharged particle radiation fields. We 3D-printed a personalized whole-body phantom of an adult female with a height of 154.8 cm, mass of 90.7 kg, and body mass index of 37.8 kg/m². The phantom comprised of a hollow plastic shell filled with water and included a watertight access conduit for positioning dosimeters. It is compatible with a wide variety of radiation dosimeters, including ionization chambers that are suitable for uncharged and charged particles. Its mass was 6.8 kg empty and 98 kg when filled with water. Watertightness and mechanical robustness were confirmed after multiple experiments and transportations between institutions. The phantom was irradiated to the cranium with therapeutic beams of 170-MeV protons, 6-MV photons, and fast neutrons. Radiation absorbed dose was measured from the cranium to the pelvis along the longitudinal central axis of the phantom. The dose measurements were made using established dosimetry protocols and well-characterized instruments. For the therapeutic environments considered in this study, stray radiation from intracranial treatment beams was the lowest for proton therapy, intermediate for photon therapy, and highest for neutron therapy. An illustrative example set of measurements at the location of the thyroid for a square field of 5.3 cm per side resulted in 0.09, 0.59, and 1.93 cGy/Gy from proton, photon, and neutron beams, respectively. In this study, we found that 3D-printed personalized phantoms are feasible, inherently reproducible, and well-suited for therapeutic radiation measurements. The measurement methodologies we developed enabled the direct comparison of radiation exposures from neutron, proton, and photon beam irradiations.

Spot-scanning proton therapy for targets with adjacent cardiac implantable electronic devices - Strategies for breast and head & neck cancer

BACKGROUND AND PURPOSE

Cardiac implantable electronic device (CIED) malfunctions can be induced by secondary neutron dose from spot-scanning proton therapy. A recent in-vitro study investigating secondary neutron dose to CIEDs up to 7 mSv per fraction found that exposure of secondary neutrons in this range was clinically manageable. This study presents decision algorithms proposed by a national expert group for selection of patients with breast and head & neck (H&N) cancer with CIEDs adjacent to target for proton therapy based on the 7 mSv threshold.

METHODS AND MATERIALS

Ten patients with breast cancer and five with H&N cancer were included in the study. Five patients with breast cancer received photon therapy with CIED and proton plans were retrospectively created. The remaining patients received proton therapy without CIED and a worst-case position of a virtual CIED was retrospectively delineated. Secondary neutron dose was estimated as ambient dose equivalent H*(10) using Monte Carlo simulations.

RESULTS

For patients with breast cancer and with contralateral CIED, the secondary neutron dose to the CIED was below 7 mSv per fraction for CTV < 1500 cm3 in 2 Gy fractions and CTV < 1000 cm3 in 2.67 Gy fractions. The secondary neutron dose to the CIED was below 7 mSv per fraction for all patients with H&N cancer.

CONCLUSIONS

Simulations of neutron exposure suggest that proton therapy is feasible for most patients with CIED adjacent to target. This forms the basis for decision algorithms for selection of patients with CIED for proton therapy.

Variation of 4 MV X-ray dose rate in fractionated irradiation strongly impacts biological endothelial cell response in vitro

PURPOSE

Even though X-ray beams are widely used in medical diagnosis or radiotherapy, the comparisons of their dose rates are scarce. We have recently demonstrated in vitro (clonogenic assay, cell viability, cell cycle, senescence) and in vivo (weight follow-up of animals and bordering epithelium staining of lesion), that for a single dose of irradiation, the relative biological effectiveness (RBE) deviates from 1 (up to twofold greater severe damage at the highest dose rate depending on the assay) when increasing the dose rate of high energy X-ray beams.

MATERIAL AND METHODS

To further investigate the impact of the dose rate on RBE, in this study, we performed in vitro fractionated irradiations by using the same two dose rates (0.63 and 2.5 Gy.min^-1) of high-energy X-rays (both at 4 MV) on normal endothelial cells (HUVECs). We investigated the viability/mortality, characterized radiation-induced senescence by using flow cytometry and measured gene analysis deregulations on custom arrays.

RESULTS

The overall results enlighten that, in fractionated irradiations when varying the dose rate of high-energy X-rays, the RBE of photons deviates from 1 (up to 2.86 for viability/mortality experiments performed 21 days postirradiation).

CONCLUSION

These results strengthen the interest of multiparametric analysis approaches in providing an accurate evaluation of the outcomes of irradiated cells in support of clonogenic assays, especially when such assays are not feasible.

Association between treatment-related lymphopenia and survival in glioblastoma patients following postoperative chemoradiotherapy

PURPOSE

Our study investigated the association between treatment-related lymphopenia and overall survival (OS) in a series of glioblastoma (GBM) patients. We also explored clinical and dosimetric predictors of lymphocytes depletion.

