ADVANTAGES OF MCNPX-BASED LATTICE TALLY OVER MESH TALLY IN HIGH-SPEED MONTE CARLO DOSE RECONSTRUCTION FOR PROTON RADIOTHERAPY

MONTE CARLO DOSE ASSESSMENT IN DENTAL CONE-BEAM COMPUTED TOMOGRAPHY

Most dental cone-beam computed tomography (CBCT) uses an x-ray beam field covering the maxillomandibular region and the width-truncated detector geometry. The spatial dose distribution in dental CBCT is analyzed in terms of local primary and remote secondary doses by using a list-mode analysis of x-ray interactions obtained from the Monte Carlo simulations. The patient-dose benefit due to the width-truncated detector geometry is also investigated for a wide range of detector offsets. The developed dose estimation agrees with the measurement in a relative error of 7.7%. The secondary dose outside of the irradiation field becomes larger with increasing tube voltage. The dose benefit with the width-truncated geometry linearly increases as the detector-offset width is decreased. Leaving the CT image quality out of the account, the MC results reveal that the operation of dental CBCT with a lower tube voltage and a smaller detector-offset width is beneficial to the patient dose.

Low- and middle-income countries can reduce risks of subsequent neoplasms by referring pediatric craniospinal cases to centralized proton treatment centers

Few children with cancer in low- and middle-income countries (LMICs) have access to proton therapy. Evidence exists to support replacing photon therapy with proton therapy to reduce the incidence of secondary malignant neoplasms (SMNs) in childhood cancer survivors. The purpose of this study was to estimate the potential reduction in SMN incidence and in SMN mortality for pediatric medulloblastoma patients in LMICs if proton therapy were made available to them. For nine children of ages 2 to 14 years, we calculated the equivalent dose in organs or tissues at risk for radiogenic SMNs from therapeutic and stray radiation for photon craniospinal irradiation (CSI) in a LMIC and proton CSI in a high-income country. We projected the lifetime risks of SMN incidence and SMN mortality for every SMN site with a widely-used model from the literature. We found that the average total lifetime attributable risks of incidence and mortality were very high for both photon CSI (168% and 41%, respectively) and proton CSI (88% and 26%, respectively). SMNs having the highest risk of mortality were lung cancer (16%), non-site-specific solid tumors (16%), colon cancer (5.9%), leukemia (5.4%), and for girls breast cancer (5.0%) after photon CSI and non-site-specific solid tumors (12%), lung cancer (11%), and leukemia (4.8%) after proton CSI. The risks were higher for younger children than for older children and higher for girls than for boys. The ratios of proton CSI to photon CSI of total risks of SMN incidence and mortality were 0.56 (95% CI, 0.37 to 0.75) and 0.64 (95% CI, 0.45 to 0.82), respectively, averaged over this sample group. In conclusion, proton therapy has the potential to lessen markedly subsequent SMNs and SMN fatalities in survivors of childhood medulloblastoma in LMICs, for example, through regional centralized care. Additional methods should be explored urgently to reduce therapeutic-field doses in organs and tissues at risk for SMN, especially in the lungs, colon, and breast tissues.

A Review of Radiotherapy-Induced Late Effects Research after Advanced Technology Treatments

The number of incident cancers and long-term cancer survivors is expected to increase substantially for at least a decade. Advanced technology radiotherapies, e.g., using beams of protons and photons, offer dosimetric advantages that theoretically yield better outcomes. In general, evidence from controlled clinical trials and epidemiology studies are lacking. To conduct these studies, new research methods and infrastructure will be needed. In the paper, we review several key research methods of relevance to late effects after advanced technology proton-beam and photon-beam radiotherapies. In particular, we focus on the determination of exposures to therapeutic and stray radiation and related uncertainties, with discussion of recent advances in exposure calculation methods, uncertainties, in silico studies, computing infrastructure, electronic medical records, and risk visualization. We identify six key areas of methodology and infrastructure that will be needed to conduct future outcome studies of radiation late effects.

Reducing the cost of proton radiation therapy: the feasibility of a streamlined treatment technique for prostate cancer

Proton radiation therapy is an effective modality for cancer treatments, but the cost of proton therapy is much higher compared to conventional radiotherapy and this presents a formidable barrier to most clinical practices that wish to offer proton therapy. Little attention in literature has been paid to the costs associated with collimators, range compensators and hypofractionation. The objective of this study was to evaluate the feasibility of cost-saving modifications to the present standard of care for proton treatments for prostate cancer. In particular, we quantified the dosimetric impact of a treatment technique in which custom fabricated collimators were replaced with a multileaf collimator (MLC) and the custom range compensators (RC) were eliminated. The dosimetric impacts of these modifications were assessed for 10 patients with a commercial treatment planning system (TPS) and confirmed with corresponding Monte Carlo simulations. We assessed the impact on lifetime risks of radiogenic second cancers using detailed dose reconstructions and predictive dose-risk models based on epidemiologic data. We also performed illustrative calculations, using an isoeffect model, to examine the potential for hypofractionation. Specifically, we bracketed plausible intervals of proton fraction size and total treatment dose that were equivalent to a conventional photon treatment of 79.2 Gy in 44 fractions. Our results revealed that eliminating the RC and using an MLC had negligible effect on predicted dose distributions and second cancer risks. Even modest hypofractionation strategies can yield substantial cost savings. Together, our results suggest that it is feasible to modify the standard of care to increase treatment efficiency, reduce treatment costs to patients and insurers, while preserving high treatment quality.

The physics of proton therapy

The physics of proton therapy has advanced considerably since it was proposed in 1946. Today analytical equations and numerical simulation methods are available to predict and characterize many aspects of proton therapy. This article reviews the basic aspects of the physics of proton therapy, including proton interaction mechanisms, proton transport calculations, the determination of dose from therapeutic and stray radiations, and shielding design. The article discusses underlying processes as well as selected practical experimental and theoretical methods. We conclude by briefly speculating on possible future areas of research of relevance to the physics of proton therapy.

Inter-Institutional Comparison of Personalized Risk Assessments for Second Malignant Neoplasms for a 13-Year-Old Girl Receiving Proton versus Photon Craniospinal Irradiation

Children receiving radiotherapy face the probability of a subsequent malignant neoplasm (SMN). In some cases, the predicted SMN risk can be reduced by proton therapy. The purpose of this study was to apply the most comprehensive dose assessment methods to estimate the reduction in SMN risk after proton therapy vs. photon therapy for a 13-year-old girl requiring craniospinal irradiation (CSI). We reconstructed the equivalent dose throughout the patient’s body from therapeutic and stray radiation and applied SMN incidence and mortality risk models for each modality. Excluding skin cancer, the risk of incidence after proton CSI was a third of that of photon CSI. The predicted absolute SMN risks were high. For photon CSI, the SMN incidence rates greater than 10% were for thyroid, non-melanoma skin, lung, colon, stomach, and other solid cancers, and for proton CSI they were non-melanoma skin, lung, and other solid cancers. In each setting, lung cancer accounted for half the risk of mortality. In conclusion, the predicted SMN risk for a 13-year-old girl undergoing proton CSI was reduced vs. photon CSI. This study demonstrates the feasibility of inter-institutional whole-body dose and risk assessments and also serves as a model for including risk estimation in personalized cancer care.