Kenneth Homann

Comparison of therapeutic dosimetric data from passively scattered proton and photon craniospinal irradiations for medulloblastoma

BackgroundFor many decades, the standard of care radiotherapy regimen for medulloblastoma has been photon (megavoltage x-rays) craniospinal irradiation (CSI). The late effects associated with CSI are well-documented in the literature and are in-part attributed to unwanted dose to healthy tissue. Recently, there is growing interest in using proton therapy for CSI in pediatric and adolescent patients to reduce this undesirable dose. Previous comparisons of dose to target and non-target organs from conventional photon CSI and passively scattered proton CSI have been limited to small populations (n ≤ 3) and have not considered the use of age-dependent target volumes in proton CSI.MethodsStandard of care treatment plans were developed for both photon and proton CSI for 18 patients. This cohort included both male and female medulloblastoma patients whose ages, heights, and weights spanned a clinically relevant and representative spectrum (age 2–16, BMI 16.4–37.9 kg/m2). Differences in plans were evaluated using Wilcoxon signed rank tests for various dosimetric parameters for the target volumes and normal tissue.ResultsProton CSI improved normal tissue sparing while also providing more homogeneous target coverage than photon CSI for patients across a wide age and BMI spectrum. Of the 24 parameters (V5, V10, V15, and V20 in the esophagus, heart, liver, thyroid, kidneys, and lungs) Wilcoxon signed rank test results indicated 20 were significantly higher for photon CSI compared to proton CSI (p ≤ 0.05) . Specifically, V15 and V20 in all six organs and V5, V10 in the esophagus, heart, liver, and thyroid were significantly higher with photon CSI.ConclusionsOur patient cohort is the largest, to date, in which CSI with proton and photon therapies have been compared. This work adds to the body of literature that proton CSI reduces dose to normal tissue compared to photon CSI for pediatric patients who are at substantial risk for developing radiogenic late effects. Although the present study focused on medulloblastoma, our findings are generally applicable to other tumors that are treated with CSI.

Predicted risks of radiogenic cardiac toxicity in two pediatric patients undergoing photon or proton radiotherapy

BackgroundHodgkin disease (HD) and medulloblastoma (MB) are common malignancies found in children and young adults, and radiotherapy is part of the standard treatment. It was reported that these patients who received radiation therapy have an increased risk of cardiovascular late effects. We compared the predicted risk of developing radiogenic cardiac toxicity after photon versus proton radiotherapies for a pediatric patient with HD and a pediatric patient with MB.MethodsIn the treatment plans, each patient’s heart was contoured in fine detail, including substructures of the pericardium and myocardium. Risk calculations took into account both therapeutic and stray radiation doses. We calculated the relative risk (RR) of cardiac toxicity using a linear risk model and the normal tissue complication probability (NTCP) values using relative seriality and Lyman models. Uncertainty analyses were also performed.ResultsThe RR values of cardiac toxicity for the HD patient were 7.27 (proton) and 8.37 (photon), respectively; the RR values for the MB patient were 1.28 (proton) and 8.39 (photon), respectively. The predicted NTCP values for the HD patient were 2.17% (proton) and 2.67% (photon) for the myocardium, and were 2.11% (proton) and 1.92% (photon) for the whole heart. The predicted ratios of NTCP values (proton/photon) for the MB patient were much less than unity. Uncertainty analyses revealed that the predicted ratio of risk between proton and photon therapies was sensitive to uncertainties in the NTCP model parameters and the mean radiation weighting factor for neutrons, but was not sensitive to heart structure contours. The qualitative findings of the study were not sensitive to uncertainties in these factors.ConclusionsWe conclude that proton and photon radiotherapies confer similar predicted risks of cardiac toxicity for the HD patient in this study, and that proton therapy reduced the predicted risk for the MB patient in this study.

Standardized treatment planning methodology for passively scattered proton craniospinal irradiation

BackgroundAs the number of proton therapy centers increases, so does the need for studies which compare proton treatments between institutions and with photon therapy. However, results of such studies are highly dependent on target volume definition and treatment planning techniques. Thus, standardized methods of treatment planning are needed, particularly for proton treatment planning, in which special consideration is paid to the depth and sharp distal fall-off of the proton distribution. This study presents and evaluates a standardized method of proton treatment planning for craniospinal irradiation (CSI).MethodsWe applied our institution’s planning methodology for proton CSI, at the time of the study, to an anatomically diverse population of 18 pediatric patients. We evaluated our dosimetric results for the population as a whole and for the two subgroups having two different age-specific target volumes using the minimum, maximum, and mean dose values in 10 organs (i.e., the spinal cord, brain, eyes, lenses, esophagus, lungs, kidneys, thyroid, heart, and liver). We also report isodose distributions and dose-volume histograms (DVH) for 2 representative patients. Additionally we report population-averaged DVHs for various organs.ResultsThe planning methodology here describes various techniques used to achieve normal tissue sparing. In particular, we found pronounced dose reductions in three radiosensitive organs (i.e., eyes, esophagus, and thyroid) which were identified for optimization. Mean doses to the thyroid, eyes, and esophagus were 0.2%, 69% and 0.2%, respectively, of the prescribed dose. In four organs not specifically identified for optimization (i.e., lungs, liver, kidneys, and heart) we found that organs lateral to the treatment field (lungs and kidneys) received relatively low mean doses (less than 8% of the prescribed dose), whereas the heart and liver, organs distal to the treatment field, received less than 1% of the prescribed dose.ConclusionsThis study described and evaluated a standardized method for proton treatment planning for CSI. Overall, the standardized planning methodology yielded consistently high quality treatment plans and perhaps most importantly, it did so for an anatomically diverse patient population.

