Evaluation of the dosimetric impact of changes in shoulder position on target coverage for spine SBRT to metastases in the lower cervical spine region.

For patients treated with SBRT for spinal metastases in the cervical area, a thermoplastic mask is the usual immobilization technique. This project investigates the impact of shoulder position variability on target coverage for such cases. Eight HN patients treated in a suite equipped with a CT-on-rails system (CTOR) were randomly chosen. Of these, three were treated with shoulder depressors. For each patient, their planning CT was used to contour spine targets at the C5, C6 and C7 levels for which two VMAT plans were developed to deliver 18 Gy to each target per the RTOG 0631 protocol. One plan used full arcs while the other used avoidance sectors around the lateral positions. For each patient, IGRT CTOR images were used to recalculate doses that would have been delivered from these plans. Target coverage and dose to the spinal cord were compared for four scenarios: full and partial arcs, with or without depressors. A Dunn test showed significant differences between groups with and without shoulder depressors, but not between those with full versus partial arcs. For most of the investigated cases, the coverage ended up being higher than planned due to the shoulder position being inferior at treatment compared to simulation. In some cases, this led to higher spinal cord doses than allowed per protocol. The results of this study confirm that, when treating lower cervical spine lesions with SBRT, special care should be taken to ensure that the shoulders are positioned as they were during planning CT acquisition.

Using failure mode and effects analysis (FMEA) to generate an initial plan check checklist for improved safety in radiation treatment.

PURPOSE: To apply failure mode and effect analysis (FMEA) to generate an effective and efficient initial physics plan checklist. METHODS: A team of physicists, dosimetrists, and therapists was setup to reconstruct the workflow processes involved in the generation of a treatment plan beginning from simulation. The team then identified possible failure modes in each of the processes. For each failure mode, the severity (S), frequency of occurrence (O), and the probability of detection (D) was assigned a value and the risk priority number (RPN) was calculated. The values assigned were based on TG 100. Prior to assigning a value, the team discussed the values in the scoring system to minimize randomness in scoring. A local database of errors was used to help guide the scoring of frequency. RESULTS: Twenty-seven process steps and 50 possible failure modes were identified starting from simulation to the final approved plan ready for treatment at the machine. Any failure mode that scored an average RPN value of 20 or greater was deemed "eligible" to be placed on the second checklist. In addition, any failure mode with a severity score value of 4 or greater was also considered for inclusion in the checklist. As a by-product of this procedure, safety improvement methods such as automation and standardization of certain processes (e.g., dose constraint checking, check tools), removal of manual transcription of treatment-related information as well as staff education were implemented, although this was not the team's original objective. Prior to the implementation of the new FMEA-based checklist, an in-service for all the second checkers was organized to ensure further standardization of the process. CONCLUSION: The FMEA proved to be a valuable tool for identifying vulnerabilities in our workflow and processes in generating a treatment plan and subsequently a new, more effective initial plan checklist was created.

An evaluation of the consistency of shifts reported by three different systems for non-coplanar treatments.

Treatment of intra-cranial lesions sometimes requires a non-coplanar beam configuration. One of the most commonly used IGRT modalities, kV conebeam CT, cannot typically be used when large couch rotations are introduced. However, multiple other systems allow for imaging/tracking the patient for such situations. This work compares shift consistency from three independent systems, namely Varian's Advanced Imaging, Brainlab's Exactrac and Varian's OSMS, all installed on the same linear accelerator. After a phantom was first positioned using conebeam CT, the three systems were used to determine shifts at different couch positions. This was done with and without intentional shifts inserted in the original phantom position. Results show that the difference in shifts between the three systems was never more than 0.7 mm (average of 0.2 mm, standard deviation of 0.2 mm). These results confirm that all three systems are equivalent to within 1 mm and may potentially be uses interchangeably, especially in cases where the PTV margin is on the order of 1 mm.

Dosimetric evaluation of abdominal compression as a method to reduce the incidence of radiation-induced pneumonitis in lung SBRT treatment.