METHODS

Between 2015 and 2019, 64 patients were treated at the same institution with postoperative chemoradiotherapy. Peripheral lymphocyte count (PLC) data and dose-volume histogram parameters were collected. Radiotherapy (RT) schedule consisted in standard total dose of 60 Gy in 30 daily fractions, with concomitant and adjuvant temozolomide (TMZ). Posttreatment acute absolute lymphopenia (nadir AAL) was calculated as a PLC lower than 1.0 × 10³/mm³. Acute relative lymphopenia (ARL) was expressed by the nadir-PLC/baseline-PLC ratio < 0.5. Nadir-PLC was the lowest PLC registered between the end of RT and the first month of follow-up. Survival rates were estimated with Kaplan-Meier curves. Clinical and dosimetric variables related to AAL/ARL and OS were identified by univariate and multivariate analyses.

RESULTS

A total of 57 patients were eligible and included in the analyses. The median PLC was significantly decreased following chemoradiotherapy (2180/mm³ vs 900/mm³). Median OS was 16 months (range 5-55 months), with no significant difference between patients who developed nadir AAL and those who did not (16 months vs 16.5 months; p = 0.304). When considering ARL vs non-ARL, median OS was 14 months vs 26 months (p = 0.013), respectively. In multivariate Cox regression only age, sex, extent of surgery, access to adjuvant chemotherapy and brain D98% were independently associated with OS.

CONCLUSION

Although iatrogenic immunosuppression could be associated with inferior clinical outcomes, our data show that treatment-related lymphopenia does not adversely affect GBM survival. Prospective studies are required to confirm these findings.

Out-of-field organ doses and associated risk of cancer development following radiation therapy with photons

Innovations in cancer treatment have contributed to the improved survival rate of these patients. Radiotherapy is one of the main options for cancer management nowadays. High doses of ionizing radiation are usually delivered to the tumor site with high energy photon beams. However, the therapeutic radiation exposure may lead to second cancer induction. Moreover, the introduction of intensity-modulated radiation therapy over the last decades has increased the radiation dose to out-of-field organs compared to that from conventional irradiation. The increased organ doses might result in elevated probabilities for developing secondary malignancies to critical organs outside the treatment volume. The organ-specific dosimetry is considered necessary for the theoretical second cancer risk assessment and the proper analysis of data derived from epidemiological reports. This study reviews the methods employed for the measurement and calculation of out-of-field organ doses from exposure to photons and/or neutrons. The strengths and weaknesses of these dosimetric approaches are described in detail. This is followed by a review of the epidemiological data associated with out-of-field cancer risks. Previously published theoretical cancer risk estimates for adult and pediatric patients undergoing radiotherapy with conventional and advanced techniques are presented. The methodology for the theoretical prediction of the probability of carcinogenesis to out-of-field sites and the limitations of this approach are discussed. The article also focuses on the factors affecting the magnitude of the probability for developing radiotherapy-induced malignancies. The restriction of out-of-field doses and risks through the use of different types of shielding equipment is presented.

Neutron and photon out-of-field doses at cardiac implantable electronic device (CIED) depths

The accuracy of an out-of-field dose from an Elekta Synergy accelerator calculated using the X-ray Voxel Monte Carlo (XVMC) dose algorithm in the Monaco treatment planning system (TPS) for both low-energy (6 MV) and high-energy (15 MV) photons at cardiac implantable electronic device (CIED) depths was investigated through a comparison between MCNPX simulated out-of-field doses and measured out-of-field doses using three high spatial and sensitive active detectors. In addition, total neutron equivalent dose and fluence at CIED depths of a 15-MV dose from an Elekta Synergy accelerator were calculated, and the corresponding CIED relative neutron damage was quantified. The results showed that for 6-MV photons, the XVMC dose algorithm in Monaco underestimated out-of-field doses in all off-axis distances (average errors: -17% at distances X < 10 cm from the field edge and -31% at distances between 10 < X ≤ 16 cm from the field edge), with an increasing magnitude of underestimation for high-energy (15 MV) photons (up to 11%). According to the results, an out-of-field photon dose at a shallower CIED depth of 1 cm was associated with greater statistical uncertainty in the dose estimate compared to a CIED depth of 2 cm and clinical depth of 10 cm. Our results showed that the relative neutron damage at a CIED depth range for 15 MV photon is 36% less than that reported for 18 MV photon in the literature.

Out-of-field dose assessment for a 1.5 T MR-Linac with optically stimulated luminescence dosimeters

PURPOSE

To assess the out-of-field surface and internal dose of the 1.5 T MR-Linac compared to the conventional external beam linac using optically stimulated luminescence dosimeters (OSLDs), and evaluate the out-of-field dose calculation accuracy of the Monaco treatment planning system (TPS) of the 1.5T MR-Linac.

METHODS

A cubic solid water phantom, with OSLDs on the surface, was vertically irradiated by MR-Linac square fields with different sizes. In addition, OSLDs were arranged out of the beam edges in four directions. An anthropomorphic adult phantom, with 125 cm³ simulated volume, was irradiated in four orthogonal directions by both MR-Linac and conventional linac at the head, thoracic, and pelvic sites. Out-of-field doses were measured by OSLDs on both the surface and internal emulational organs at risk (OARs). The results were compared to the simulated dose from Monaco TPS.

