Evaluation of dosimetric variance in whole breast forward intensity modulated radiotherapy based on 4D CT and 3D CT.

Abstract

e13530 Background: To explore and compare the dosimetric variance in forward intensity modulated radiotherapy (IMRT) based on 4D CT and 3D CT after breast conserving surgery. Methods: Seventeen patients after breast conserving surgery underwent the 3D CT simulation scans followed by respiration-synchronized 4D CT simulation scans on the state of free breathing. The treatment plan constructed using the end inspiration (EI) scan was then copied and applied to the end expiration (EE) and 3D scans and the dose distribution was calculated separately. Dose–volume histograms (DVHs) parameters for the CTV, PTV, ipsilateral lung (IPSL) and heart were evaluated and compared. Results: The CTV volume amplitude was 11.93 ± 28.64 cm3, and volume of the CTV receiving 95%, 100%, and 103% prescription dose among different scans were all differed by < 0.4%. Mean PTV dose at EE was lower than EI (t = 2.87, p = 0.011), but there were no statistice significance between 3D CT scan and EI, EE scans (t = 1.06, -1.59; p = 0.304, 0.132). The homogeneity index (HI) at EI, EE, 3D plans were 0.156 ± 0.02, 0.162 ± 0.02, 0.161 ± 0.02, respectively, and difference only between EI and EE (t = -2.56, p = 0.021). The highest conformal index (CI) was at EI phase (t = 4.55, 2.70; p = 0.000, 0.016), and there was no significant difference between EE and 3D (t = 0.04, p = 0.967). The V20, V30, V40, V50 and Dmean of IPSL at EE phase were lower than EI (t = 2.39~5.54, p = 0.000~0.030). There were no significant differences in all the indexes for heart (t = -1.77~1.40, p = 0.128~0.693). Conclusions: The breast deformation during respiration may be disregarded in whole breast IMRT; PTV dose distribution was changed significantly between EI and EE phase, and the differentiation of the lung high dose area between EI and EE phase may induced by thorax expansion. 3D treatment planning is sufficient for whole breast forward IMRT, but 4D CT scans assist with respiratory gating ensure precise delivery of radiation dose.