Assessing the performance of an automated breast treatment planning software.

PURPOSE: To assess the dosimetric performance of an automated breast planning software. METHODS: We retrospectively reviewed 15 breast cancer patients treated with tangent fields according to the RTOG 1005 protocol and 30 patients treated off-protocol. Planning with electronic compensators (eComps) via manual, iterative fluence editing was compared to an automated planning program called EZFluence (EZF) (Radformation, Inc.). We compared the minimum dose received by 95% of the volume (D95%), D90%, the volume receiving at least 105% of prescription (V105%), V95%, the conformity index of the V95% and PTV volumes (CI95%), and total monitor units (MUs). The PTV_Eval structure generated by EZF was compared to the RTOG 1005 breast PTV_Eval structure. RESULTS: The average D95% was significantly greater for the EZF plans, 95.0%, vs. the original plans 93.2% (P = 0.022). CI95% was less for the EZF plans, 1.18, than the original plans, 1.48 (P = 0.09). D90% was only slightly greater for EZF, averaging at 98.3% for EZF plans and 97.3% for the original plans (P = 0.0483). V105% (cc) was, on average, 27.8cc less in the EZF breast plans, which was significantly less than for those manually planned. The average number of MUs for the EZF plans, 453, was significantly less than original protocol plans, 500 (P = 8 × 10-6 ). The average difference between the protocol PTV volume and the EZF PTV volume was 196 cc, with all but two cases having a larger EZF PTV volume (P = 0.020). CONCLUSION: EZF improved dose homogeneity, coverage, and MU efficiency vs. manually produced eComp plans. The EZF-generated PTV eval is based on the volume encompassed by the tangents, and is not appropriate for dosimetric comparison to constraints for RTOG 1005 PTV eval. EZF produced dosimetrically similar or superior plans to manual, iteratively derived plans and may also offer time and efficiency benefits.

Benchmarking failure mode and effects analysis of electronic brachytherapy with data from incident learning systems.

PURPOSE: Failure modes and effects analysis (FMEA) is a prospective risk assessment tool for identifying failure modes in equipment or processes and informing the design of quality control systems. This work aims to benchmark the performance of FMEAs for electronic brachytherapy (eBT) of the skin and for breast by comparing predicted versus actual failure modes reported in multiple incident learning systems (ILS). METHODS AND MATERIALS: Two public and our institution's internal ILS were queried for Xoft Axxent eBT-related events over 9 years. The failure modes and Risk Priority Numbers (RPNs) were taken from FMEAs previously performed for Xoft eBT of nonmelanoma skin cancer and breast intraoperative radiation therapy (IORT). For each event, the treatment site and primary failure mode was compared with the failure modes and RPNs from that site's FMEA. RESULTS: 49 events involving Xoft eBT were identified. Thirty-one (63.3%) involved breast IORT, and 18 (36.7%) involved the skin. Three events could not be linked to an FMEA failure mode. In 87.7% of events, the primary failure mode ranked in the FMEA top 10 by RPNs. In 83.3% of skin events, the failure modes ranked in the top 10 by RPN or severity. In 90.3% of IORT events, the failure modes ranked within the top 10 by RPN or severity. CONCLUSIONS: Evaluating FMEA failure modes against ILS data demonstrates that FMEA is effective at predicting failure modes but can be dependent on user experience. ILS data can improve FMEA by identifying potential failure modes and suggesting realistic occurrence, detectability, and severity values.

Evaluation of effective treatment depth in skin cancer treatments with electronic brachytherapy.

PURPOSE: To evaluate changes in the percent depth dose (PDD) and effective depth of treatment due to force applied by the applicator during treatments of nonmelanoma skin cancer with the Xoft electronic brachytherapy system. METHODS: To simulate compressible tissue, a 5-mm tissue-equivalent bolus was used. A soft x-ray ion chamber was used for output measurements, which were performed for all Xoft surface applicators with plastic endcaps in place. Output was first measured at 5 mm depth with minimal pressure from the applicator on the bolus and then repeated after applying uniform pressure on the applicator to calculate the change in PDD and effective treatment depth. RESULTS: For the 10-mm cone, a moderate force of 5 N changed the PDD by more than 20%. The effect was also pronounced for the 20-mm cone, while minimal for the 35- and 50-mm cones. Even when only a moderate force was applied, the effective prescription depth changed by several millimeters, on the order of a typical prescription depth. CONCLUSION: Based on the results of this simulation, excessive pressure applied on the skin by the applicator can drastically alter the PDD and effective treatment depth. The effect is most pronounced for the 10- and 20-mm cones, which tend to be used most frequently. Inappropriate applicator placement may therefore result in significant consequences such as excessive dose to the target, severe skin reaction, permanent discoloration, skin indentation, and poor overall cosmesis upon completion of treatment.

