Purpose: To validate the dosimetric robustness of surface guided radiation therapy (SGRT) for breast radiotherapy using cone-beam CT (CBCT) dose calculations.
Methods: A retrospective sample of 15 breast radiotherapy patients (40-50 Gy in 15-25 fractions) was selected for this analysis. All patients were set up using SGRT (VisionRT, London, UK) with weekly CBCT verification (chest wall alignment, 3DoF shifts, >75 CBCT datasets). The online matched CBCT images were imported into the treatment planning system (Raystation 8A) to compute CBCT fraction doses using a previously validated framework. Fraction doses with SGRT alone were simulated by repeating all CBCT dose calculations without any couch shifts. The differences between the doses calculated after CBCT and simulated SGRT alignments were evaluated to validate the dosimetric feasibility of using SGRT alone for setup. Observed trends between dose differences and CBCT shifts were assessed to determine minimum thresholds for CBCT shifts and SGRT monitoring.
Results: The majority (>70%) of CBCT shifts were <5 mm. Minimal changes were observed between the CBCT and SGRT-aligned dose distributions (breast ΔD95 < 1% for > 60% of fractions, < 3% for > 80% of fractions).
Conclusion: The differences between the CBCT and SGRT-aligned doses increased with the magnitude of CBCT shifts and were minimal for small shifts (breast ΔD95 <1% for shifts <3 mm, <3% for shifts <5 mm), which supports the use of 3 mm thresholds for CBCT matching and SGRT monitoring. The use of SGRT alone was also found to be dosimetrically feasible for breast radiotherapy based on minimal degradation of delivered dose with SGRT alone (breast D95 > 95% or ΔD95 < 3% for > 95% of fractions).