Author: Charmainne Cruje, Maria Dumol, Nawroz Fatima, Marisa Finlay, Kalaina Johnson, Raman Mohla, Jasleen Uppal 👨🔬
Affiliation: Trillium Health Partners, Carlo Fidani Regional Cancer Centre 🌍
Purpose: To evaluate the robustness of a breast SBRT protocol in achieving target coverage by utilizing online- and retrospectively-matched CBCT-to-CT images.
Methods: The first pilot patient was scanned head-first supine under deep inspiration breath hold. The patient has a metastatic 26-cc CTV centered in the upper quadrants of the right breast. A 5-mm expansion was applied to the CTV, resulting in a 59-cc PTV. A VMAT plan was prepared to deliver 40 Gy in 5 fractions using two 6 MV partial arcs. The plan was accepted based on dose coverage and OAR DVH requirements meeting a previously published dose table and passing patient specific QA results.
Daily setup CBCT images were acquired, followed by couch shifts and rotations. Post-shift images were also acquired. All images were checked for Hounsfield unit accuracy.
The CTV was contoured in the CBCT images. Daily setup images were retrospectively matched offline by three radiation therapists, by matching the visible lesion in the CBCT to CT. The dose delivered to the target volume was calculated using retrospectively-matched and post-shift CBCT images.
Results: The CTV coverage requirement of V100%Rx > 95% was achieved according to all acquired CBCT images. Slice-by-slice isodose line evaluation demonstrate marginal coverage of the CTV in the superior aspect of the lesion. In other directions, the PTV expansion of 5 mm proved to be adequate. CT images acquired four months post-radiation show a significant reduction of the lesion, from a maximum of 4 cm in axial slices down to 9 mm.
Conclusion: For our first pilot breast SBRT patient, sufficient coverage to the target volume was achieved with our preliminary planning and treatment protocols. Follow up imaging four months post-radiation demonstrated promising local control of the lesion. The same evaluation will be done in additional cases, to be presented in the conference.