Radiotherapy of pelvic malignancies: impact of two types of rigid immobilisation devices on localisation errors

Background and purpose: To determine the distribution of set-up errors for patients treated with and without two rigid partial immobilisation devices for pelvic malignancies. Materials and methods: 30 patients receiving pelvic irradiation with a four field technique underwent a total of 524 portal films. The patients are di vided into 3 cohorts of 10 patients. The first cohort is treated on a stand ard treatment couch without immobilisation device (NI); the second and thir d cohorts are treated with a custom-made immobilisation device used in an a ttempt to improve set-up accuracy: an Alpha-Cradle mattress (1) (AM) or an Orfit cast (2) (OC). Set-up deviations are analysed in the X, Y, Z directio ns of a fixed coordinate system, corresponding to the lateral, cranio-cauda l and antero-posterior direction, respectively. Results: Considering the percentage of discrepancies less than or equal to 5 mm between the simulation films and the portal films as a measure of set- up accuracy, immobilisation devices seem to increase accuracy: 88.5% (X) 79 % (Y) and 100% (Z) with AM; 84% (X-Y), 97.5% (Z) with OC and only 76.5% (X) , 40% (Y) and 65.5% (Z) far NI. The distribution of the systematic set-up e rrors for the three patient cohorts, defined as the mean patient displaceme nt for the treatment course, had a mean and a standard deviation of (0.7 +/ - 2.7) mm in the X-axis, (-5.5 +/- 2.6) mm in the Y-axis and (-0.9 +/- 2.2) mm in the Z-axis when no immobilisation is addled; (0.8 +/- 1.7) mm, (-2 /- 2.7) mm and (0.3 +/- 0.4) mm for the Alpha-Cradle group; (0.3 +/- 1.4) m m, (0.5 +/- 1.1) mm and (0.5 +/- 0.6) mm for the Orfit cast group. The dist ribution of random errors about the mean approximated a normal distribution and the standard deviations are 4.4 mm (X), 4.2 mm (Y) and 4.8 mm (Z) for NI; 3.3, 3.5 and 2.5 mm for the AM; 3.4, 3.3 and 2.7 mm for the OC. Conclusions: The two rigid immobilisation devices improve the reproducibili ty of a given pelvic held but there is a small benefit comparative to the c ost and the cumbersome place of the devices. (C) 1999 Elsevier Science Irel and Ltd. All rights reserved.