Purpose/Objective(s): We report on PTV margins for breast cancer patients, while testing the hypothesis that arm positioning for treatment affects humeral head reproducibility.Materials/Methods: 14 post-mastectomy patients undergoing 3DCRT to a median dose of 50.4Gy plus 10Gy were studied for 134 filmed fractions. Chestwall (n=14) and regional lymph node (n=12) target volumes were contoured per RTOG atlas. Patients were immobilized with customized upper molds. Alignment was verified weekly using orthogonal kV imaging and online matching of bony landmarks and surgical clips when present. Treatment fields were also verified with MV portal imaging. To calculate PTV margins, initial couch shifts after kV imaging were analyzed. Offline matches were used to calculate 2D offset of the humeral head in the initial AP film. PTV margins were computed following van Herk’s methodology. Patients were positioned daily using skin marks and surface imaging. Arm position was quantified by centering a field on the humeral head and calculating the couch and gantry angles required to intersect the humeral length. For a humerus extended laterally, gantry = 90o and couch = 0o. As the arm is raised above the head, the couch angle increases.Results: Table 1 shows the calculated PTV margins assuming that no online corrections are made for daily treatments. A PTV margin > 1 cm is required to position the humeral head, a bony surrogate for nodal targets. We found positive correlations between BMI (17.3-51.7) and both the average 3D kV shifts (r=0.67) and their standard deviations (r=0.78). There was no correlation between BMI and average 2D positioning errors of the humeral head. There was a positive correlation (r=0.77) between average 2D errors in humeral head setup and the sum of the couch and gantry angles characterizing the arm position. Rotational adjustments of patients were required in 40% ±12% of filmed sessions.Conclusions: For 3DCRT, the use of a non-uniform PTV margin of 0.7cm AP and 1cm SI/LR should be considered to cover post-mastectomy targets in the absence of daily imaging. The margin should be increased to 1.5cm to ensure adequate nodal coverage. The correlation between arm positioning and 2D errors in humeral head positioning, which may be related to post-surgical discomfort, could be harnessed to reduce errors. Even with the use of a PTV margin, patient rotations would remain uncorrected during 40% of treatments by forgoing daily kV imaging. Table 1PTV margin (cm)Anterior-Posterior (AP)Superior-Inferior (SI)Left-Right (LR)Overall0.710.90.96Humeral Head1.581.33
Hubert, G,
IGRT in Post-Mastectomy Breast Cancer Patients Identifies Significant Rotations and Errors in Humeral Head Positioning. Radiological Society of North America 2014 Scientific Assembly and Annual Meeting, - ,Chicago IL.
http://archive.rsna.org/2014/14043751.html