Abdominal compression has been shown to reduce the extent of lung tumor motion but the dosimetric impact of the approach is still in need of investigation. The current work analyzes the impact of various changes in PTV volume on key metrics of the final dose distribution to normal lung. To add clinical perspective, we also provide NTCP calculations for grade 2+ pneumonitis for each case. For a total of seventeen cases, the original ITV/PTV was reduced by systematically varied amounts and SBRT plans using dynamic conformal arc and VMAT techniques were created. DVH analysis for the normal lung comparing the original plan to the one with the ITV reduced by up to 10 mm shows that the average reduction of V5, V20 and mean lung dose is 3.8%, 2.0% and 1.1 Gy, respectively, for the conformal arc plans. Corresponding values for the VMAT plans were 3.9%, 1.9% and 1.2 Gy respectively. The mean NTCP drop for the conformal arc plans was 2.0% while it was 1.9% for the VMAT plans. These results suggest that abdominal compression has a modest impact on NTCP and on dosimetric parameters typically used to predict the risk of radiation pneumonitis in patients undergoing lung SBRT.

Head to head comparison of two commercial phantoms used for SRS QA.

Multiple commercial phantoms are now available for performing end-to-end QA testing for stereotactic procedures. This project aims at directly comparing one of the newest phantoms on the market against a more established one by performing similar tests to determine whether results are similar and they can be used interchangeably. Both phantoms were used to evaluate the coincidence of radiation and laser isocenters of a linear accelerator. End-to-end dosimetric tests were also performed using both an ion chamber and film. As part of the testing, both phantoms were also evaluated in terms of their efficiency of setup as well as the time required to switch inserts for different tests. Results showed that the laser/radiation isocenter coincidence as determined from each phantom was highly correlated. Ion chamber results were within 0.5% of the expected values. Gamma (2%, 2mm) pass rates of corresponding films were within 2% between phantoms. These results show that both phantoms are capable of producing equivalent results for the QA tests evaluated here.

Evaluation of alignment error due to a speed artifact in stereotactic ultrasound image guidance.

Ultrasound (US) image guidance systems used in radiotherapy are typically calibrated for soft tissue applications, thus introducing errors in depth-from-transducer representation when used in media with a different speed of sound propagation (e.g. fat). This error is commonly referred to as the speed artifact. In this study we utilized a standard US phantom to demonstrate the existence of the speed artifact when using a commercial US image guidance system to image through layers of simulated body fat, and we compared the results with calculated/predicted values. A general purpose US phantom (speed of sound (SOS) = 1540 m s(-1)) was imaged on a multi-slice CT scanner at a 0.625 mm slice thickness and 0.5 mm x 0.5 mm axial pixel size. Target-simulating wires inside the phantom were contoured and later transferred to the US guidance system. Layers of various thickness (1-8 cm) of commercially manufactured fat-simulating material (SOS = 1435 m s(-1)) were placed on top of the phantom to study the depth-related alignment error. In order to demonstrate that the speed artifact is not caused by adding additional layers on top of the phantom, we repeated these measurements in an identical setup using commercially manufactured tissue-simulating material (SOS = 1540 m s(-1)) for the top layers. For the fat-simulating material used in this study, we observed the magnitude of the depth-related alignment errors resulting from the speed artifact to be 0.7 mm cm(-1) of fat imaged through. The measured alignment errors caused by the speed artifact agreed with the calculated values within one standard deviation for all of the different thicknesses of fat-simulating material studied here. We demonstrated the depth-related alignment error due to the speed artifact when using US image guidance for radiation treatment alignment and note that the presence of fat causes the target to be aliased to a depth greater than it actually is. For typical US guidance systems in use today, this will lead to delivery of the high dose region at a position slightly posterior to the intended region for a supine patient. When possible, care should be taken to avoid imaging through a thick layer of fat for larger patients in US alignments or, if unavoidable, the spatial inaccuracies introduced by the artifact should be considered by the physician during the formulation of the treatment plan.