Enhancing inter-fraction set-up accuracy and margin minimization (E-SAMM): a comparison of maskless surface-guided and mask-based laser alignment positioning in pelvic radiotherapy.

PURPOSE: Surface-guided radiation therapy (SGRT) is a non-invasive, tattoo-less technique that enables real-time optical surface monitoring during radiotherapy. This prospective E‑SAMM study compared inter-fraction set-up accuracy, estimated PTV margin requirements derived from CBCT-based pre-correction setup deviations, and set-up time between maskless SGRT and mask-based setup in patients receiving pelvic radiotherapy. METHODS: One hundred patients with pelvic malignancies (cervical, endometrial, rectal, and prostate cancers) undergoing radical or neoadjuvant radiotherapy (45-68 Gy delivered in 25-28 fractions, five fractions per week) were prospectively enrolled. A total of 2376 fractions were analysed. Patients were randomised to either maskless SGRT with AlignRT, knee support, and intrafraction monitoring (Group A, n = 50) or six-clamp mask-based laser alignment with image-guided radiotherapy (Group B, n = 50). All patients underwent online cone-beam computed tomography (CBCT) verification on a TrueBeam SVC linear accelerator with a six-degree-of-freedom couch. Inter-fraction translational (medio-lateral [ML], cranio-caudal [CC], antero-posterior [AP]) and rotational set-up errors were derived from CBCT corrections. PTV margin estimates were calculated using the van Herk formula from CBCT-recorded pre-correction setup deviations. Statistical comparisons were performed using analysis of variance and the Mann-Whitney U test. RESULTS: SGRT demonstrated significantly improved translational accuracy in the ML (1.97 ± 1.54 mm vs 2.63 ± 2.06 mm, P = 0.03) and CC (3.13 ± 2.34 mm vs 4.03 ± 2.67 mm, P = 0.001) directions compared with mask-based alignment, whereas larger shifts were observed in the AP direction (P = 0.001). Rotational differences were not statistically significant (P = 0.14). The mean three-dimensional vector displacement was significantly lower with SGRT (1.91 ± 0.36 mm) than with mask-based positioning (2.26 ± 0.48 mm; P < 0.05). Estimated PTV margins were smaller with SGRT in all translational directions. Mean set-up time was significantly reduced with SGRT (2.51 ± 0.81 min vs 3.55 ± 0.99 min, P = 0.007). CONCLUSIONS: Maskless SGRT provided improved inter-fraction set-up accuracy, lower estimated PTV margin requirements based on setup deviations, and shorter set-up times compared with mask-based laser alignment for pelvic radiotherapy.