RESULTS

At different field sizes (5 × 5 to 20 × 20 cm² ) and distances (1 to 10 cm) to beam edge, the out-of-field surface dose measured on MR-Linac varied from 0.16 % (10 cm to 5 × 5 cm² edge) to 7.02 % (1 cm to 20 × 20 cm² edge) of the maximum dose laterally and from 0.14 % (10 cm to 5 × 5 cm² edge) to 8.56 % (1 cm to 20 × 20 cm² edge) of the maximum dose longitudinally. Compared to the OSLDs measured data, the Monaco TPS presented an overestimate of the out-of-field dose of OARs at 0-2 % isodose area on both surface and internal check points, and the overestimation gets greater as the distance increases. The underestimation was found to be 0-35% at 2-5% isodose area on both surface and internal check points. Compared to the conventional linac, MR-Linac delivered higher average values of out-of-field dose on surface check points (20%, 19%, 21%) and internal simulated OARs (42%, 37%, 9%) of the anthropomorphic phantom at head, thoracic, and pelvic irradiations, respectively.

CONCLUSIONS

Compared to the conventional linac, MR-Linac has the same out-of-field dose distribution. However, considering the absolute dose values, MR-Linac delivered relatively higher out-of-field doses on both surface and internal OARs. Additional radiation shielding to patients undergoing MR-Linac may provide protection from out-of-field exposure.

Measurement of the photon and thermal neutron doses of contralateral breast surface in breast cancer radiotherapy

Introduction and purpose:

During the radiation therapy of tumoral breast, the contralateral breast (CB) will receive scattered doses. In the present study, the photon and thermal neutron dose values received by CB surface during breast cancer radiation therapy were measured.

Materials and methods:

The right breast region of RANDO phantom was considered as CB, and the measurements of photon and thermal neutron dose values were carried out on this region surface. The phantom was irradiated with 18 MV photon beams, and the dose values were measured with thermoluminescent dosimeter (TLD-600 and TLD-700) chips for 11 × 13, 11 × 17 and 11 × 21 cm2 field sizes in the presence of physical and dynamic wedges.

Results:

The total dose values (photon + thermal neutron) received by the CB surface in the presence of physical wedge were 12·06%, 15·75% and 33·40% of the prescribed dose, respectively, for 11 × 13, 11 × 17 and 11 × 21 cm2 field sizes. The corresponding dose values for dynamic wedge were 9·18%, 12·92% and 29·26% of the prescribed dose, respectively. Moreover, the results showed that treatment field size and wedge type affect the received photon and thermal neutron doses at CB surface.

Conclusion:

According to our results, the total dose values received at CB surface during breast cancer radiotherapy with high-energy photon beams are remarkable. In addition, the dose values received at CB surface when using a physical wedge were greater than when using a dynamic wedge, especially for medial tangential fields.

PERIPHERAL SURFACE DOSE FROM A LINEAR ACCELERATOR: RADIOCHROMIC FILM EXPERIMENTAL MEASUREMENTS OF FLATTENING FILTER FREE VERSUS FLATTENED BEAMS

There is a growing interest in the use of flattening filter free (FFF) beams due to the shorter treatment times. The reduction of head scatter suggests a better radiation protection to radiotherapy patients, considering the expected decrease in peripheral surface dose (PSD). In this work, PSD of flattened (FF) and FFF-photon beams was compared. A radiochromic film calibration method to reduce energy dependence was used. PSD was measured at distances from 2 to 50 cm to the field border for different square field sizes, modifying relevant clinical parameters. Also, clinical breast and prostate stereotactic body radiotherapy (SBRT) plans were studied. For square beams, FFF PSD is lower compared with FF PSD (differences ranging from 3 to 64%) and 10 MV FFF yields to the lowest value, for distances greater than 5 cm. For SBRT plans, near and far away from the field border, there is a reduction of PSD for FFF-beams, but the behavior at intermediate distances should be checked depending on the case.

Variation of 4 MV X-ray dose rate strongly impacts biological response both in vitro and in vivo

Whereas an RBE > 1 is described for very low-energy X-ray beams (in the range of 25–50 kV), there is a consensus that the RBE of X-rays (from 0.1 to 3 MeV) is equal to 1, whatever the energy or dose rate of the beam. Comparisons of X-ray beam dose rates are scarce even though these beams are widely used in medical diagnosis or radiotherapy. By using two dose rates (0.63 and 2.5 Gy.min⁻¹) of high-energy X-rays on normal endothelial cells (HUVECs), we have studied the clonogenic assay, but also viability/mortality, cell cycle analysis and measured cellular senescence by flow cytometry, and have performed gene analysis on custom arrays. In order to consolidate these data, we performed localized irradiation of exteriorized small intestine at 0.63 and 2.5 Gy.min⁻¹. Interestingly, in vivo validation has shown a significantly higher loss of weight at the higher dose when irradiating to 19 Gy a small fragment of exteriorized small intestine of C57Bl6J mice. Nevertheless, no significant differences were observed in lesioned scores between the two dose rates, while bordering epithelium staining indicated twofold greater severe damage at 2.5 Gy.min⁻¹ compared to 0.63 Gy.min⁻¹ at one week post-irradiation. Taken together, these experiments systematically show that the relative biological effectiveness of photons is different from 1 when varying the dose rate of high-energy X-rays. Moreover, these results strongly suggest that, in support of clonogenic assay, multiparametric analysis should be considered to provide an accurate evaluation of the outcome of irradiated cells.

