Full
Outline
To evaluate the clinical efficacy of an optimized pelvic immobilization protocol—combining a locked headrest with knee wedge removal—on setup accuracy and planning target volume (PTV) margins in rectal cancer patients undergoing VMAT in the supine position.
We retrospectively analyzed 159 fractions of cone-beam CT (CBCT) registration data from 53 patients with rectal cancer treated with pelvic volumetric modulated arc therapy (VMAT) between December 2020 and December 2025. Patients were assigned to three cohorts: Group A (n=15) used the “non-fixed headrest + knee wedge” fixation scheme; Group B (n=18) used the “locked headrest + knee wedge” fixation scheme; Group C (n=20) used the “locked headrest + knee wedge removed” fixation scheme. Six-dimensional (Vrt, Lng, Lat, Pitch, Roll, Rtn) setup errors were collected. One-way ANOVA with LSD post-hoc tests and Mann-Whitney U tests were employed to compare individual systematic errors and random errors among the groups. Abnormal setup was defined as any linear shift >5.00 mm or rotation >2.00°. PTV margins in three translational directions were determined using the van Herk formula (Margin = 2.5Σ + 0.7σ).
One-way ANOVA revealed significant differences in systematic Vrt (F = 15.89, P<0.01) and Roll (F = 3.87, P = 0.03) bias among the three groups with Group C demonstrating systematic means closest to the absolute zero center (Vrt: -0.28 mm, Roll: 0.01°). Group C achieved the lowest group systematic error (Σ) in Lng (1.14 mm vs. 1.79 mm in Group A and 1.98 mm in Group B) and Lat directions (1.43 mm vs. 1.54 mm in Group A and 2.84 mm in Group B). For random errors, Group C achieved significantly smaller Pitch rotational variation compared to Group A (Z = -2.47, P = 0.01). The abnormal setup rate in Group C was 3.33%, significantly lower than 31.11% in Group A and 22.22% in Group B (χ2 = 14.87, P<0.01). PTV margins for Group C in the Vrt, Lng, and Lat directions were safely contracted to 3.58 mm, 5.65 mm, and 6.47 mm, respectively, with the maximum narrowing of 35.50% compared to Group A.
The optimized pelvic immobilization protocol utilizing a locked headrest and removing the knee wedge significantly enhances the geometric reproducibility of VMAT setups for rectal cancer by stabilizing the spinal kinetic chain and engaging the knee screwing-home mechanism. This cost-free approach effectively compresses PTV margins and enhances organ-at-risk (OAR) sparing, making it highly recommended as a standardized protocol for supine pelvic radiotherapy.
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| 科 Family | 属数 Number of genus | 种数 Number of species | 占总种数比例 Percentage of total species (%) | 属 Genus | 种数 Number of species | 占总种数比例 Percentage of total species (%) |
|---|---|---|---|---|---|---|
| 鹅膏菌科Amanitaceae | 2 | 11 | 5.26 | 鹅膏菌属 Amanita | 10 | 4.78 |
| 小菇科 Mycenaceae | 2 | 12 | 5.74 | 丝盖伞属 Inocybe | 5 | 2.39 |
| 多孔菌科 Polyporaceae | 8 | 14 | 6.70 | 蜡蘑属 Laccaria | 5 | 2.39 |
| 红菇科 Russulaceae | 3 | 23 | 11.00 | 小皮伞属 Marasmius | 6 | 2.87 |
| 小菇属 Mycena | 11 | 5.26 | ||||
| 光柄菇属 Pluteus | 5 | 2.39 | ||||
| 红菇属 Russula | 17 | 8.13 | ||||
| 栓菌属 Trametes | 5 | 2.39 |
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