ESTRO 2024 - Abstract Book

S5511

RTT - Patient care, preparation, immobilisation and IGRT verification protocols

ESTRO 2024

neuro navigation protocol, was performed in conjunction with the CT simulation scans. Planned Target Volume (PTV) included GTV with a 1-2mm margin.

Daily CBCT scans were obtained before each treatment session to ensure patient setup accuracy. These scans were subsequently registered with the planning CT to compute the necessary translational and rotational adjustments. Any rotational shifts less than 2 degrees (°) were addressed using the hexapod robotic couch. When a setup exhibited shifts greater than 2 degrees in the rotational directions, patient was repositioned. Key metrics included absolute random shifts in translational and rotational directions, number of repeat CBCTs, and the time interval between CBCTs. The secondary objective was to compare accuracy rates in terms of PTV between the two groups in both translational (defined as shift ≤ 1 mm; shift ≤ 2 mm). Chi square test or Fisher’s exact test was used, as applicable. A p-value <0.05 was deemed statistically significant.

Results:

The demographic and clinical characteristics of the participants were well-balanced between the two groups The Shim mask with mouth bite significantly reduced the mean setup errors in the lateral translation (p=0.022) from 0.17 cm (SD=0.10) to 0.10 cm (SD=0.10), and in X-axis rotation (p=0.030) from 0.79° (SD=0.43) to 0.47° (SD=0.47). (Table 1).

Table 1. Random Setup error in standard versus SHIM mobilization techniques

Standard (N=20)

Shim (N=20)

p value

Plan

Direction

Mean

SD

Mean

SD

Lateral

0.17 0.17 0.13 0.79 0.77 0.77

0.10 0.11 0.08 0.43 0.46 0.40

0.10 0.14 0.09 0.47 0.63 0.58

0.10 0.12 0.07 0.47 0.53 0.49

0.022*

Translational

Longitudinal

0.414 0.180

Vertical

X Y Z

0.030*

Rotational

0.362 0.196

Values are average absolute setup errors for translational shifts (in cm) and in rotational shifts (in °).

SD: Standard deviation; * statistically significant difference (p<0.05)

In terms of PTV, the Shim group was significantly more accurate, at ≤1 mm, in the lateral direction, with accuracy rate at 70% versus 20% compared to standard techniques (p=0.004). Considering the same accuracy cutoff of ≤1mm, and although not statistically significant, Shim immobilization showed higher accuracy rates in the longitudinal (50% vs 35%, p=0.337) and vertical (70% vs 40%, p=0.057) directions as well as in the X-axis (90.0% vs 70.0%, p=0.235. By using the cutoff ≤2 mm in the translational directions, accuracy rates did not differ significantly between the two techniques (p>0.05). All shifts were implemented with couch displacement. Moreover, while 70% of patients in the standard group required repeat CBCT scans, none in the Shim group did, resulting in an average time saving of 29.3 minutes per patient. The data reported were the final offset between the best-achieved position and the planned treatment position.