D. Sasaki

Low-cost optical scanner and 3-dimensional printing technology to create lead shielding for radiation therapy of facial skin cancer: First clinical case series

Purpose Three-dimensional printing has been implemented at our institution to create customized treatment accessories, including lead shields used during radiation therapy for facial skin cancer. To effectively use 3-dimensional printing, the topography of the patient must first be acquired. We evaluated a low-cost, structured-light, 3-dimensional, optical scanner to assess the clinical viability of this technology. Methods and materials For ease of use, the scanner was mounted to a simple gantry that guided its motion and maintained an optimum distance between the scanner and the object. To characterize the spatial accuracy of the scanner, we used a geometric phantom and an anthropomorphic head phantom. The geometric phantom was machined from plastic and included hemispherical and tetrahedral protrusions that were roughly the dimensions of an average forehead and nose, respectively. Polygon meshes acquired by the optical scanner were compared with meshes generated from high-resolution computed tomography images. Most optical scans contained minor artifacts. Using an algorithm that calculated the distances between the 2 meshes, we found that most of the optical scanner measurements agreed with those from the computed tomography scanner within approximately 1 mm for the geometric phantom and approximately 2 mm for the head phantom. We used this optical scanner along with 3-dimensional printer technology to create custom lead shields for 10 patients receiving orthovoltage treatments of nonmelanoma skin cancers of the face. Patient, tumor, and treatment data were documented. Results Lead shields created using this approach were accurate, fitting the contours of each patients face. This process added to patient convenience and addressed potential claustrophobia and medical inability to lie supine. Conclusions The scanner was found to be clinically acceptable, and we suggest that the use of an optical scanner and 3-dimensional printer technology become the new standard of care to generate lead shielding for orthovoltage radiation therapy of nonmelanoma facial skin cancer.

Creation of knowledge‐based planning models intended for large scale distribution: Minimizing the effect of outlier plans

Abstract Knowledge‐based planning (KBP) can be used to estimate dose–volume histograms (DVHs) of organs at risk (OAR) using models. The task of model creation, however, can result in estimates with differing accuracy; particularly when outlier plans are not properly addressed. This work used RapidPlan™ to create models for the prostate and head and neck intended for large‐scale distribution. Potential outlier plans were identified by means of regression analysis scatter plots, Cooks distance, coefficient of determination, and the chi‐squared test. Outlier plans were identified as falling into three categories: geometric, dosimetric, and over‐fitting outliers. The models were validated by comparing DVHs estimated by the model with those from a separate and independent set of clinical plans. The estimated DVHs were also used as optimization objectives during inverse planning. The analysis tools lead us to identify as many as 7 geometric, 8 dosimetric, and 20 over‐fitting outliers in the raw models. Geometric and over‐fitting outliers were removed while the dosimetric outliers were replaced after re‐planning. Model validation was done by comparing the DVHs at 50%, 85%, and 99% of the maximum dose for each OAR (denoted as V50, V85, and V99) and agreed within −2% to 4% for the three metrics for the final prostate model. In terms of the head and neck model, the estimated DVHs agreed from −2.0% to 5.1% at V50, 0.1% to 7.1% at V85, and 0.1% to 7.6% at V99. The process used to create these models improved the accuracy for the pharyngeal constrictor DVH estimation where one plan was originally over‐estimated by more than twice. In conclusion, our results demonstrate that KBP models should be carefully created since their accuracy could be negatively affected by outlier plans. Outlier plans can be addressed by removing them from the model and re‐planning.

Analysis of First Case Series in the World of Skin Cancer Patients Where Lead Shielding for Radiation Therapy of Facial Skin Cancer was Designed Utilizing Optical Scanner and 3D Printer Technology

Purpose Radiation is one of the modalities used to treat non-melanoma skin cancers. For facial lesions; ortho-voltage radiotherapy (RT) can require the creation of lead shielding to protect vulnerable organs at risk (OAR). Creating a lead shield is often difficult due to the complex contours of the face. The traditional method involves creating a plaster mould of a patients face to use as a template for creating a shield. This requires another patient visit, and for patients who are claustrophobic or medically unable to lie flat, this strategy is not ideal. We address this by utilizing optical scanner and 3D printer technology to create lead shields and report the first case series in the English literature here.

Sci-Fri PM: Radiation Therapy, Planning, Imaging, and Special Techniques - 11: Quantification of chest wall motion during deep inspiration breast hold treatments using cine EPID images and a physics based algorithm

Purpose: This work presents an algorithm used to quantify intra-fraction motion for patients treated using deep inspiration breath hold (DIBH). The algorithm quantifies the position of the chest wall in breast tangent fields using electronic portal images. Methods: The algorithm assumes that image profiles, taken along a direction perpendicular to the medial border of the field, follow a monotonically and smooth decreasing function. This assumption is invalid in the presence of lung and can be used to calculate chest wall position. The algorithm was validated by determining the position of the chest wall for varying field edge positions in portal images of a thoracic phantom. The algorithm was used to quantify intra-fraction motion in cine images for 7 patients treated with DIBH. Results: Phantom results show that changes in the distance between chest wall and field edge were accurate within 0.1 mm on average. For a fixed field edge, the algorithm calculates the position of the chest wall with a 0.2 mm standard deviation. Intra-fraction motion for DIBH patients was within 1 mm 91.4% of the time and within 1.5 mm 97.9% of the time. The maximum intra-fraction motion was 3.0 mm. Conclusions: A physics based algorithm was developed and can be used to quantify the position of chest wall irradiated in tangent portal images with an accuracy of 0.1 mm and precision of 0.6 mm. Intra-fraction motion for patients treated with DIBH at our clinic is less than 3 mm.

SU‐E‐T‐92: Creation of a Comprehensive Head and Model Using Knowledge Based Planning

PURPOSE The purpose of this study was to evaluate if a commercial implementation of Knowledge Based Planning (KBP) software can be used to estimate Dose Volume Histogram (DVHs) on a comprehensive data set of Head and Neck (HN) patients. METHODS KBP is a tool capable of estimating DVHs for Organs At Risk (OARs) based on the DVHs of plans of similar patients treated in the past. This study used a newly developed commercial implementation of KBP to create a HN model. The model was trained using a database of retrospectively treated HN patients. This database covered the spectrum of cases expected to be found in the clinic, including multiple targets and 18 different dose prescription combinations. A set of independent validation patients was used to quantify the accuracy of DVHs estimated using the model and covered the same spectrum of HN cases. RESULTS The accuracy of the model was calculated by comparing the volumes of the estimated and clinical DVHs at doses equal to 50%, 85% and 99% of the maximum OAR dose. This allowed us to quantify the accuracy of the estimated DVHs even in cases when the OAR was receiving a low dose. The highest accuracy was obtained in the estimation of the DVHs for the Brain (<1% on average). The accuracy for the Brainstem, Cord, Mandible, Oral Cavity, Parotids and the Pharyngeal Constrictor ranged from -2% to 9% on average. CONCLUSION This study shows the feasibility to estimate DVHs in a wide range of HN cases using a novel KBP algorithm. The use of a comprehensive set of patients results in a robust HN model which can be used in a wide range of clinical cases. Further planning is required to confirm if the current accuracy is sufficient to guide the planning process. The authors are Clinical Evaluators/Consultants for Varian Medical Systems. This study was partially funded by Varian Medical Systems.