Set-up Variation in Palliative Radiotherapy: One Versus Three Skin Localisation Marks

Background: Accuracy and reproducibility of the patient's position is crucial for successful delivery of radiotherapy (RT). Data on palliative patients' set-up uncertainties are sparse. The aim of this study was to calculate set-up errors observed for palliative patients positioned using one skin mark (Group 1) versus three skin marks (Group 2) and to assess the accuracy of both approaches. Methods: Displacements in the left-right (L-R) and superior-inferior (S-I) directions were retrospectively analysed for 175 sites treated with a course of fractionated palliative RT. Population mean, systematic and random errors were calculated in both directions for patients positioned with one and three skin marks. Frequency of deviations was also examined for both groups. Results: The population mean, systematic and random errors for Group 1 and 2 for the L-R direction were 0.0, 4.4, 4.8 and 0.4, 3.1 and 3.3 mm, respectively, and in the S-I direction: 0.1, 3.4, 4.2 and 1.2, 2.7 and 3.3 mm, respectively. Frequency of images within the clinical tolerance of 5mm was 47.1% for Group 1 and 65.9% for Group 2. Conclusion: Three skin marks are recommended for patients receiving a fractionated course of palliative RT, as it reduces set-up error, reduces the number of gross displacements (>10 mm) and increases the number of displacements within the clinically acceptable tolerance of 5 mm.