ESTRO 2022 - Abstract Book

S1653

Abstract book

ESTRO 2022

Our current prone rectum radiotherapy patients are aligned using traditional three-points skin markings plane. However, the setup errors differ hugely from day to day. We would like to study if the new technology surface-guided positioning approach could mitigate this challenge. Materials and Methods Two groups of patients were included for comparison. In Group 1, 30 prone rectum patients were selected randomly. All patients were aligned using surface-guided system (Varian OSMS, Palo Alto, CA). All 6 degree of freedoms were corrected to nearly zero using ‘Move Couch’ function on the OSMS. 230 of CBCT images were taken, the 6D shift errors were recorded. Retrospectively, 30 prone rectum patients who were treated using skin-marking positioning (Group 2) between July 2021 to September 2021 were extracted randomly. Similarly, 230 of CBCT images were retrieved. The shifts errors were recorded. The data were analysed using SPSS19. Both shift errors were compared in terms of mean, standard deviations, maximum displacement, and residual errors. Residual error was defined as the shift error that was more than 3° on based on CBCT image registration. Results The result showed that the mean and standard deviations for Group 1 and 2 were (-0.06±0.18)cm, (0.04± 0.24)cm, (- 0.01±0.18)cm, (-0.14±0.81)°, (0.28±1.06)°, (-0.01±0.87)° and (0.02±0.42)cm, (-0.06± 0.47)cm, (0.05±0.33)cm, (0.04±1.07)°, (1.56±1.43)°, (-0.43±1.06)° in vertical, lateral, longitudinal, roll, pitch and rotation directions. Data analysis (Table 1) has shown statistically significant (p<0.05) in longitudinal, roll and pitch directions. The maximum displacements for Group 1 and 2 were (- 0.58/0.58)cm, (-0.66/0.57)cm, (-0.56/0.69)cm, (-2.8/2.7)°, (-2.6/2.8)°, (-2.7/2.6)° and (- 0.98/0.92)cm, (-1.02/0.87)cm, (-0.86/0.91)cm, (-3.8/2.7)°, (-2.3/5.7)°, (-3.5/2.6)° in vertical, lateral, longitudinal, roll, pitch and rotation directions. In terms of residual error rates, Group 1 had no patient with the discrepancies that were more than 3° in all three rotational directions; while there were 9.6% and 3.9% of CBCT images were larger than 3° in pitch and roll direction in Group 2. The average positioning time using surface-guided was (102.5±12.9)s. Table 1: The mean and standard deviations of SGRT-based positioning (Group 1) method and skin marking-based positioning (Group 2) method.

Conclusion Surface-guided positioning approach could improve the positioning accuracy and reduce large shift errors in both translational and rotational directions. The positioning time using surface-guided method is acceptable in our institution as well.

PO-1865 A review of the image quality (IQ) of dose optimised pelvic CBCT protocols.

R. Crouch 1

1 Weston Park Cancer Centre, Radiotherapy, Sheffield, United Kingdom

Purpose or Objective This service evaluation project compared the IQ of dose optimised pelvic CBCT protocols, developed in house at Weston Park Cancer Centre, UK, based on work by Wood et al (2015), with that of the manufacturer’s (Varian) standard pelvis protocol. Materials and Methods 20 patients receiving daily treatment verification imaging with CBCT as part of their radiotherapy for localised prostate cancer, were selected for inclusion in the service evaluation project. Within the existing imaging schedule, the patients were scanned using the standard pelvis CBCT protocol for fractions one and two, and the dose optimised pelvis CBCT protocol was used for fractions three and four. For the remaining fractions the standard protocol was used. There was no additional radiation exposure to the patient than what was already set out within the agreed imaging protocols. The IQ of the CBCTs was assessed by two independent observers. The Likert scale used in a study by Langmack, Newton, Jordan & Smith, (2016) was adapted and used to assess IQ: 3 = Excellent, no artefacts, 2 = Good, few artefacts, 1 = Poor, just about able to match. Statistical analysis was performed to compare the IQ of the dose optimised and the standard pelvis CBCT protocols. Results No CBCT image, either standard protocol or dose optimised protocol, was deemed to be of too poor IQ to be used clinically. Statistical analysis on the differences in the results of the two imaging protocols was performed using the paired t test and the Wilcoxon signed rank test. Both tests returned p values of 0.8 -a p value of <0.05 is required to show statistical significance. Therefore there was not enough evidence to prove that the CBCT protocols gave different IQ results. Kappa tests gave results close to zero, implying no better than random agreement between the raters.