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

Purpose: This study investigates (i) the effect of verification protocols on treatment accuracy and PTV margins for partial breast and boost breast radiotherapy with short fractionation schema (15 fractions), (ii) the effect of deformation of the excision cavity (EC) on PTV margin size, (iii) the imaging dose required to achieve specific PTV margins. Methods and materials: Verification images using implanted EC markers were studied in 36 patients. Target motion was estimated for a 15 fraction partial breast regimen using imaging protocols based on on-line and off-line motion correction strategies (No Action Level (NAL) and the extended NAL (eNAL) protocols). Target motion was used to estimate a PTV margin for each protocol. To evaluate treatment errors due to deformation of the excision cavity, individual marker positions were obtained from 11 patients. The mean clip displacement and daily variation in clip position during radiotherapy were determined and the contribution of these errors to PTV margin calculated. Published imaging dose data were used to estimate total dose for each protocol. Finally the number of images required to obtain a specific PTV margin was evaluated and hence, the relationship between PTV margins and imaging dose was investigated. Results: The PTV margin required to account for excision cavity motion, varied between 10.2 and 2.4 mm depending on the correction strategy used. Average clip movement was 0.8 mm and average variation in clip position during treatment was 0.4 mm. The contribution to PTV margin from deformation was estimated to be small, less than 0.2 mm for both off-line and on-line correction protocols. Conclusion: A boost or partial breast PTV margin of ∼10 mm, is possible with zero imaging dose and workload, however, patients receiving boost radiotherapy may benefit from a margin reduction of ∼4 mm with imaging doses from 0.4 cGy to 25 cGy using an eNAL protocol. PTV margin contributions from deformation errors are likely to be small in comparison to other sources of error, i.e., set up or delineation.