Implementation of an Analytical Model for Leakage Neutron Equivalent Dose in a Proton Radiotherapy Planning System

Equivalent dose from neutrons produced during proton radiotherapy increases the predicted risk of radiogenic late effects. However, out-of-field neutron dose is not taken into account by commercial proton radiotherapy treatment planning systems. The purpose of this study was to demonstrate the feasibility of implementing an analytical model to calculate leakage neutron equivalent dose in a treatment planning system. Passive scattering proton treatment plans were created for a water phantom and for a patient. For both the phantom and patient, the neutron equivalent doses were small but non-negligible and extended far beyond the therapeutic field. The time required for neutron equivalent dose calculation was 1.6 times longer than that required for proton dose calculation, with a total calculation time of less than 1 h on one processor for both treatment plans. Our results demonstrate that it is feasible to predict neutron equivalent dose distributions using an analytical dose algorithm for individual patients with irregular surfaces and internal tissue heterogeneities. Eventually, personalized estimates of neutron equivalent dose to organs far from the treatment field may guide clinicians to create treatment plans that reduce the risk of late effects.

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, , 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.

Use of a matchline dosimetry analysis tool (MDAT) to quantify dose homogeneity in the region between abutting tangential and supraclavicular radiation fields

In this work, we develop and test a matchline dosimetry analysis tool (MDAT) to examine the dose distribution within the abutment region of two or more adjoining radiotherapy fields that employ different blocking mechanisms and geometries in forming a match. This objective and quantitative tool uses calibrated radiographic film to measure the dose in the abutment region, and uses a frequency distribution of area versus dose (a dose‐area histogram) to visualize the spatial dose distribution. We tested the MDATs clinical applicability and parameters by evaluating the dose between adjacent photon fields incident on a flat phantom using field‐matching techniques employing collimator‐jaw and multileaf collimator (MLC) configurations. Additionally, we evaluated the dose in the abutment regions of four different clinical tangential‐breast and supraclavicular matching techniques using various combinations of collimator and MLC matches. Using the MDAT tool, it was determined that a 1 cm abutment region width (centered about the theoretical matchline between fields) is the most appropriate width to determine dose homogeneity in a field matching region. Using the MDAT, both subtle and large differences were seen between fields that used MLCs to form the match, compared to flat edge devices such as collimators and external cerrobend blocks. We conclude that the MDAT facilitates a more precise evaluation of the distribution of dose within the region of abutment of radiotherapy fields. PACS number: 87.55.dk

SU‐E‐T‐275: Voxelized Second Cancer Risk Calculation — Comparison of Proton and Photon Radiotherapies for Hodgkin Lymphoma

Purpose: The purpose of this study was to compare the risk of developing a second lung or breast cancer for a female Hodgkin Lymphoma patient treated with passively scattered proton therapy (PSPT) and photon intensity modulated radiotherapy (IMRT). Particular objectives of this work were to fully account for stray dose, perform voxel by voxel risk calculations, and to visually display risk in a manner analogous to dose visualization used in commercial treatment planning systems (CTPS). Methods: Clinically approved PSPT and IMRT treatment plans were designed using a CTPS and prescribed 36 Gy to the target volume. Neutron and stray photon dose were calculated using an in house Monte Carlo code and a TLD measurement fitted analytical model for PSPT and IMRT, respectively. For lung and breast, therapeutic and stray doses were summed and input into an in‐house program that calculates Excess Absolute Risk (EAR) per voxel in the patient based on the RadRAT risk model. The radiation weighting factor (Wr) varied for neutron doses (Wr =10 and 30) to account for biological uncertainties. Results: Neutron equivalent doses added an additional 3% (Wr =10)/9% (Wr =30) of the prescribed dose to the CTV and near field organs. Scatter dose added an additional 5% (Wr =10 )/15% (Wr =30) of the prescribed dose to organ portions far out of field. PSPT registered a lower EAR compared to IMRT except for lung when Wr =30. Larger differences were seen in low dose regions of organs. Conclusion: EAR was lower for PSPT versus IMRT unless neutron Wr values were very high, e.g.> ∼20. More work incorporating uncertainties must be done before definitive answers are determined. In the future, similar types of risk visualization tools could be incorporated into treatment plan analysis to prospectively select plans with the lowest predicted risk of second cancers. This research was supported in part, by the National Cancer Institute award 1R01CA131463‐01A1 (W.D. Newhauser, P.I.) and a subcontract of that award (R.M. Howell, P.I). No conflicts of interest to disclose.

