Room: AAPM ePoster Library
Purpose: Hypofractionated whole-breast radiation therapy (WBRT) is widely used to manage early stage post-lumpectomy breast cancer. Halcyon 2.0 offers low dose (<1mGy CTDIvol) cone-beam CT, which is used for daily image-guidance for WBRT at our institution. In this study, volumetric online images were used to quantify the dose deviations in patients receiving WBRT.
Methods: Ten patients undergoing supine-position tangential beam WBRT treated on Halcyon 2.0 were retrospectively studied. Planning CT was deformed to daily CBCT to generate synthetic CT using MIM Maestro v6.9. Synthetic CT images were then imported into Varian Eclipse v15.6, where a daily PTV_Eval contour was propagated rigidly and then cropped 0.5cm from the skin, consistent with original PTV generation. The original plan was applied to daily synthetic CT via online registration used in treatment and recalculated to provide daily dose. Absolute dosimetric quantities, Dmax, D95%, V90%, and V105%, as well as their deviation from planned (?) were measured. These were compared to couch shifts and changes in patient weight and PTV volume between fractions.
Results: We found that the metrics of target coverage, D95% and V90%, consistently fell within clinically acceptable limits for a majority of the treatments (98.8% and 100% of 160 fractions acceptable, respectively), while metrics of hotspots, Dmax and V105%, deviated moderately (87.5% and 78.8% acceptable, respectively). Delivered dose hotspot V105% was larger than planned (PTV_Eval ?V105%[%]=3.88±3.42%, p<0.0001), while target coverage (D95% and V90%) was indistinguishable from planned (p>0.05). Some patients were more affected by hotspots than others, with 3/10 patients having V105% greater than acceptable (>15%) for >50% of their treatments.
Conclusions: Daily target coverage during WBRT on Halcyon 2.0 was sufficient and consistent. However, hotspots within the breast, which could cause undesirable cosmesis outcome, vary substantially day-to-day and patient-to-patient. Study suggests that online adaptive WBRT may offer benefit for select patients.