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Unlike other body sites, extremities patient positioning is challenging due to lack of comprehensive immobilisation devices as well as familiarity of the staff experience. We would like to investigate if the newly installed SGRT system could be clinically useful for extremities patients in terms of accuracy and PTV margins reduction.

24 random patients who were treated upper or lower extremities between September 2020 and August 2021 were analysed retrospectively. 12 patients in the control group (Group A) used the traditional setup method: lasers and skin markings (one anterior and two lateral). While the other 12 patients in the experimental group (Group B) used SGRT system (AlignRT, Vision RT) for patient setup. In Group B, all patients were aligned within 1mm and 1° tolerance. A total of 209 and 168 CBCT images were acquired for Group A and B respectively. Both translational and rotational shifts were recorded. The positioning errors (Σ±σ): systematic errors (Σ) and random errors (σ) were calculated. Systematic error was defined as the standard deviation of individual mean positioning error; random error was defined as the root mean square of individual standard deviation positioning error. A formula of 2.5Σ +0.7σ was applied for CTV to PTV margins calculation.

The positioning errors for Group A and B were (2.52±3.08)mm and (1.14±1.43)mm, (2.26±2.51)mm and (1.38±1.88)mm, (4.80±3.41)mm and (1.26±1.76)mm in x (lateral), y (longitudinal) and z(vertical) directions; (0.82±1.12)° and (0.71±0.69)°, (0.52±1.18)° and (0.89±0.82)°, (1.49±1.14)° and (1.11± 1.53)° in Rx (Pitch), Ry (Roll) and Rz (Yaw) directions. The percentages of translational and rotational distribution errors were shown in the table 1. The calculated CTV-PTV margins were 8.5mm, 7.4mm, 14.4mm and 3.9mm, 4.8mm, 4.4mm in x, y and z directions for Group A and B respectively.

Table 1: The distribution errors of translational and rotational shifts.

The use of SGRT can improve the positioning accuracy especially in lateral and vertical direction. Accurate and reproducible positioning could potentially reduce the PTV margins significantly for better side effects management.