Image-Guided Study of Interfraction and Intrafraction Set-up Variability and Margins in Reverse Semi-Decubitus Breast Radiation Therapy

Abstract

Reverse semi-decubitus (RSD) technique could reduce cardiac dose during breast radiotherapy for patients unable to tolerate breath hold. This study aimed to report the interfraction and intrafraction set-up variability for RSD technique and to determine an appropriate clinical tumor volume (CTV) to planning target volume (PTV) margin to account for motion and positional uncertainties. Twenty-five consecutive patients were prospectively enrolled in the protocol of RSD breast radiotherapy. The prescribed dose was 40 Gy in 15 fractions delivered by opposed tangential field or volumetric arc therapy with 7-mm PTV margin. The pre- and post-fraction cone-beam CT (CBCT) scans were acquired at fractions 1, 4, 7, 8, 11 and 14. CBCT data were matched to planning CT data using (i) chest wall and (ii) clips. Mean displacement (MD), systematic (Σ), and random (σ) translational errors in 3-dimensions (LR: left-right; AP: anterior-posterior; and CC: cranial-caudal) were analyzed. Positive values for LR, AP, and CC pointed at motion in right, anterior, cranial directions, respectively. Body weight and body mass index (BMI) were recorded to examine predictive factors for set-up variability. The CTV to PTV margins were calculated based on the van-Herk formula (2.5Σ + 0.7σ). A total of 300 CBCT scans were analyzed for 25 patients. The population MD, systematic, and random errors based on each of chest wall and clip matches were shown in the Table 1. The calculated interfraction and intrafraction CTV to PTV margins were 8.8/12.9/11.2 and 8.7/13.5/11.4 mm (LR/AP/CC) for the chest wall and clip matches, respectively. A correlation was observed between interfraction setup variability in the AP direction and body weight (R2 = 0.32; p = 0.004) and BMI (R2 = 0.61; p < 0.001). To calculate the AP margin for patients with heavier body weight and higher BMI, we divided the patients into two groups for each median value. The AP margins were 9.2 and 8.4 mm for patients with lighter body weight, and lower BMI, respectively. The AP margins were 11.6 and 10.9 mm for patients with heavier body weight and higher BMI, respectively. This is the first study to determine the PTV margins for interfraction and intrafraction set-up variability for reverse semi-decubitus breast radiotherapy using pre- and post-fraction CBCT scans. A uniform 7-mm PTV margin is insufficient in the absence of CBCT. Table 1 Mean displacement (MD), systematic (Σ) and random (σ) translational errors, and calculated margins in 3-dimensions for chest wall and clip based cone-beam CT versus planning CT matches in reverse semi-decubitus positions (mm).Abstract 272; TableChest wall matchClip based matchInterfractionIntrafractionInterfractionIntrafractionLRMD0.3-0.20.7-0.2Σ1.90.91.80.9σ1.71.11.61.1Margin5.92.95.73.0APMD-2.3-0.5-1.6-0.6Σ3.30.93.61.0σ1.81.41.61.4Margin9.53.410.13.4CCMD0.80.30.40.3Σ2.51.02.60.9σ2.21.52.11.4Margin7.73.58.13.3 Open table in a new tab