The Role of Optical Surface Imaging Systems in Radiation Therapy.

Optical surface imaging is a nonradiographic, noninvasive technology for continuous localization of patients during radiation therapy. Surface-guided radiation therapy (SGRT) has been applied to many treatment sites including breast, intracranial, head and neck, and extremities. SGRT enables a reduction of initial setup variability, provides verification of immobilization continuously during treatment including at noncoplanar linac gantry angles, and provides dynamic surface information for use in gated and breath-hold treatment techniques, all of which can permit reductions in the margins required to account for target localization uncertainty. Ancillary benefits from surface imaging include the ability to use immobilization techniques that confer greater comfort to patients, a reduction in imaging dose through reduced radiographic localization requirements, and improvements to the speed, efficiency, and safety of clinical workflows. This review will describe the objectives of SGRT, review the commercially available surface imaging systems, and provide an overview of SGRT applications by treatment site. Limitations and future applications of surfacing imaging systems are also discussed.

Dosimetric benefit of adaptive re-planning in pancreatic cancer stereotactic body radiotherapy.

Stereotactic body radiotherapy (SBRT) shows promise in unresectable pancreatic cancer, though this treatment modality has high rates of normal tissue toxicity. This study explores the dosimetric utility of daily adaptive re-planning with pancreas SBRT. We used a previously developed supercomputing online re-planning environment (SCORE) to re-plan 10 patients with pancreas SBRT. Tumor and normal tissue contours were deformed from treatment planning computed tomographies (CTs) and transferred to daily cone-beam CT (CBCT) scans before re-optimizing each daily treatment plan. We compared the intended radiation dose, the actual radiation dose, and the optimized radiation dose for the pancreas tumor planning target volume (PTV) and the duodenum. Treatment re-optimization improved coverage of the PTV and reduced dose to the duodenum. Within the PTV, the actual hot spot (volume receiving 110% of the prescription dose) decreased from 4.5% to 0.5% after daily adaptive re-planning. Within the duodenum, the volume receiving the prescription dose decreased from 0.9% to 0.3% after re-planning. It is noteworthy that variation in the amount of air within a patient׳s stomach substantially changed dose to the PTV. Adaptive re-planning with pancreas SBRT has the ability to improve dose to the tumor and decrease dose to the nearby duodenum, thereby reducing the risk of toxicity.

Improving linear accelerator service response with a real- time electronic event reporting system.

To track linear accelerator performance issues, an online event recording system was developed in-house for use by therapists and physicists to log the details of technical problems arising on our institution's four linear accelerators. In use since October 2010, the system was designed so that all clinical physicists would receive email notification when an event was logged. Starting in October 2012, we initiated a pilot project in collaboration with our linear accelerator vendor to explore a new model of service and support, in which event notifications were also sent electronically directly to dedicated engineers at the vendor's technical help desk, who then initiated a response to technical issues. Previously, technical issues were reported by telephone to the vendor's call center, which then disseminated information and coordinated a response with the Technical Support help desk and local service engineers. The purpose of this work was to investigate the improvements to clinical operations resulting from this new service model. The new and old service models were quantitatively compared by reviewing event logs and the oncology information system database in the nine months prior to and after initiation of the project. Here, we focus on events that resulted in an inoperative linear accelerator ("down" machine). Machine downtime, vendor response time, treatment cancellations, and event resolution were evaluated and compared over two equivalent time periods. In 389 clinical days, there were 119 machine-down events: 59 events before and 60 after introduction of the new model. In the new model, median time to service response decreased from 45 to 8 min, service engineer dispatch time decreased 44%, downtime per event decreased from 45 to 20 min, and treatment cancellations decreased 68%. The decreased vendor response time and reduced number of on-site visits by a service engineer resulted in decreased downtime and decreased patient treatment cancellations.

A method for assessing voxel correspondence in longitudinal tumor imaging.

