Quality improvement process to assess tattoo alignment, set-up accuracy and isocentre reproducibility in pelvic radiotherapy patients

Abstract

Introduction: This quality improvement study tested three methods of tattoo alignment and isocentre definition to investigate if aligning lateral tattoos to minimise pitch, roll and yaw decreased set-up error, and if defining the isocentre using the lateral tattoos for cranio-caudal (CC) position improved isocentre reproducibility. The study population was patients receiving curative external beam radiotherapy (EBRT) for prostate cancer. The results are applicable to all supine pelvic EBRT patients.

Methods: The three sequential cohorts recruited 11, 11 and 10 patients respectively. A data set of 20 orthogonal pairs of electronic portal images (EPI) was acquired for each patient. EPIs were matched offline to digitally reconstructed radiographs. In cohort 1, lateral tattoos were adjusted to minimise roll. The anterior tattoo was used to define the isocentre. In cohort 2, lateral tattoos were aligned to minimise roll and yaw. Isocentre was defined as per cohort 1. In cohort 3, lateral tattoos were aligned as per cohort 2 and the anterior tattoo was adjusted to minimise pitch. Isocentre was defined by the lateral tattoos for CC position and the anterior tattoo for the left-right position.

Results: Cohort 3 results were superior as CC systematic and random set-up errors reduced from -1.3 mm to -0.5 mm, and 3.1 mm to 1.4 mm respectively, from cohort 1 to cohort 3. Isocentre reproducibility also improved from 86.7% to 92.1% of treatment isocentres within 5 mm of the planned isocentre.

Conclusion: The methods of tattoo alignment and isocentre definition in cohort 3 reduced set-up errors and improved isocentre reproducibility.

Keywords: Isocentre definition; pitch; roll; yaw.

Figure 1

Demonstrates the tattoo alignment of all cohorts and illustrates pitch and yaw. The crosses depict tattoo locations and the circles represent the tattoos used for isocentre definition. (a) computed tomography (CT) simulation – Three set‐up tattoos administered in the same transverse plane. (b) Cohort 1 – Lateral tattoos are levelled to horizontal lasers only, to minimise roll. Note that this image demonstrates significant yaw. Isocentre is defined by the anterior–posterior tattoo. (c) Cohort 2 – Lateral tattoos are levelled to horizontal and vertical lasers to minimise roll and yaw. Note that this image demonstrates pelvic tilt i.e., pitch as the anterior–posterior tattoo is out of alignment with the laterals. Isocentre is defined by the anterior–posterior tattoo. (d) Cohort 3 – Lateral tattoos are levelled to horizontal and vertical lasers to minimise roll and yaw, and pitch is adjusted so that the anterior tattoo is within 5 mm of the lateral tattoos in the cranio‐caudal (CC) plane. Isocentre is defined by the lateral tattoos for the CC position and the anterior tattoo for the left–right position.

Figure 2

Illustrates pitch, yaw and roll as rotation about three axes.

Figure 3

(a) Lateral tattoos were not aligned in the cranio‐caudal (CC) direction in cohort 1 patients. This shows that the greater the distance between the lateral tattoos in a CC direction, the greater the yaw as measured on AP EPIs. (b) Lateral tattoos were aligned in the CCdirection for cohort 2 patients resulting in reduced yaw measured on anterior–posterior (AP) electronic portal images (EPI). Rt, right; Lt, left