ESTRO 2022 - Abstract Book

S168

Abstract book

ESTRO 2022

Purpose or Objective In the field of pelvic MRI-only treatment the use of coil bridges for the anterior MRI receiver coil is recommended to avoid deformation of the external contour. The increased distance between patient and coil will decrease the SNR. Development in coil technology has enabled lightweight, blanket-like coils to be commercially available (air coils, GE healthcare). This work aimed to evaluate the benefits and effects of air coil technology in a pelvic MRI-only workflow, with and without coil bridges. Anatomical and dosimetric quality measures were assessed. Materials and Methods Six patients referred to HYPO (42.7 Gy in 7 fractions) prostate MRI-only radiotherapy were included in the analysis. One large field of view T2 weighted sequence (T2 cli ), for synthetic CT (sCT) generation and consequently treatment planning, was acquired for each patient using coil bridges according to clinical routine. The coil bridge was then removed, and the air coil was placed directly on the patient. A second identical T2 was added to the examination (T2 ncb ). A 3T GE scanner, large anterior air coil and a posterior built-in spine coil were used for all acquisitions. Anatomical difference between the T2 cli and T2 ncb was assessed to evaluate potential deformation from the on-patient air coil placement by measuring the absolute maximum anterior-posterior thickness (AMT) of the patient in the same left-right position on a slice in the middle of the prostate. The T2 cli and T2 ncb were used to generate sCTs (sCT cli and sCT ncb ). The clinically approved treatment plan, created on sCT cli , was transferred and recalculated on the 6D rigidly registered sCT ncb , using the same number of monitor units. The clinical dose metrics for the mean PTV dose, rectum V 96% , V 89% and V 77% , and mean bladder dose were analysed using the clinical structure set. A paired Wilcoxon signed-rank test was used for dose metric comparisons, p < 0.05 was considered statistically significant. The SNR using coil bridges was measured in a homogenous phantom where the air coil and bridge was placed at a 5, 10, 15 and 20 cm distance from the surface of the phantom. Results Median AMT difference between T2 cli and T2 ncb was 1.9 mm, range [0.5 to 8.9 mm]. sCTs for the worst case (8.9 mm) is presented in Figure 1.

For the PTV no significant difference was seen (p = 0.16) and the median mean dose difference was -0.03 Gy (range -0.20 to 0.03 Gy). No significant differences were found for either rectal or bladder dose metrics (Table 1).