PURPOSE: Tumor characterization employing a voxel-wise analysis of image signal facilitates the determination of the spatial distribution of tumor attributes, and when employed for therapy response assessment offers the promise of greater sensitivity to change than conventional approaches. However, the accuracy of a voxel-wise analysis of change is limited by local registration uncertainties that can disrupt the spatiotemporal correspondence between assessed voxels. We present a method for assessing voxel correspondence strength using a multiresolution local histogram-based measure of image structure similarity. When employed in a longitudinal tumor imaging context, a voxel similarity measure must be robust to intensity variations that can arise from the image acquisition, treatment effects, or changes in underlying disease processes. Consequently, the local histogram-based similarity measure is evaluated for sensitivity to structural change and robustness to intensity variation and is compared against normalized mutual information and normalized cross-correlation. METHODS: T1-weighted (T1W) magnetic resonance (MR) images of glioblastoma acquired as part of a longitudinal response assessment study are first rigidly registered, and then similarity between spatially corresponding voxels is evaluated using multiresolution local histograms. Region-based and nonuniform intensity changes of varying magnitude as well as deformations to image structure are applied individually and in combination to the test images. Statistical analysis is used to test for interaction effects between the applied perturbations and the value of the local histogram similarity function. Pair-wise voxel similarity maps are computed for pairs of longitudinal clinical MR image volumes and compared with observed patterns of change on conventional imaging. RESULTS: The simulations demonstrated that the local histogram measure was robust to intensity modulations applied to increasing region sizes and exhibited a strong negative correlation with the magnitude of local deformation. No statistically significant interaction effects were observed upon the value of the local histogram similarity function when deformation was applied in conjunction with a nonuniform intensity change. Pair-wise voxel similarity maps were consistent with image change observed on T1W MR imaging and revealed patterns of change not apparent in conventional image sequences. CONCLUSIONS: A measure of local histogram image structure similarity can be used to assess the strength of voxel to voxel correspondences independently of intensity nonuniformities. The metric can provide a local estimate of the limits of achievable correspondence underlying the registration and voxel-wise comparison of signal in longitudinal imaging used for assessing tumor response to treatment.

A study of tumor motion management in the conformal radiotherapy of lung cancer.

PURPOSE: To assess the benefit derived from the reduction of planning target volumes (PTVs) afforded by tumor motion management in treatment planning for lung cancer. METHODS: We use a simple formula that combines measurements of tumor motion and set-up error for 7 patients to determine PTVs based on the following scenarios: standard uniform 15 mm margin, individualized PTVs (no gating), spirometry-based gating, and active breath-control (ABC). We compare the percent volumes of lung receiving at least 20 Gy (V20) for a standard prescription, and the maximum tolerated doses (MTDs) at fixed V20. In anticipation of improvements in set-up accuracy, we repeat the analysis assuming a reduced set-up margin of 3mm. RESULTS: Relative to the standard, the average percent reductions in V20 (+/- 1 standard deviation) for the ungated and gated scenarios are 17+/-5 and 21+/-8; the percent gains in MTD are 25+/-12 and 33+/-11, respectively. For the 3mm set-up margin, the corresponding results for V20 are 28+/-7 and 36+/-7, and for MTD are 57+/-23 and 79+/-31. CONCLUSIONS: Any form of motion management provides a benefit over the use of a standard margin. The benefit derived from gating compared to the use of ungated individualized PTVs increases with tumor mobility but is generally modest. While motion management may benefit patients with highly mobile tumors, we expect efforts to reduce set-up error to be of greater overall significance. The practical limit for lung PTV margins is likely around 4-5mm, provided set-up error can be reduced sufficiently.

Quantitative PET comparing gated with nongated acquisitions using a NEMA phantom with respiratory-simulated motion.

