Simultaneous integrated boost in breast conserving radiotherapy: is replanning necessary following tumor bed change?

Tumor bed (TB) change is often seen during the whole breast irradiation (WBI) in early breast cancer patients. The aims of this study were to evaluate change in seroma volume on repeat CTs and to explore whether replanning is necessary in breast conserving radiotherapy (RT) using the intensity modulated radiotherapy (IMRT) with simultaneous integrated boost (SIB). Thirty patients underwent WBI with 84 CT scans (24 with 3 CTs (CT1, CT2 and CT3) and 6 without CT3) during the 6 weeks of RT were reviewed. TB and other target volumes on all CTs were delineated and compared. IMRT-SIB treatment plans with 50.68 Gy to the whole breast and 64.4 Gy to the boost in 28 fractions were constructed in the CT1. Replan and hybrid plan (without replanning) in the CT2 were reproduced. Dosimetric differences between the replans and hybrid plans were also compared. The mean TB volumes for CT1, CT2 and CT3 were 42.1 cm(3), 20.1 cm(3), 17.0 cm(3), respectively. The mean TB volume reduction was 40.5% from the CT1 to CT2 and 4.3% from the CT2 to CT3. The difference of TB volumes between the CT1 and CT2 was statistically significance (p < 0.001), but not significant between the CT2 and CT3 (p = 1.000). For all patients, target coverage remained adequate with either hybrid plans or replans. However, replanning can significantly decrease the whole breast mean dose (35.2 vs. 35.6 Gy, p = 0.026) and breast volume outside the boost receiving 95% of the boost prescribed dose (39.5 vs. 68.2 cm(3), p < 0.001). In summary, boost volume could irradiate adequately after the significance shrinkage of tumor bed with the fractionated schedule of IMRT-SIB, but replanning on a second CT could avoid the undesired high dose irradiated breast tissue outside boost.