Investigation on the performance of dedicated radiotherapy positioning devices for MR scanning for prostate planning

Abstract

The purpose of this study was to investigate performance of the couch and coil mounts designed for MR-simulation prostate scanning using data from ten volunteers. Volunteers were scanned using the standard MR scanning protocol with the MR coil directly strapped on the external body and the volunteer lying on the original scanner table. They also were scanned using a MR-simulation table top and pelvic coil mounts. MR images from both setups were compared in terms of body contour variation and image quality effects within particular organs of interest. Six-field conformal plans were generated on the two images with assigned bulk density for dose calculation. With the MR-simulation devices, the anterior skin deformation was reduced by up to 1.7 cm. The hard tabletop minimizes the posterior body deformation which can be up to 2.3 cm on the standard table, depending on the weight of volunteer. The image signal-to-noise ratio reduced by 14% and 25% on large field of view (FOV) and small FOV images, respectively, after using the coil mount; the prostate volume contoured on two images showed difference of 1.05 ± 0.66 cm3. The external body deformation caused a mean dose reduction of 0.6 ± 0.3 Gy, while the coverage reduced by 22% ± 13% and 27% ± 6% in V98 and V100, respectively. A dedicated MR simulation setup for prostate radiotherapy is essential to ensure the agreement between planning anatomy and treatment anatomy. The image signal was reduced after applying the coil mount, but no significant effect was found on prostate contouring.

Figure 1

Conventionally the MRI surface coil is strapped onto a patient's pelvis using two pairs of straps (left). The coil induced anterior body deformation can be eliminated by fixing the coil to coil mounts (right).

Figure 2

MRI Large field of view (LFOV) (top) and small field of view (SFOV) (bottom) images acquired using conventional scanning protocol, I_standard (left) and radiotherapy‐dedicated protocol, I_MRISim (right). Both images with the same FOV were output with the same window/level setting.

Figure 3

Organ signal comparison on LFOV and SFOV images acquired by the standard setup and MRISim setup. $\text{LHOF}=\text{left head of femur}$, $\text{RHOF}=\text{right head of femur}$, $\text{PP}=\text{prostate peripheral zone}$, $\text{PC}=\text{prostate central zone}$.

Figure 4

DVH diagram of the one volunteer comparing target dose in P_actual and P_MRISim (reference).