UNLABELLED: This study evaluated the use of gated versus nongated PET acquisitions for absolute quantification of radioisotope concentration (RC) in a respiratory motion-simulated moving phantom filled with radioactive spheres and background for both 2-dimensional (2D) and 3-dimensional (3D) acquisitions. METHODS: An image-quality phantom with all 6 spheres filled with the same (18)F RC (range, 19-62 kBq/mL) was scanned with PET/CT at rest and in motion with and without gating. The background was filled with (18)F solution to yield sphere-to-background ratios of approximately 5, 10, 15, and 20 to 1. Both 2D and 3D acquisitions were used for all combinations. Respiratory motion was simulated by using a motor-driven plastic platform to move the phantom periodically with a displacement of 2 cm and a cycle time of 5.8 s. For gated acquisitions, the phantom was tracked using a real-time position management system. Images were reconstructed, and regions of interest with the same sizes as the actual spheres were manually placed on axial slices to determine maximum and mean pixel RC. A threshold method (70% and 94% for 2D and 3D modes) was also used to determine a mean voxel RC. All values were compared with the expected RC; percentage differences were calculated for each sphere. To reduce partial-volume effects, only data for the 4 largest spheres were analyzed. RESULTS: The mean pixel method was the only method with linear responses for all 3 scan types, enabling direct comparisons. The ranges of RC percentage differences were underestimated for all scan types (using the mean pixel method). The overall mean percentage differences were 37, 49, and 41 in 2D mode and 40, 51, and 41 in 3D mode for static, nongated, and gated acquisitions, respectively. Gated acquisitions improved quantification (by reducing underestimation) over nongated acquisitions by 8% and 10% for 2D and 3D modes. CONCLUSION: In the presence of motion, the use of gated PET acquisitions appears to improve quantification accuracy over nongated acquisitions, almost restoring the results to those observed when the phantom is static.

Prediction of lung tumour position based on spirometry and on abdominal displacement: accuracy and reproducibility.

BACKGROUND AND PURPOSE: A simulation investigating the accuracy and reproducibility of a tumour motion prediction model over clinical time frames is presented. The model is formed from surrogate and tumour motion measurements, and used to predict the future position of the tumour from surrogate measurements alone. PATIENTS AND METHODS: Data were acquired from five non-small cell lung cancer patients, on 3 days. Measurements of respiratory volume by spirometry and abdominal displacement by a real-time position tracking system were acquired simultaneously with X-ray fluoroscopy measurements of superior-inferior tumour displacement. A model of tumour motion was established and used to predict future tumour position, based on surrogate input data. The calculated position was compared against true tumour motion as seen on fluoroscopy. Three different imaging strategies, pre-treatment, pre-fraction and intrafractional imaging, were employed in establishing the fitting parameters of the prediction model. The impact of each imaging strategy upon accuracy and reproducibility was quantified. RESULTS: When establishing the predictive model using pre-treatment imaging, four of five patients exhibited poor interfractional reproducibility for either surrogate in subsequent sessions. Simulating the formulation of the predictive model prior to each fraction resulted in improved interfractional reproducibility. The accuracy of the prediction model was only improved in one of five patients when intrafractional imaging was used. CONCLUSIONS: Employing a prediction model established from measurements acquired at planning resulted in localization errors. Pre-fractional imaging improved the accuracy and reproducibility of the prediction model. Intrafractional imaging was of less value, suggesting that the accuracy limit of a surrogate-based prediction model is reached with once-daily imaging.

Correlation of lung tumor motion with external surrogate indicators of respiration.

PURPOSE: To assess the correlation of respiratory volume and abdominal displacement with tumor motion as seen with X-ray fluoroscopy. Measurements throughout the patient's treatment course allowed an assessment of the interfractional reproducibility of this correlation. METHODS AND MATERIALS: Data were acquired from 11 patients; 5 were studied over multiple days. Measurements of respiratory volume by spirometry and abdominal displacement by a real-time position tracking system were correlated to simultaneously acquired X-ray fluoroscopy measurements of superior-inferior tumor displacement. The linear correlation coefficient was computed for each data acquisition. The phase relationship between the surrogate and tumor signals was estimated through cross-correlation delay analysis. RESULTS: Correlation coefficients ranged from very high to very low (0.99-0.39, p < 0.0001). The correlation between tumor displacement and respiratory volume was higher and more reproducible from day to day than between tumor displacement and abdominal displacement. A nonzero phase relationship was observed in nearly all patients (-0.65 to +0.50 s). This relationship was observed to vary over inter- and intrafractional time scales. Only 1 of 5 patients studied over multiple days had a consistent relationship between tumor motion and either surrogate. CONCLUSIONS: Respiratory volume has a more reproducible correlation with tumor motion than does abdominal displacement. If forming a tumor-surrogate prediction model from a limited series of observations, the use of surrogates to guide treatment might result in geographic miss.