ESTRO 2024 - Abstract Book

S5574

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

ESTRO 2024

A retrospective chart review was undertaken to assess intrafractional shifts in 10 successive paediatric cancer patients. Patient setup was done with SGRT using AlignRT (Vision RT Ltd., UK) and the PTV was aligned with the help of CBCT. Following the acquisition of a new SGRT reference image, patients were continuously monitored during treatment. If SGRT detected any intrafractional shift exceeding 3 mm in any direction, the Auto Beam hold feature paused treatment. Subsequently, CBCT scans were repeated, and shift measurements were recorded. These patients were evaluated for intrafractional shifts greater than 3 mm, based on the standard Gamma index criteria (3%, 3mm), to assess potential dosimetric impacts on PTV coverage and the minimum and maximum PTV doses.

Results:

The mean PTV coverage significantly changes due to intrafractional motion ranging within 3 - 5 mm. The planned gamma index passing criteria i.e. (3%, 3mm) were compared with the shift of intrafractional motion ranging from 3 - 5mm in portal dosimetry. The shifts of 3-5 mm were detected by SGRT system thereby activating the auto beam hold causing temporary cessation of radiation delivery. These were then corrected, and 95% gamma index was achieved after application of shifts. The median Planned Gamma Index value was 98.5%, while 3mm shift was 93.8%, 5mm shift was 85.9% and 7mm shift was 81.2%. A Wilcoxon signed-rank test showed that there was a significant difference between the Planned Gamma Index and 3 mm shift (p-value = 0.005) and between the Planned Gamma Index and 5 mm shift (p-value = 0.005).

Conclusion:

In conclusion, intrafraction motion even in the range of 3-5 mm, leads to significant underdosing of the target volumes. The SGRT system was valuable in detecting these motions and the automated beam hold helps avoid these errors. In paediatric patients, SGRT system is useful to prevent dose delivery errors.