Radiation Physics

PV QA 3 - Poster Viewing Q&A 3

TU_10_3216 - Can Spine SBRT Patients Shrug Off Their PTV Coverage?

Tuesday, October 23
1:00 PM - 2:30 PM
Location: Innovation Hub, Exhibit Hall 3

Can Spine SBRT Patients Shrug Off Their PTV Coverage?
V. Sarkar1, L. Huang1, S. Lloyd1, A. Paxton1, and B. J. Salter2; 1University of Utah Huntsman Cancer Institute, Salt Lake City, UT, 2Huntsman Cancer Hospital, University of Utah, Salt Lake City, UT

Purpose/Objective(s): To investigate how shoulder position can affect PTV coverage in patients undergoing SBRT treatment of bony mets to the C-spine.

Materials/Methods: We randomly chose a patient treated for a Head and Neck malignancy with CT-on-rails (CTOR) image guidance. Given the patient was scanned with CTOR during treament, we had multiple high-quality fan-beam CT datasets of the same patient in treatment position. An attending physician contoured the C5, C6 and C7 vertebral bodies to mimic three PTVs as if the patient had bony mets to those targets. A separate plan was created in a treatment planning system for each PTV using two co-planar VMAT arcs and the RTOG 0631 planning criteria. The plans were normalized to deliver 16Gy to 90% of the target. Each of the CTOR datasets obtained for IGRT purposes was then fused to the planning image-set within the treatment planning system. The fusion was box-based and focused on the target and one vertebral body superior and inferior using the bone window. Once the fusion was checked and deemed appropriate, the original structure set was copied onto the CTOR dataset and all three treatment plans were recalculated on the new images using the original MUs from each plan (3 calculations per CTOR dataset). This was repeated for ten CTOR sets and DVH metrics evaluated included PTV coverage (V16Gy) as well as V10Gy for the partial cord for each target.

Results: For the C5 target, the average V16Gy was 93.8% (Range: 90.9%-95.7%). For the C6 PTV, the average V16Gy was 87.9% (Range: 75.2%-94.3%). For the C7 PTV, the average V16Gy was 85.7% (Range: 77.2%-91.0%). The dose covering 90% of the PTV for each of these cases averaged 16.33 Gy (range 16.06 - 16.57 Gy), 15.96 Gy (range 15.61 - 16.21 Gy) and 15.75 Gy (range 15.31 - 16.07 Gy) for the C5, C6 and C7 targets respectively. All of the partial cord doses met the RTOG 0631 criteria.

Conclusion: Our results show that the PTV coverage was very adversely affected in several cases. When reviewing the fusion results, it was obvious that most of this difference in coverage was due to slightly different shoulder positions. The fusion process was used to mimic the very focused fusion done on the vertebral bodies around the target without necessarily paying much attention to the shoulder but that sometimes lead to more shoulder (or humeral head) being within the treatment field. These sub-optimal geometries may very well have been the one on treatment day for that patient given our patients with C-level SBRTs are typically only immobilized with a mask, just as this patient was. If accepted we will present further results from the same type of analysis using static-gantry IMRT plans as well as an investigation of whether a less target-focused fusion strategy can lead to a smaller loss of target coverage. We will also extend all analyses to include more patients.

Author Disclosure: V. Sarkar: None. L. Huang: None. S. Lloyd: Honoraria; Sirtex. B.J. Salter: Honoraria; RO-HAC. Advisory Board; RO-HAC.

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