Geometric impact and dose estimation of on-patient placement of a lightweight receiver coil in a clinical magnetic resonance imaging-only radiotherapy workflow for prostate cancer

Abstract

Background and purpose: For pelvic magnetic resonance imaging (MRI)-only radiotherapy the use of receiver coil bridges (CB) is recommended to avoid deformation of the patient. Development in coil technology has enabled lightweight, flexible coils. In this work we evaluate the effects of a lightweight coil in a pelvic MRI-only radiotherapy workflow.

Materials and methods: Twenty-one patients, referred to prostate MRI-only radiotherapy, were included. Images were acquired with and without CB. Anatomical deformation from the on-patient coil placement was measured in the anterior-posterior (AP) and left-right (LR) direction. The change in signal-to-noise ratio (SNR) was measured in phantom and in vivo.The clinical treatment plan, created on the image with CB, was transferred and recalculated on the image without the CB. Dose metrics for the targets (planning- and clinical target volume) and organs at risks (OAR) were analyzed.

Results: There was a statistically significant increase in SNR in-vivo (median 21 %, p = 0.002) when removing the CB. Anatomical differences after removing the CB in patients were -1.5 mm in AP (median change) and + 2.5 mm in LR direction. Dosimetric differences for the target structures were clinically negligible, but statistically significant. The difference in target mean doses were 0.2 % (both p = 0.004) of the prescribed dose. No dosimetric differences were observed for the OAR, except for the penile bulb.

Conclusions: We concluded that anatomical change and dosimetric differences, originating from scanning without CB were minor. The CB can thereby be removed from the workflow, enabling easier patient positioning and increased SNR when using lightweight coils.

Keywords: Coil bridge; MRI-only; Prostate cancer; Radiotherapy.

Fig. 1

Measured difference from first scan (at 0 min). Measurements in anterior posterior (AP) and left right (LR) for all the T2 weighted (T2w) transversal scans performed. Note that two patients are removed from graph with LR measurements (removed due to tightening gluteus maximus in one sCT but not the other (see Figure S6, patient 17 and patient 18)). Graph with all patients for LR measurements are available in the Supplementary materials, Figure S5. Squares indicates large T2w scans used for synthetic CT generation. Filled circles indicates first scan without the coil bridges. Unmarked points indicate fast T2w scans. The dashed lines indicate mean deviation for all patients. Each colored line represents an individual patient.

Fig. 2

Boxplot of the measured mean Hounsfield Units (HU) in both sCT (with and without coil bridges (CB)) for the four difference anatomical regions evaluated. P-values from paired two-sided Wilcoxon signed rank test is presented on each subplot. Note the difference y-scales.

Fig. 3

Dose estimations of the planning target volume (PTV) and clinical target volume (CTV). White boxplots for synthetic CTs with no coil bridges (CB) and grey boxplots with CB. P-values from a paired two-sided Wilcoxon signed rank test is presented on each subplot. Note the different Y-scales. CB – Coil bridges, CTV – Clinical target volume, DXX% - Dose to the XX% volume of the structure, HI – Homogeneity index, PCI – Paddicks conformity index, PTV – Planning target volume.

Fig. 4

Dose estimations of the organs at risk. White boxplots for synthetic CTs with no coil bridges (CB) and grey boxplots with CB. P-values from a paired two-sided Wilcoxon signed rank test is presented on each subplot. Note the different y-scales. CB – Coil bridges, FH – Femoral head, L – left, PB – Penile Bulb, R – right, VXXGy – Volume of the organ receiving XX Gy or less.