Can surface imaging improve the patient setup for proton postmastectomy chest wall irradiation?

Purposes/objectives: For postmastectomy radiation therapy by proton beams, the usual bony landmark based radiograph setup technique is indirect because the target volumes are generally superficial and far away from major bony structures. The surface imaging setup technique of matching chest wall surface directly to treatment planning computed tomography was evaluated and compared to the traditional radiograph-based technique.

Methods And Materials: Fifteen postmastectomy radiation therapy patients were included, with the first 5 patients positioned by standard radiograph-based technique; radiopaque makers, however, were added on the patient's skin surface to improve the relevance of the setup. AlignRT was used to capture patient surface images at different time points along the process, with the calculated position corrections recorded but not applied. For the remaining 10 patients, the orthogonal x-ray imaging was replaced by the AlignRT setup procedure followed by a beamline radiograph at the treatment gantry angle only as confirmation. The position corrections recorded during all fractions for all patients (28-31 each) were analyzed to evaluate the setup accuracy. The time spent on patient setup and treatment delivery was also analyzed.

Results: The average position discrepancy over the treatment course relative to the planning computed tomography was significantly larger in the radiograph only group, particularly in translations (3.2 ± 2.0 mm in vertical, 3.1 ± 3.0 mm in longitudinal, 2.6 ± 2.5 mm in lateral), than AlignRT assisted group (1.3 ± 1.3 mm in vertical, 0.8 ± 1.2 mm in longitudinal, 1.5 ± 1.4 mm in lateral). The latter was well within the robustness limits (±3 mm) of the pencil beam scanning treatment established in our previous studies. The setup time decreased from an average of 11 minutes using orthogonal x-rays to an average of 6 minutes using AlignRT surface imaging.

Conclusions: The use of surface imaging allows postmastectomy chest wall patients to be positioned more accurately and substantially more efficiently than radiograph only-based techniques.