Management of radiotherapy patients with implanted cardiac pacemakers and defibrillators: A Report of the AAPM TG-203†

Managing radiotherapy patients with implanted cardiac devices (implantable cardiac pacemakers and implantable cardioverter-defibrillators) has been a great practical and procedural challenge in radiation oncology practice. Since the publication of the AAPM TG-34 in 1994, large bodies of literature and case reports have been published about different kinds of radiation effects on modern technology implantable cardiac devices and patient management before, during, and after radiotherapy. This task group report provides the framework that analyzes the potential failure modes of these devices and lays out the methodology for patient management in a comprehensive and concise way, in every step of the entire radiotherapy process.

An objective method to evaluate radiation dose distributions varying by three orders of magnitude

PURPOSE

Modern radiotherapy practices typically report the absorbed dose (D) within the 5% relative isodose volume (i.e., the therapeutic dose region) to an accuracy of 3%-5%. Gamma-index analysis, the most commonly used method to evaluate dosimetric accuracy, has low sensitivity to discrepancies that occur outside of this region. The objective of this study was to develop an evaluation method with high sensitivity across dose distributions spanning three orders of magnitude.

METHODS

We generalized the gamma index to include an additional criterion, the absolute absorbed dose difference, specifically for the low-dose region (i.e., D ≤ 5%). We also proposed a method to objectively select the appropriate magnitudes for relative-dose-difference, absolute-dose-difference, and distance-to-agreement criteria. We demonstrated the generalized gamma-index method by first finding the appropriate generalized gamma-index agreement criteria at an interval of specified passing rates. Next, we used the generalized gamma index to evaluate one-, two-, and three-dimensional absorbed dose distributions in a water-box phantom and voxelized patient geometry.

RESULTS

Generalized gamma-index passing rates for one-, two-, and three-dimensional dose distributions were 55.4%, 44.5%, and 8.9%, respectively. Traditional gamma-index passing rates were 100%, 97.8%, and 96.4%, respectively. These results reveal that the generalized method has adequate sensitivity in all regions (i.e., therapeutic and low dose). Additionally, the algorithmic determination of triplets of agreement criteria revealed that they are strong functions of the specified passing rate.

CONCLUSIONS

The major finding of this work is that the proposed method provides an objective evaluation of the agreement of dose distributions spanning three orders of magnitude. In particular, this generalized method correctly characterized dosimetric agreement in the low-dose region, which was not possible by traditional methods. The proposed algorithmic selection of agreement criteria decreased subjectivity and requirements of user judgment and skill. This method could find utility in a variety of applications including dose-algorithm development and translation.

Report of a National Cancer Institute special panel: Characterization of the physical parameters of particle beams for biological research

PURPOSE

To define the physical parameters needed to characterize a particle beam in order to allow intercomparison of different experiments performed using different ions at the same facility and using the same ion at different facilities.

METHODS

At the request of the National Cancer Institute (NCI), a special panel was convened to review the current status of the field and to provide suggested metrics for reporting the physical parameters of particle beams to be used for biological research. A set of physical parameters and measurements that should be performed by facilities and understood and reported by researchers supported by NCI to perform pre-clinical radiobiology and medical physics of heavy ions were generated.

RESULTS

Standard measures such as radiation delivery technique, beam modifiers used, nominal energy, field size, physical dose and dose rate should all be reported. However, more advanced physical measurements, including detailed characterization of beam quality by microdosimetric spectrum and fragmentation spectra, should also be established and reported. Details regarding how such data should be incorporated into Monte Carlo simulations and the proper reporting of simulation details are also discussed.

CONCLUSIONS

In order to allow for a clear relation of physical parameters to biological effects, facilities and researchers should establish and report detailed physical characteristics of the irradiation beams utilized including both standard and advanced measures. Biological researchers are encouraged to actively engage facility staff and physicists in the design and conduct of experiments. Modeling individual experimental setups will allow for the reporting of the uncertainties in the measurement or calculation of physical parameters which should be routinely reported.

A novel pencil beam model for carbon-ion dose calculation derived from Monte Carlo simulations

An accurate kernel model is of vital importance for pencil-beam dose algorithm in charged particle therapy using precise spot-scanning beam delivery, in which an accurate depiction of the low dose envelope is especially crucial. Based on the Monte Carlo method, we investigated the dose contribution of secondary particles to the total dose and proposed a novel beam model to depict the lateral dose distribution of carbon-ion pencil beam in water. We demonstrated that the low dose envelope in single-spot profiles in water could be adequately modelled with the addition of a logistic distribution to a double Gaussian one, which was verified in both single carbon-ion pencil beam and superposed fields of different sizes with multiple pencil beams. Its superiority was mainly manifested at medium depths especially for high-energy beams with small fields compared with single, double and triple Gaussian models, where the secondary particles influenced the total dose considerably. The double Gaussian-logistic model could reduce the deviations from 4.1%, 1.7% to 0.3% in the plateau and peak regions, and from 19.2%, 4.9% to 1.2% in the tail region compared for the field size factor (FSF) calculations of 344 MeV/u carbon-ion pencil beam with the single and double Gaussian models. Compared with the triple Gaussian one, our newly-proposed model was on a par with it, even better than it in the plateau and peak regions. Thus our work will be helpful for improving the dose calculation accuracy for carbon-ion therapy.

