Improving fiducial and prostate capsule visualization for radiotherapy planning using MRI

Abstract

Background and purpose Intraprostatic fiducial markers (FM) improve the accuracy of radiotherapy (RT) delivery. Here we assess geometric integrity and contouring consistency using a T2*‐weighted (T2*W) sequence alone, which allows visualization of the FM. Material and methods Ten patients scanned within the Prostate Advances in Comparative Evidence (PACE) trial (NCT01584258) had prostate images acquired with computed tomography (CT) and Magnetic Resonance (MR) Imaging: T2‐weighted (T2W) and T2*W sequences. The prostate was contoured independently on each imaging dataset by three clinicians. Interobserver variability was assessed using comparison indices with Monaco ADMIRE (research version 2.0, Elekta AB) and examined for statistical differences between imaging sets. CT and MR images of two test objects were acquired to assess geometric distortion and accuracy of marker positioning. The first was a linear test object comprising straight tubes in three orthogonal directions, the second was a smaller test object with markers suspended in gel. Results Interobserver variability for prostate contouring was lower for both T2W and T2*W compared to CT, this was statistically significant when comparing CT and T2*W images. All markers are visible in T2*W images with 29/30 correctly identified, only 3/30 are visible in T2W images. Assessment of geometric distortion revealed in‐plane displacements were under 0.375 mm in MRI, and through plane displacements could not be detected. The signal loss in the MR images is symmetric in relation to the true marker position shown in CT images. Conclusion Prostate T2*W images are geometrically accurate, and yield consistent prostate contours. This single sequence can be used to identify FM and for prostate delineation in a mixed MR‐CT workflow.

Comparison of prostate delineation on multimodality imaging for MR-guided radiotherapy

“…All observers, both clinicians and radiographers, are from the same institution, which will influence both consistency and accuracy, as assessed by the overlap and distance measurements here and previously. 9…”

“…The patient population and image acquisition have previously been described. 9 10 patients receiving treatment within the Prostate Advances in Comparative Evidence (PACE) trial (NCT01584258) at the Royal Marsden Hospital NHS Foundation Trust, Sutton, had RT planning CT and MRI scans acquired on the same day. The PACE trial has two parallel randomisations; PACE A randomises between prostatectomy or stereotactic body radiotherapy (SBRT) to a dose of 36.25 Gy in five fractions, and PACE B randomised patients between SBRT or conventionally fractionated RT, either 62 Gy in 20 fractions or 78 Gy in 39 fractions.…”

“…The interobserver variability for these clinician contours has previously been reported with median Dice similarity co-efficient (DSC) of 0.95 (interquartile range 0.94–0.96), 0.97 (0.96–0.97) and 0.97 (0.96–0.97) for CT, T 2 W and T 2 *W imaging respectively. 9…”

“…One such sequence, T 2 *-weighted ( T 2 *W) MRI, uses multiple echo times 8 resulting in a more defined prostate capsule as well as a reliable depiction of fiducials; geometric accuracy and clinician contouring consistency on this type of sequence has previously been assessed. 9…”

Prostate cancer – Advantages and disadvantages of MR-guided RT

“…MR for radiotherapy planning has been shown to decrease target volume, although latterly some studies in MR-experienced centres have shown no difference [8]. MR also reduces inter-observer variability in target contouring [9]. The ESTRO ACROP consensus guidelines on CT- and MRI-based target volume delineation for primary radiotherapy of localised prostate cancer [10] aim to improve consistency and reliability of prostate contours in what remains the weak link in radiation therapy.…”