ESTRO 2024 - Abstract Book

S5486

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

ESTRO 2024

The biologically effective doses in hypo fractionated treatments such as SABR have demonstrated improved outcomes, this is especially applicable to prostate cancers, which have been found to be more responsive to higher doses per fraction compared to normal tissues (1). Accurate image guided radiotherapy is integral to these advancements. The use of TPUS could aid in this accuracy, as the system can be used for both inter-fraction set-up and/or intra-fraction monitoring (2, 3). Fiducial insertion with CBCT is regarded the ‘gold standard’ in terms of IGRT, but application must be considered carefully as the procedure can be invasive, time-consuming, and reliant on consultant expertise. Precision of the fiducials is dependent on these markers remaining in the same location and on the prostate not changing shape during the course treatment.

Material/Methods:

Verification translations were evaluated for 120 scans relating to patients who underwent EBRT for localised prostate cancer as part of the PACE-C trial. All scans were retrospectively evaluated by three imaging specialists to limit bias.

Compensation of the setup error was based on CBCT imaging, with a Clarity image captured in quick succession to review offline and allow for intra-fractional imaging. Registration translations for each matched pair, for each observer, were then compared and analysed separately for each direction. The overall mean was calculated from the absolute differences between CBCT and TPUS. These differences were then analysed in a correlation and Bland Altman plot. The limits of agreement indicate if the two methods are interchangeable and if the difference between the upper and lower limit is within a clinically acceptable range. (Mean ± 2*SD)

Results:

Left/Right

Difference between set up error measurements as a function of the mean using Bland Altman analysis were 0.37, 0.28 mm

TPUS agreed with CBCT 88.3% of the time within 3mm, and 100% of the time between 5mm

Median absolute difference between TPUS and CBCT was was 1.1 mm

Average distance between CBCT and TPUS was -0.04± 3.2 mm

Pearson R- 0.821

Sup/Inf

Difference between set up error measurements as a function of the mean using Bland Altman analysis were -0.15, 0.42 mm

TPUS agreed with CBCT 86.7% of the time within 3mm, and 98.3% of the time between 5mm

Median absolute difference between TPUS and CBCT was 1.0mm