COMPARISON OF RESPONSE OF PASSIVE DOSIMETRY SYSTEMS IN SCANNING PROTON RADIOTHERAPY-A STUDY USING PAEDIATRIC ANTHROPOMORPHIC PHANTOMS

Proton beam therapy has advantages in comparison to conventional photon radiotherapy due to the physical properties of proton beams (e.g. sharp distal fall off, adjustable range and modulation). In proton therapy, there is the possibility of sparing healthy tissue close to the target volume. This is especially important when tumours are located next to critical organs and while treating cancer in paediatric patients. On the other hand, the interactions of protons with matter result in the production of secondary radiation, mostly neutrons and gamma radiation, which deposit their energy at a distance from the target. The aim of this study was to compare the response of different passive dosimetry systems in mixed radiation field induced by proton pencil beam inside anthropomorphic phantoms representing 5 and 10 years old children. Doses were measured in different organs with thermoluminescent (MTS-7, MTS-6 and MCP-N), radiophotoluminescent (GD-352 M and GD-302M), bubble and poly-allyl-diglycol carbonate (PADC) track detectors. Results show that RPL detectors are the less sensitive for neutrons than LiF TLDs and can be applied for in-phantom dosimetry of gamma component. Neutron doses determined using track detectors, bubble detectors and pairs of MTS-7/MTS-6 are consistent within the uncertainty range. This is the first study dealing with measurements on child anthropomorphic phantoms irradiated by a pencil scanning beam technique.

Impact of irradiation setup in proton spot scanning brain therapy on organ doses from secondary radiation

A Monte Carlo model of a proton spot scanning pencil beam was used to simulate organ doses from secondary radiation produced from brain tumour treatments delivered with either a lateral field or a vertex field to one adult and one paediatric patient. Absorbed doses from secondary neutrons, photons and protons and neutron equivalent doses were higher for the vertex field in both patients, but the differences were low in absolute terms. Absorbed doses ranged between 0.1 and 43 μGy.Gy-1 in both patients with the paediatric patient receiving higher doses. The neutron equivalent doses to the organs ranged between 0.5 and 141 μSv.Gy-1 for the paediatric patient and between 0.2 and 134 μSv.Gy-1 for the adult. The highest neutron equivalent dose from the entire treatment was 7 mSv regardless of field setup and patient size. The results indicate that different field setups do not introduce large absolute variations in out-of-field doses produced in patients undergoing proton pencil beam scanning of centrally located brain tumours.

Proton therapy for hepatocellular carcinoma: Current knowledges and future perspectives

Hepatocellular carcinoma (HCC) is the second leading cause of cancer-related death, as few patients can be treated with currently available curative local modalities. In patients with HCC where curative modalities are not feasible, radiation therapy (RT) has emerged as an alternative or combination therapy. With the development of various technologies, RT has been increasingly used for the management of HCC. Among these advances, proton beam therapy (PBT) has several unique physical properties that give it a finite range in a distal direction, and thus no exit dose along the beam path. Therefore, PBT has dosimetric advantages compared with X-ray therapy for the treatment of HCC. Indeed, various reports in the literature have described the favorable clinical outcomes and improved safety of PBT for HCC patients compared with X-ray therapy. However, there are some technical issues regarding the use of PBT in HCC, including uncertainty of organ motion and inaccuracy during calculation of tissue density and beam range, all of which may reduce the robustness of a PBT treatment plan. In this review, we discuss the physical properties, current clinical data, technical issues, and future perspectives on PBT for the treatment of HCC.

Dose distribution of secondary radiation in a water phantom for a proton pencil beam-EURADOS WG9 intercomparison exercise

Systematic 3D mapping of out-of-field doses induced by a therapeutic proton pencil scanning beam in a 300  ×  300  ×  600 mm³ water phantom was performed using a set of thermoluminescence detectors (TLDs): MTS-7 (⁷LiF:Mg,Ti), MTS-6 (⁶LiF:Mg,Ti), MTS-N (^natLiF:Mg,Ti) and TLD-700 (⁷LiF:Mg,Ti), radiophotoluminescent (RPL) detectors GD-352M and GD-302M, and polyallyldiglycol carbonate (PADC)-based (C₁₂H₁₈O₇) track-etched detectors. Neutron and gamma-ray doses, as well as linear energy transfer distributions, were experimentally determined at 200 points within the phantom. In parallel, the Geant4 Monte Carlo code was applied to calculate neutron and gamma radiation spectra at the position of each detector. For the cubic proton target volume of 100  ×  100  ×  100 mm³ (spread out Bragg peak with a modulation of 100 mm) the scattered photon doses along the main axis of the phantom perpendicular to the primary beam were approximately 0.5 mGy Gy^-1 at a distance of 100 mm and 0.02 mGy Gy^-1 at 300 mm from the center of the target. For the neutrons, the corresponding values of dose equivalent were found to be ~0.7 and ~0.06 mSv Gy^-1, respectively. The measured neutron doses were comparable with the out-of-field neutron doses from a similar experiment with 20 MV x-rays, whereas photon doses for the scanning proton beam were up to three orders of magnitude lower.

