How does imaging frequency and soft tissue motion affect the PTV margin size in partial breast and boost...

Area of Science:

• Radiation Oncology
• Medical Physics

Background:

• Accurate targeting is crucial in partial breast and boost radiotherapy.
• Short fractionation schemas (15 fractions) require precise planning target volume (PTV) margins.
• Excision cavity (EC) deformation and motion introduce uncertainties in radiotherapy delivery.

Purpose of the Study:

• To assess the impact of verification protocols on PTV margins and treatment accuracy in short-course breast radiotherapy.
• To quantify the effect of EC deformation on PTV margin size.
• To determine the imaging dose necessary for achieving specific PTV margins.

Main Methods:

• Investigated 36 patients using implanted EC markers for verification imaging.
• Estimated target motion using on-line and off-line motion correction strategies (NAL, eNAL protocols).
• Calculated PTV margins based on motion and deformation data, and estimated imaging doses.

Main Results:

• PTV margins varied from 2.4mm to 10.2mm based on correction strategy.
• Average clip displacement was 0.8mm, with daily variation of 0.4mm.
• Deformation contributed less than 0.2mm to the PTV margin.

Conclusions:

• A PTV margin of ~10mm is feasible with minimal imaging dose.
• The extended No Action Level (eNAL) protocol may reduce PTV margins by ~4mm with doses of 0.4-25cGy.
• Deformation errors have a minor impact on PTV margins compared to setup or delineation errors.