Improving staging of rectal cancer in the pelvis: the role of PET/MRI

Eur J Nucl Med Mol Imaging. 2021 Apr;48(4):1235-1245. doi: 10.1007/s00259-020-05036-x. Epub 2020 Oct 9.


Purpose: The role of positron emission tomography/magnetic resonance (PET/MR) in evaluating the local extent of rectal cancer remains uncertain. This study aimed to investigate the possible role of PET/MR versus magnetic resonance (MR) in clinically staging rectal cancer.

Methods: This retrospective two-center cohort study of 62 patients with untreated rectal cancer investigated the possible role of baseline staging PET/MR versus stand-alone MR in determination of clinical stage. Two readers reviewed T and N stage, mesorectal fascia involvement, tumor length, distance from the anal verge, sphincter involvement, and extramural vascular invasion (EMVI). Sigmoidoscopy, digital rectal examination, and follow-up imaging, along with surgery when available, served as the reference standard.

Results: PET/MR outperformed MR in evaluating tumor size (42.5 ± 21.03 mm per the reference standard, 54 ± 20.45 mm by stand-alone MR, and 44 ± 20 mm by PET/MR, P = 0.004), and in identifying N status (correct by MR in 36/62 patients [58%] and by PET/MR in 49/62 cases [79%]; P = 0.02) and external sphincter infiltration (correct by MR in 6/10 and by PET/MR in 9/10; P = 0.003). No statistically significant differences were observed in relation to any other features.

Conclusion: PET/MR provides a more precise assessment of the local extent of rectal cancers in evaluating cancer length, N status, and external sphincter involvement. PET/MR offers the opportunity to improve clinical decision-making, especially when evaluating low rectal tumors with possible external sphincter involvement.

Keywords: MRI; PET; PET/MR; PET/MRI; Rectal cancer; Staging.

MeSH terms

  • Cohort Studies
  • Humans
  • Magnetic Resonance Imaging*
  • Neoplasm Staging
  • Pelvis / diagnostic imaging
  • Pelvis / pathology
  • Positron-Emission Tomography
  • Rectal Neoplasms* / diagnostic imaging
  • Rectal Neoplasms* / pathology
  • Retrospective Studies