SU‐E‐T‐283; Risks of Cardiac Toxicity in Pediatric Patients Receiving Photon Or Proton Radiotherapy

PURPOSE To compare the predicted risk of developing radiogenic cardiac toxicity after photon versus proton radiotherapies for a pediatric patient with Hodgkin disease (HD) and a pediatric patient with medulloblastoma (MB). METHODS In the treatment plans, each patient heart was contoured in fine detail, including substructures of the pericardium and myocardium. Risk calculations took into account both therapeutic and stray radiation doses. We calculated the relative risk (RR) of cardiac toxicity using a linear risk model and the normal tissue complication probability (NTCP) values using relative seriality and Lyman models. Uncertainty analyses were also performed Results: The RR values of cardiac toxicity for the HD patient were 7.27 (95% confidence interval (CI), 3.09 to 27.12) (proton) and 8.37 (95% CI, 3.46 to 31.70) (photon), respectively; the RR values for the MB patient were 1.28 (95% CI, 1.09 to 2.18) (proton) and 8.39 (95% CI, 3.46 to 31.78) (photon), respectively. The predicted NTCP values for the HD patient were 2.17% (proton) and 2.67% (photon) for the myocardium, and were 2.11% (proton) and 1.92% (photon) for the whole heart. The predicted ratios of NTCP values (proton/photon) for the MB patient were much less than unity. Uncertainty analyses revealed that the predicted ratio of risk between proton and photon therapies was sensitive to uncertainties in the NTCP model parameters and the mean radiation weighting factor for neutrons, but was not sensitive to heart structure contours. The qualitative findings of the study were not sensitive to uncertainties in these factors. CONCLUSION We conclude that proton and photon radiotherapies confer similar predicted risks of cardiac toxicity for the HD patient and that proton therapy reduced the predicted risk for the MB patient relative to photon therapy.

SU-E-T-278: Risk of Developing a Second Cancer in the Breast for Hodgkin Lymphoma Patients Receiving Carbon Ion Therapy Versus Proton Therapy

PURPOSE Although radiation therapy is frequently used to cure Hodgkin lymphoma (HL), young patients treated with radiation have an increased risk to develop secondary malignant neoplasms. The purpose of this study was to determine whether using carbon ion therapy instead of proton therapy would show a difference in the predicted risk of radiation-induced second cancers in the breast for female HL patients. METHODS We retrospectively selected 6 female HL patients who were previously treated with proton therapy and limited our study to patients with supradiaphragmatic disease. We designed scanned proton and scanned carbon ion treatment plans to deliver 36 Gy (RBE) to the HL target using a single anterior-posterior beam for each patient. We calculated relative predicted risks of second cancer in the breast using a linear-no-threshold tumor induction model and a linear-quadratic breast cell inactivation model, and we explicitly modeled RBE values for both of these competing processes. RESULTS For all patients, we found that the predicted risk for second cancer incidence in the breast was slightly lower using carbon ions instead of protons. For individual patients, we observed large variations in predicted risk given the plausible range of RBE for cancer induction for protons and carbon ions. CONCLUSION Our findings are indicative that a higher or lower risk of second cancer in the breast might be expected for some patients using carbon therapy instead of proton therapy, depending on RBE for tumor induction, which implicitly depends on the α/β ratio of breast tissue, as well as on the physical dose distribution. UT MD Anderson Rosalie B. Hite Fellowship.

SU-FF-T-196: Evaluation of the Dose Within the Abutment Region Between Tangential and Supraclavicular Fields for Various Breast Irradiation Techniques

Purpose: To compare the uniformity of dose in the abutment region (matchline) between breast tangential and supraclavicular fields produced by irradiation techniques having different means of forming the match (e.g. external block, MLC, collimator jaw). Method and Materials: A film dosimetry system was developed and validated by comparing lateral and percent-depth-dose (PDD) profiles measured with EDR2 film in a plastic water phantom to ion chamber measurements made in water. Field sizes of 5×5, 10×10, and 20×20 cm2 were examined covering a range of depths. Once established, EDR2 film was placed in solid phantom at various depths and used for characterization of the abutment region produced by several three-field intact-breast irradiations. The abutment regions (+/− 2.5 mm on either side of the central matchline) of the various techniques were analyzed via a Dose Area Histogram (DAH) and compared to each other. Preliminary data are presented, in which DAHs are used to evaluate the abutment region between a field collimated with a jaw and another field collimated with the configurations mentioned above. Results: Characterization of the film dosimetry system demonstrates that, for all field sizes and depths, all lateral profiles and depth-dose curves measured agree with ion chamber measurements to within < 3% in low dose gradient regions and < 2 mm distance to agreement (DTA) in high dose gradient regions (≥ 30% cm−1). Inspection of preliminary abutment region data indicates that the DAH clearly and sensitively demonstrates uniformity differences between the various methods used to create the match. Conclusion: We have developed a film-based dosimetry tool that uses DAHs to accurately portray relative dose distributions within the abutment regions of smooth-edge and MLC-produced fields. This tool will be used, subsequently, in an anthropomorphic phantom using actual breast treatment fields. Conflict of Interest: Research supported in part by Varian Medical Systems.