SGRT for set up followed by IGRT with cone-beam computed tomography (CBCT) is used in our clinic as a standard technique for patients (pts) receiving SBRT to the lung. SGRT is also utilized for continuous monitoring during treatment to detect intrafraction motion during SBRT. We previously reported the reliability of SGRT in detection and quantification of intrafraction motion for SBRT treatments in the lung. The purpose of this study is to analyze the potential dosimetric effects on target coverage and spinal cord dose of observed intrafraction motion. An IRB approved retrospective chart review was conducted to evaluate IGRT shift information for 58 pts with malignant thoracic tumors treated with SBRT. SGRT was utilized for patient set up and CBCT was taken for volumetric ITV matching. After a new SGRT reference image was captured, pts were then monitored continuously with SGRT during treatment. If an intrafractional shift in any direction >2mm for longer than 2 seconds was detected by SGRT, then the treatment was stopped, CBCT was repeated, and any IGRT shifts were recorded. Of the 25 pts and 34 fractions that had detected intrafractional shifts, the 11 highest shift quantities were analyzed for potential dosimetric effects on target (ITV and PTV) coverage and minimum dose, as well as max spinal cord dose. Paired t-tests were used to compare the differences in these parameters and were considered significant if the p-value was less than α=0.05. In addition, plans were recreated for PTV margins of 2mm and 4mm to analyze the potential effects of intrafraction motion with reduced PTV margins. 11 shifts during SBRT treatments of the lung were included in the analysis. The mean vector intrafraction shift was 0.51 cm. Mean PTV coverage was significantly reduced due to intrafraction motion (95% vs. 88.2%, p=<0.001). Mean PTV minimum dose was lower (4509 cGy planned vs. 4192 cGy, p=0.017). Mean ITV coverage was not changed due to intrafraction motion and remained at 100%. Mean ITV minimum dose was also not significantly reduced (5944 cGy vs. 5785, p=0.078). Mean spinal cord maximum dose was not significantly changed (990 cGy vs. 987.8 cGY, p=0.864). As PTV margins decreased from 5mm to 2mm, ITV coverage started to decline but remained >95%. SGRT is valuable in detecting potentially clinically meaningful intrafraction motion that may result in decreased PTV coverage if not accounted for during treatment. Accurate treatments with reduced PTV margins may be achievable when continuous monitoring or imaging of pts is performed.

Triggered imaging is a tool used to automatically detect intrafraction motion of implanted fiducial markers. kV images are taken with a defined frequency and the beam is gated if the fiducials move out of the set tolerance; if the beam is held, additional images are taken to determine if the patient needs to be shifted. The objective of this study was to determine the dosimetric effects of intrafraction shifts detected by triggered imaging during prostate SBRT, as well as investigating the effect of PTV margin reduction. 9 prostate SBRT patients were treated with 1-2 VMAT arcs, each receiving 3625cGy/5fx. A planar kV image was acquired every 15 seconds. If the fiducial positions were more than 3mm (our CTV to PTV margin) from their expected positions, the software automatically gated the beam and either a kV pair (8/9 patients) or a CBCT was done to reposition the patient. These IGRT shifts were reviewed retrospectively and each patients' largest shifts in a single day were recorded; 3D vector shifts ranged from 0.9mm to 12.6mm (rotations were ignored). The shifts were used to move isocenter in the planning software and recalculate dose for 2 scenarios: 1. Assuming only 1 fraction was affected and 4 were treated accurately (1:4 plans), and 2. Assuming all 5 fractions were affected by this worst-case magnitude shift (5fx plans). Next, PTV margins were reduced from 3mm (1 patient had 5mm initially) to 2mm and 1mm and similar evaluations were done with the shifted isocenters. Finally, robustness was tested by normalizing the original plans such that the CTV in the shifted plans received at least the original plan's coverage and evaluating the effect on normal tissues. The mean V100 PTV was significantly reduced for 2 and 3mm margins for the 1:4 plans (95.9% vs 89.4 and 89.1% with p=0.020 and p=0.004 for 2 and 3mm margins, respectively). For the 5 fx plans, the mean V100 PTV and the D100 CTV were significantly reduced from the original plan, regardless of the margin used (p=0.004). As the margins decreased, the dosimetric impact of the shifts on V100 PTV for 1:4 plans decreased; there was no trend with the targets coverage for margin reduction otherwise. Interestingly, the normal tissue effects were not as dramatic: only the urethra in the 5 fx plans showed a significant difference in maximum dose. For the robustness plans, only in the 5 fx original margin plans were the urethra max dose and the rectal volume receiving 1810cGy significantly different. Triggered imaging is a useful technology for ensuring accurate prostate SBRT treatments. Intrafraction monitoring with triggered imaging can prevent loss of target coverage and unnecessary additional dose to normal tissues and may allow for margin reductions for SBRT prostate plans. In addition, it may be possible to preemptively increase the robustness of plans without adversely affecting normal tissues even in the presence of intrafraction motion.