Assessment of leakage dose in vivo in patients undergoing radiotherapy for breast cancer

Background and purpose

Accurate quantification of the relatively small radiation doses delivered to untargeted regions during breast irradiation in patients with breast cancer is of increasing clinical interest for the purpose of estimating long-term radiation-related risks. Out-of-field dose calculations from commercial planning systems however may be inaccurate which can impact estimates for long-term risks associated with treatment. This work compares calculated and measured dose out-of-field and explores the application of a correction for leakage radiation.

Materials and methods

Dose calculations of a Boltzmann transport equation solver, pencil beam-type, and superposition-type algorithms from a commercial treatment planning system (TPS) were compared with in vivo thermoluminescent dosimetry (TLD) measurements conducted out-of-field on the contralateral chest at points corresponding to the thyroid, axilla and contralateral breast of eleven patients undergoing tangential beam radiotherapy for breast cancer.

Results

Overall, the TPS was found to under-estimate doses at points distal to the radiation field edge with a modern linear Boltzmann transport equation solver providing the best estimates. Application of an additive correction for leakage (0.04% of central axis dose) improved correlation between the measured and calculated doses at points greater than 15 cm from the field edge.

Conclusions

Application of a correction for leakage doses within peripheral regions is feasible and could improve accuracy of TPS in estimating out-of-field doses in breast radiotherapy.

MEASUREMENT OF THE CONTRALATERAL BREAST PHOTON AND THERMAL NEUTRON DOSES IN BREAST CANCER RADIOTHERAPY: A COMPARISON BETWEEN PHYSICAL AND DYNAMIC WEDGES

This research aimed to measure the received photon and thermal neutron doses to contralateral breast (CB) in breast cancer radiotherapy for various field sizes in presence of physical and dynamic wedges. The measurement of photon and thermal neutron doses was carried out on right breast region of RANDO phantom (as CB) for 18 MV photon beams. The dose measurements were performed by thermoluminescent dosimeter chips. These measurements obtained for various field sizes in presence of physical and dynamic wedges. The findings of this study showed that the received doses (both of the photon and thermal neutron) to CB in presence of physical wedge for 11 × 13, 11 × 17 and 11 × 21 cm2 field sizes were 5.92, 6.36 and 6.77% of the prescribed dose, respectively as well as for dynamic wedge were 2.92, 4.63 and 5.60% of the prescribed dose, respectively. The results showed that the received photon and thermal neutron doses to CB increase with increment of field sizes. The received photon and thermal neutron doses to CB in presence of physical wedge were more than dynamic wedge. According to obtained findings, it is suggested that using a dynamic wedge is preferable than physical wedge, especially for medial tangential field.

Measurement and modeling of out-of-field doses from various advanced post-mastectomy radiotherapy techniques

More and more advanced radiotherapy techniques have been adopted for post-mastectomy radiotherapies (PMRT). Patient dose reconstruction is challenging for these advanced techniques because they increase the low out-of-field dose area while the accuracy of out-of-field dose calculations by current commercial treatment planning systems (TPSs) is poor. We aim to measure and model the out-of-field radiation doses from various advanced PMRT techniques. PMRT treatment plans for an anthropomorphic phantom were generated, including volumetric modulated arc therapy with standard and flattening-filter-free photon beams, mixed beam therapy, 4-field intensity modulated radiation therapy (IMRT), and tomotherapy. We measured doses in the phantom where the TPS calculated doses were lower than 5% of the prescription dose using thermoluminescent dosimeters (TLD). The TLD measurements were corrected by two additional energy correction factors, namely out-of-beam out-of-field (OBOF) correction factor K _OBOF and in-beam out-of-field (IBOF) correction factor K _IBOF, which were determined by separate measurements using an ion chamber and TLD. A simple analytical model was developed to predict out-of-field dose as a function of distance from the field edge for each PMRT technique. The root mean square discrepancies between measured and calculated out-of-field doses were within 0.66 cGy Gy^-1 for all techniques. The IBOF doses were highly scattered and should be evaluated case by case. One can easily combine the measured out-of-field dose here with the in-field dose calculated by the local TPS to reconstruct organ doses for a specific PMRT patient if the same treatment apparatus and technique were used.

A descriptive and broadly applicable model of therapeutic and stray absorbed dose from 6 to 25 MV photon beams

PURPOSE

To develop a simple model of therapeutic and stray absorbed dose for a variety of treatment machines and techniques without relying on proprietary machine-specific parameters.

METHODS

Dosimetry measurements conducted in this study and from the literature were used to develop an analytical model of absorbed dose from a variety of treatment machines and techniques in the 6 to 25 MV interval. A modified one-dimensional gamma-index analysis was performed to evaluate dosimetric accuracy of the model on an independent dataset consisting of measured dose profiles from seven treatment units spanning four manufacturers.

