Optimization of CBCT-guided imaging schedules for left-sided postmastectomy radiotherapy using VMAT: A dosimetric, radiobiological, and cost analysis.

PURPOSE: This study aimed to determine an appropriate frequency of image-guided (IG) schedules for left-sided postmastectomy radiotherapy by comprehensively evaluating dosimetric, radiobiological, imaging dose, and imaging-related cost outcomes.

METHODS

A retrospective analysis of 20 patients treated with volumetric modulated arc therapy (VMAT) was conducted. Virtual CT images were generated by deforming the pCT to daily cone-beam computed tomography (CBCT) using deformable image registration. Accumulated dose, tumor control probability (TCP), normal tissue complication probability (NTCP), imaging dose, and imaging-related cost were compared among six IG schedules: no image-guided (NIG), weekly image-guided (WIG), twice-weekly image-guided (TIG), thrice-weekly image-guided (THRIG), initial 3 days then weekly image-guided (3D + WIG), and daily image-guided (DIG).

RESULTS

Increasing the frequency of IG schedules significantly improved planning target volume (PTV) dose coverage (D95) and TCP (P < 0.001), with DIG achieving optimal target dose delivery. Although no statistically significant differences were observed in dose to organs at risk (OARs) or NTCP, higher-frequency schedules showed a trend toward reduced variability. However, cumulative imaging dose increased linearly with the frequency of IG schedules (e.g., contralateral breast: 1.26-31.50 mGy; ipsilateral lung: 2.85-71.25 mGy), and imaging-related cost increased substantially from NIG to DIG, with differences of approximately 1.7- to 25-fold. THRIG demonstrated a favorable balance, with minimal deviations in PTV D95 (1.38%) and TCP (0.83%) relative to DIG, while reducing imaging dose and cost to approximately 60% of DIG.

CONCLUSION

While high frequency IG schedules improve target dose coverage, they are associated with increased imaging dose and imaging-related cost. Within the CBCT-guided VMAT workflow evaluated in this study, THRIG may provide a balanced trade-off between treatment precision, safety, and cost.