The value and importance of peer review in radiation oncology has been recognized in the field, but published studies focus on physician peer review of contouring and plan goals. The Radiation Oncology Incident Learning System (ROILS) aggregate reports indicate that the most frequently identified workflow step where events occur is treatment planning. To our knowledge, there is no published report describing the implementation and analysis of a dosimetrist peer review process. The purpose of this study is to examine the effectiveness of a dosimetrist peer review implemented within a commercially available record and verify system. A checklist/questionnaire was created within our R&V system containing commonly verified plan parameters and tasks. In our clinic, the peer review is first completed by a dosimetrist who did not create the plan and then is double checked by a physicist during the initial plan check. Items that are identified as needing correction are noted first by the peer review dosimetrist and then any remaining items are noted by the physicist. A report is generated that contains all the identified items, which are then categorized and analyzed. We analyzed the items that were caught during the peer review and the items that were caught during the subsequent physicist chart check. For calendar year 2016, dosimetrists made 121 total entries in the peer review questionnaire while physicists made 370 total entries, but each entry could contain more than one item needing correction. While the dosimetrist peer review was effective at identifying incorrect charges, the number of items detected by physicists for other categories outnumber those detected by dosimetrists by a margin of 3 to 1. The table below contains the top 5 most identified items by the dosimetrists and physicists. If billing is removed, then the 5th most detected item by dosimetrists is errors in shifts from the simulation isocenter. Dosimetrists performed peer reviews on 84% of new patient charts for 2016. Reasons for a missing peer review include dosimetrist time off and urgent sim and start patients.Abstract 3310RankDosimetristPhysicist1BillingDRR Issue2Setup notes2nd MU Calc Issue3ContouringTolerance Table4Prescription IssuePrescription Issue5Tolerance TableImaging instructions Open table in a new tab We believe this is the first reported implementation and analysis of a formal dosimetrist peer review. While the dosimetrist peer review allowed 75% of the total items through, it was effective at catching certain items, such as billing errors and errors in setup notes. As this is our first analysis of the dosimetrist peer review, this data will serve as our baseline for future comparisons. These results have been shared with the dosimetry staff and additional guidance has been given to improve the detection rate of the dosimetry peer review. We will report updated results at the annual meeting.