RESULTS

The average difference between the calculated and measured absorbed dose values was 9.9% for those datasets on which the model was trained. Additionally, these results indicate that the model can provide accurate calculations of both therapeutic and stray radiation dose from a wide variety of radiotherapy units and techniques.

CONCLUSIONS

We have developed a simple analytical model of absorbed dose from external beam radiotherapy treatments in the 6 to 25 MV beam energy range. The model has been tested on measured data from multiple treatment machines and techniques, and is broadly applicable to contemporary external beam radiation therapy.

Measuring dose from radiotherapy treatments in the vicinity of a cardiac pacemaker

This study investigated the dose absorbed by tissues surrounding artificial cardiac pacemakers during external beam radiotherapy procedures. The usefulness of out-of-field reference data, treatment planning systems, and skin dose measurements to estimate the dose in the vicinity of a pacemaker was also examined. Measurements were performed by installing a pacemaker onto an anthropomorphic phantom, and using radiochromic film and optically stimulated luminescence dosimeters to measure the dose in the vicinity of the device during the delivery of square fields and clinical treatment plans. It was found that the dose delivered in the vicinity of the cardiac device was unevenly distributed both laterally and anteroposteriorly. As the device was moved distally from the square field, the dose dropped exponentially, in line with out-of-field reference data in the literature. Treatment planning systems were found to substantially underestimate the dose for volumetric modulated arc therapy, helical tomotherapy, and 3D conformal treatments. The skin dose was observed to be either greater or lesser than the dose received at the depth of the device, depending on the treatment site, and so care should be if skin dose measurements are to be used to estimate the dose to a pacemaker. Square field reference data may be used as an upper estimate of absorbed dose per monitor unit in the vicinity of a cardiac device for complex treatments involving multiple gantry angles.

Comparative Risk Predictions of Second Cancers After Carbon-Ion Therapy Versus Proton Therapy

PURPOSE

This work proposes a theoretical framework that enables comparative risk predictions for second cancer incidence after particle beam therapy for different ion species for individual patients, accounting for differences in relative biological effectiveness (RBE) for the competing processes of tumor initiation and cell inactivation. Our working hypothesis was that use of carbon-ion therapy instead of proton therapy would show a difference in the predicted risk of second cancer incidence in the breast for a sample of Hodgkin lymphoma (HL) patients.

METHODS AND MATERIALS

We generated biologic treatment plans and calculated relative predicted risks of second cancer in the breast by using two proposed methods: a full model derived from the linear quadratic model and a simpler linear-no-threshold model.

RESULTS

For our reference calculation, we found the predicted risk of breast cancer incidence for carbon-ion plans-to-proton plan ratio, <Rc/Rp>, to be 0.75 ± 0.07 but not significantly smaller than 1 (P=.180).

CONCLUSIONS

Our findings suggest that second cancer risks are, on average, comparable between proton therapy and carbon-ion therapy.

PHITS simulations of absorbed dose out-of-field and neutron energy spectra for ELEKTA SL25 medical linear accelerator

Monte Carlo (MC) based calculation methods for modeling photon and particle transport, have several potential applications in radiotherapy. An essential requirement for successful radiation therapy is that the discrepancies between dose distributions calculated at the treatment planning stage and those delivered to the patient are minimized. It is also essential to minimize the dose to radiosensitive and critical organs. With MC technique, the dose distributions from both the primary and scattered photons can be calculated. The out-of-field radiation doses are of particular concern when high energy photons are used, since then neutrons are produced both in the accelerator head and inside the patients. Using MC technique, the created photons and particles can be followed and the transport and energy deposition in all the tissues of the patient can be estimated. This is of great importance during pediatric treatments when minimizing the risk for normal healthy tissue, e.g. secondary cancer. The purpose of this work was to evaluate 3D general purpose PHITS MC code efficiency as an alternative approach for photon beam specification. In this study, we developed a model of an ELEKTA SL25 accelerator and used the transport code PHITS for calculating the total absorbed dose and the neutron energy spectra infield and outside the treatment field. This model was validated against measurements performed with bubble detector spectrometers and Boner sphere for 18 MV linacs, including both photons and neutrons. The average absolute difference between the calculated and measured absorbed dose for the out-of-field region was around 11%. Taking into account a simplification for simulated geometry, which does not include any potential scattering materials around, the obtained result is very satisfactorily. A good agreement between the simulated and measured neutron energy spectra was observed while comparing to data found in the literature.

