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Advances in Motion

PET/MRI Superior to MRI Alone for Staging of Rectal Cancer

Journal

Originally published on European Journal of Nuclear Medicine and Molecular Imaging

Key findings

  • This retrospective study compared positron emission tomography/magnetic resonance imaging (PET/MRI) with stand-alone MRI for staging rectal cancer in 62 patients
  • Average tumor length was 43 mm by sigmoidoscopy, 54 mm by MRI and 44 mm by PET/MRI (P=0.004)
  • Extent of sphincter infiltration was correct by MRI in 6/10 patients and by PET/MRI in 9/10 (P=0.003)
  • For assessing T stage and extramural vascular invasion (EMVI), PET/MRI showed a non-significant trend toward better performance than MRI
  • PET/MRI correctly assigned N stage in 79% of cases vs. 58% for standalone MRI (P=0.02)

Both MRI and PET/CT have limitations for locoregional staging of rectal cancer. Onofrio A. Catalano, MD, PhD, medical director of PET/MR at the Martinos Center for Biomedical Imaging at Massachusetts General Hospital, and Bruce Rosen, MD, PhD, director of the Martinos Center, and colleagues have observed that hybrid PET/MRI is more precise than MRI for assessing the extent of rectal cancer, N status and sphincter involvement.

Their report appears in the European Journal of Nuclear Medicine and Molecular Imaging.

Study Methods

The researchers reviewed images from 62 patients with untreated locally advanced rectal cancer who underwent PET/MRI between January 2016 and August 2019. Follow-up ended in March 2020.

MR and PET/MRI images were evaluated separately at least four weeks apart. When evaluating MR images, readers did not have access to co-acquired PET data.

Primary Tumor

  • The average tumor length was 43 mm by sigmoidoscopy, 54 mm by MRI and 44 mm by PET/MRI (P=0.004)
  • The extent of sphincter infiltration was correct by MRI in 6/10 patients and by PET/MRI in 9/10 (P=0.003), with the reference being surgical pathology or the combination of follow-up imaging, digital rectal examination and post-chemotherapy surgical pathology
  • The modalities performed similarly in assessing mesorectal fascia involvement and extramural vascular invasion, though there was a non-significant trend toward higher performance with PET/MRI: 11/12 detected as opposed to 8/12 in the case of MRI alone
  • For T stage and EMVI, PET/MRI showed a non-significant trend toward better performance.

In three low rectal cancer cases, external sphincter infiltration was missed by MRI and therefore erroneously classified as T3.

Lymph Nodes

MRI correctly assigned N stage in 58% of cases vs. 79% for PET/MRI (P=0.02).

Conclusion

PET/MRI can help improve clinical decision-making, especially when low rectal tumors have possible external sphincter involvement.

90%
of patients with rectal cancer had sphincter infiltration correctly assessed by PET/MRI

79%
of patients with rectal cancer had N stage correctly assessed by PET/MRI

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Journal Article Published: Dates of Study: -

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