OSMS followed by IGRT with cone-beam computed tomography (CBCT) is used in our clinic as a standard technique for initial setup of patients (pts) receiving SBRT. OSMS is also utilized for continuous monitoring during treatment to detect intrafraction motion during SBRT. The purpose of this study is to analyze daily patient shift "deltas" determined during the initial phase of the IGRT process to assess the accuracy of OSMS at limiting interfraction patient set up error during SBRT and to measure the reliability of OSMS in detection and quantification of intrafraction motion for SBRT treatments of the lung and abdomen. An IRB approved retrospective chart review was conducted to evaluate IGRT shift information for 58 pts with malignant thoracic or abdominal tumors treated with SBRT. A 2-step IGRT procedure was used for all reviewed therapy fractions- an initial setup using OSMS followed by CBCT for more accurate soft tissue and tumor volumetric localization. The "delta", represented by the additional CBCT translational and rotational shift measurements after initial setup from each treatment was recorded to assess interfraction setup error. After a new reference image was captured, pts were then monitored continuously with OSMS during treatment. If an intrafractional shift in any direction >2mm for longer than 2 seconds was detected by OSMS, then the treatment was stopped, and CBCT was repeated and the recorded deltas were compared to those detected by OSMS. A paired t-test was used to compare the difference in vector delta between the methods and considered significant if the p-value was less than α = 0.05. 58 pts were identified with 71 lesions. The majority of lesions were located in the lung. Additional interfractional deltas after OSMS setup and soft tissue match with CBCT were small in all directions with mean values of 0.29 cm vertically, 0.41 cm in the longitudinally, 0.22 cm laterally, 0.44 degrees pitch, 0.40 degrees roll, and 0.39 degrees rotation. Additionally, 13 pts had intrafraction deltas during treatment detected with OSMS where repeat CBCT was performed. This resulted in 11 (84.6%) clinically meaningful additional shifts of at least 2 mm on subsequent CBCT. When comparing the vector delta for each method, no significant difference was found between the average delta detected by OSMS and the average delta detected by CBCT (0.30 cm versus 0.32 cm, p=0.608). OSMS provides a means of ensuring accurate initial setup of SBRT prior to volumetric IGRT with limited additional interfractional shifts. In addition, continuous monitoring with OSMS during treatment was valuable in detecting potentially clinically meaningful intrafraction motion and was comparable in magnitude to additional CBCT imaging suggesting good correlation. Additional data is being acquired to provide correlative statistics for detected intrafraction motion in each direction.

Matching of orthogonal kV planar x-ray images is used in our clinic as a standard image guided radiation therapy (IGRT) technique for initial setup of patients receiving stereotactic body radiation therapy (SBRT). 3D surface-based matching to a reference dataset, in contrast to kV/kV matching, is performed without the delivery of ionizing radiation. 3D surface mapping can also be utilized in situations in which kV images cannot be obtained due to collisions with the patient or the treatment couch. The purpose of this study is to analyze daily patient shift "deltas" determined during the initial phase of the IGRT process to determine if 3D surface mapping provides comparable accuracy for initial setup of SBRT patients to that provided by kV/kV matching. An IRB-approved retrospective chart review was conducted to evaluate IGRT shift information for 46 patients with a pathologic or clinical/radiographic diagnosis of malignant thoracic or abdominal tumor and treated with SBRT. A 2-step IGRT procedure was used for all reviewed therapy fractions for accurate patient setup—an initial setup using an alternating scheme of kV/kV matching or 3D surface mapping followed by cone beam computed tomography (CBCT) for more accurate soft tissue and tumor volumetric localization. The "delta," represented by the additional CBCT translational and rotational shift measurements recorded after initial setup from each treatment, were compared for each of the methods. Linear mixed models for repeated measures were used to analyze the rotational and translational shift measurements. Measurements were modeled with main effects for method of setup and temporal treatment period, as well as a lesion-specific identifier as a random factor to account for within-lesion correlation. A total of 46 patients were identified with 58 lesions. The majority of lesions were located in the lung (69.0%), followed by liver (17.2%), adrenal (5.2%), spine (1.7%), and other sites (6.9%). Except for the longitudinal direction (P=0.019), there was no significant association between method of setup and the shift outcomes. For the longitudinal shifts, the kV/kV method resulted in smaller CBCT shifts on average (0.28 cm) than the 3D surface mapping method (0.41 cm). However, the larger estimated shift for the 3D surface mapping method is still much smaller than the clinical action level of 0.8 cm for which a repeat IGRT study would typically be required at our institution. 3D surface mapping provides a means of ensuring accurate initial setup of SBRT patients that is comparable to that provided by our clinical standard IGRT method, kV/kV matching. The effect of baseline patient and disease characteristics (e.g., BMI, site and location of tumor) on the association between quantitative shift outcomes and set up method was also investigated and will be presented.