A simple and fast physics-based analytical method to calculate therapeutic and stray doses from external beam, megavoltage x-ray therapy

State-of-the-art radiotherapy treatment planning systems provide reliable estimates of the therapeutic radiation but are known to underestimate or neglect the stray radiation exposures. Most commonly, stray radiation exposures are reconstructed using empirical formulas or lookup tables. The purpose of this study was to develop the basic physics of a model capable of calculating the total absorbed dose both inside and outside of the therapeutic radiation beam for external beam photon therapy. The model was developed using measurements of total absorbed dose in a water-box phantom from a 6 MV medical linear accelerator to calculate dose profiles in both the in-plane and cross-plane direction for a variety of square field sizes and depths in water. The water-box phantom facilitated development of the basic physical aspects of the model. RMS discrepancies between measured and calculated total absorbed dose values in water were less than 9.3% for all fields studied. Computation times for 10 million dose points within a homogeneous phantom were approximately 4 min. These results suggest that the basic physics of the model are sufficiently simple, fast, and accurate to serve as a foundation for a variety of clinical and research applications, some of which may require that the model be extended or simplified based on the needs of the user. A potentially important advantage of a physics-based approach is that the model is more readily adaptable to a wide variety of treatment units and treatment techniques than with empirical models.

Configuration and validation of an analytical model predicting secondary neutron radiation in proton therapy using Monte Carlo simulations and experimental measurements

PURPOSE

This study focuses on the configuration and validation of an analytical model predicting leakage neutron doses in proton therapy.

METHODS

Using Monte Carlo (MC) calculations, a facility-specific analytical model was built to reproduce out-of-field neutron doses while separately accounting for the contribution of intra-nuclear cascade, evaporation, epithermal and thermal neutrons. This model was first trained to reproduce in-water neutron absorbed doses and in-air neutron ambient dose equivalents, H*(10), calculated using MCNPX. Its capacity in predicting out-of-field doses at any position not involved in the training phase was also checked. The model was next expanded to enable a full 3D mapping of H*(10) inside the treatment room, tested in a clinically relevant configuration and finally consolidated with experimental measurements.

RESULTS

Following the literature approach, the work first proved that it is possible to build a facility-specific analytical model that efficiently reproduces in-water neutron doses and in-air H*(10) values with a maximum difference less than 25%. In addition, the analytical model succeeded in predicting out-of-field neutron doses in the lateral and vertical direction. Testing the analytical model in clinical configurations proved the need to separate the contribution of internal and external neutrons. The impact of modulation width on stray neutrons was found to be easily adjustable while beam collimation remains a challenging issue. Finally, the model performance agreed with experimental measurements with satisfactory results considering measurement and simulation uncertainties.

CONCLUSION

Analytical models represent a promising solution that substitutes for time-consuming MC calculations when assessing doses to healthy organs.

Dose distribution outside the target volume for 170-MeV proton beam

Dose delivered outside the proton field during radiotherapy can potentially lead to secondary cancer development. Measurements with a 170-MeV proton beam were performed with passive detectors (track etched detectors and thermoluminescence dosemeters) in three different depths along the Bragg curve. The measurement showed an uneven decrease of the dose outside of the beam field with local enhancements. The major contribution to the delivered dose is due to high-energy protons with linear energy transfer (LET) up to 10 keV µm(-1). However, both measurement and preliminary Monte Carlo calculation also confirmed the presence of particles with higher LET.

Radiation burden from secondary doses to patients undergoing radiation therapy with photons and light ions and radiation doses from imaging modalities

Ionising radiation is increasingly used for the treatment of cancer, being the source of a considerable fraction of the medical irradiation to patients. With the increasing success rate of cancer treatments and longer life expectancy of the treated patients, the issue of secondary cancer incidence is of growing concern, especially for paediatric patients who may live long after the treatment and be more susceptible to carcinogenesis. Also, additional imaging procedures like computed tomography, kilovoltage and megavoltage imaging and positron emission tomography, alone or in conjunction with radiation therapy, may add to the radiation burden associated with the risk of occurrence of secondary cancers. This work has been based on literature studies and is focussed on the assessment of secondary doses to healthy tissues that are delivered by the use of modern radiation therapy and diagnostic imaging modalities in the clinical environment.

Monte Carlo and analytical model predictions of leakage neutron exposures from passively scattered proton therapy

PURPOSE

Stray neutron radiation is of concern after radiation therapy, especially in children, because of the high risk it might carry for secondary cancers. Several previous studies predicted the stray neutron exposure from proton therapy, mostly using Monte Carlo simulations. Promising attempts to develop analytical models have also been reported, but these were limited to only a few proton beam energies. The purpose of this study was to develop an analytical model to predict leakage neutron equivalent dose from passively scattered proton beams in the 100-250-MeV interval.

METHODS

To develop and validate the analytical model, the authors used values of equivalent dose per therapeutic absorbed dose (H∕D) predicted with Monte Carlo simulations. The authors also characterized the behavior of the mean neutron radiation-weighting factor, wR, as a function of depth in a water phantom and distance from the beam central axis.

RESULTS

The simulated and analytical predictions agreed well. On average, the percentage difference between the analytical model and the Monte Carlo simulations was 10% for the energies and positions studied. The authors found that wR was highest at the shallowest depth and decreased with depth until around 10 cm, where it started to increase slowly with depth. This was consistent among all energies.

CONCLUSION

Simple analytical methods are promising alternatives to complex and slow Monte Carlo simulations to predict H∕D values. The authors' results also provide improved understanding of the behavior of wR which strongly depends on depth, but is nearly independent of lateral distance from the beam central axis.