Dosimetric Advantages of Lumpectomy Cavity Guided Patient Positioning in Partial Breast Irradiation

Abstract

position were used in this study to compare dosimetric effects of commonly-used four image registration methods for patient repositioning: registration of the CT of the day with the planning CT based on the center of mass (COM) of lumpectomy cavity, and registration of the daily DRR (portal image for the day) with the planning DRR based on chest wall, surgical clips, or breast contours. AnIMRTplan with 5-9 beams wasgenerated for each patientsbasedon the planningCT. Thisplan was applied toeach daily CT to reconstruct the four plans for the four repositioning methods. Various dose-volume parameters including D95-PTV (dose covering 95% of PTV), D95-CTV and mean dose of the whole breast were compared. Results: The repositioning shifts consist systematic and random components (S, s) and were (4.5, 4.0), (3.2, 3.9), and (3.2, 4.5 mm), respectively, for clip, breast contour, and chest wall based alignments relative to those for the COM alignment. The average daily D95-PTVreductionfrom its planningvalue was1.0, 4.4, 4.0and 6.2%for COM, surgicalclips, breastcontour and chest-wall alignments respectively. The daily variation in the D95-PTV reduction was large for the DRR based methods, especially for the chest wall alignment, as compared to the COM method. Similar findings were obtained for D95-CTV. Conclusions: Comparing the various clinically-used image guidance methods in partial breast irradiation, the high-quality CT guided repositioning based on lumpectomy cavity is preferred for correcting interfractional setup errors and anatomic changes. The repositioning based on chest wall using 2D images should be avoided.