EQD2 Dose Accumulation Based on Deformable Image Registration of HR-CTV, Bladder, Rectum and Bowel for Combined External-Beam Radiotherapy and Brachytherapy for Cervical Cancer

PURPOSE/OBJECTIVE(S) Evaluate the accuracy of DVH parameter summation between brachytherapy (BT) and external-beam radiotherapy (EBRT) for cervical cancer, by comparing deformable image registration (DIR) EQD2 dose accumulation. DVH summation of dosimetric parameters can overestimate cumulative organ at risk (OAR) doses due to daily variations in relative OAR position. MATERIALS/METHODS Ten patients were studied. Nine out of 10 patients received BT+EBRT. All women were treated with intracavitary tandem and ring BT. In phase I, CT simulation images from each BT fraction were rigidly fused with first fraction BT images, (BT1) focusing on the applicator and uterus. Hybrid-based DIR was then conducted using MIM software. Contours used for DIR were CTV_HR, bladder, rectum and bowel space. For phase II, EBRT and BT1 utero-cervix contours were rigidly fused, followed by contour-based DIR using utero-cervix, bladder and rectum. Each BT fraction and sum EBRT dose was accumulated onto BT1 and converted to EQD2 (Gy) via the linear quadratic equation with a/b of 10 for tumor and 3 for OARs. The absolute dose differences were reported for D0.1cc, D1cc and D2cc (OARs) and D90% (CTV_HR). The dice similarity coefficient (DSC) of all contours was reported. For phase I, the D2cc isodose level and OAR overlap structure was assessed for each BT fraction and the initial and deformed D2cc reported. The mean distance-to-agreement (MDA) and DSC of D2cc were reported for DIR accuracy assessment. RESULTS For phase I, average DSC values for CTV_HR, bladder, rectum and bowel were 0.84 ± 0.07, 0.86 ± 0.08, 0.82 ± 0.05, 0.81 ± 0.07 and average DSC of initial vs. deformed D2cc were 0.88 ± 0.02, 0.89 ± 0.02, 0.88 ± 0.03 respectively. Average D2cc MDA values were 0.28 ± 0.04, 0.29 ± 0.05, 0.33 ± 0.06 for the bladder, rectum and bowel, respectively with absolute D2cc differences in Gy of 0.04 ± 0.04, 0.01 ± 0.01, and 0.03 ± 0.03. For CTV_HR, absolute D90 difference in EQD2 was 2.69 ± 1.98 Gy. For bladder, rectum and bowel, absolute EQD2 differences in Gy for D0.1cc were 5.57 ± 3.73, 2.78 ± 2.27, 5.10 ± 2.70 respectively. For D1cc, values in Gy were 3.27 ± 1.80, 1.62 ± 1.04, and 4.20 ± 2.51. For D2cc, 2.42 ± 0.80 Gy, 1.16 ± 0.72 Gy, and 2.83 ± 2.23 Gy. For phase II, average DSC values for utero-cervix, bladder and rectum were 0.82 ± 0.12, 0.80 ± 0.08, and 0.82 ± 0.17 respectively. Average absolute EQD2 difference in Gy for CTV_HR D90 was 7.43 ± 3.10 and 12.87 ± 6.84 and 6.02 ± 4.80 respectively for bladder and rectum D0.1cc. For D1cc and D2cc, absolute EQD2 differences in Gy were 10.46 ± 5.10 and 8.10 ± 4.48 respectively for bladder and 3.84 ± 2.95 and 3.21 ± 2.17 for rectum. CONCLUSION Arithmetic DVH addition over-estimated OAR D2cc in 7 of 10 patients. D2cc overestimation of bladder occurred in 6 of 9 patients and 7 of 9 patients for rectum. Our results show good estimation of accumulated dose with DVH parameter addition for both BT only and BT + EBRT.