MR imaging for rectal cancer: the role in staging the primary and response to neoadjuvant therapy

Abstract

Pre-operative staging is an essential aspect of modern rectal cancer management and radiological assessment is central to this process. An ideal radiological assessment should provide sufficient information to reliably guide pre-operative decision-making. Technical advances allow high-resolution imaging to not only provide prognostic information but to define the anatomy, helping the surgeon to anticipate potential pitfalls during the operation. The main imaging modality for local staging of rectal cancer is Magnetic Resonance Imaging (MRI), as it defines the tumour and relevant anatomy providing the most detail on the important prognostic factors that influence treatment choice. In addition, there is an emerging role for MRI in the assessment of the response to neoadjuvant therapy. This article is an evidence-based review of rectal cancer staging focusing on post-treatment assessment of response using MRI. The discussion extends into the implications for reliably assessing response and how this may influence future rectal cancer management.

Keywords::

• MRI low rectal cancer staging system
• MRI tumour regression grade
• rectal cancer
• response
• staging

Financial & competing interests disclosure

This work was supported by a research grants from NIHR BRC Royal Marsden and the Pelican Cancer Foundation, UK. The authors have no other relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript apart from those disclosed.

No writing assistance was utilized in the production of this manuscript.

Key issues

• Staging the primary rectal cancer & assessing response to neoadjuvant therapy

  • Without optimal imaging, optimal preoperative staging cannot be achieved.

  • Careful attention to the MRI technique is required.

  • Based on poor results from ultrasound under- and overstaging early tumous, it is likely that MRI may prove to be more reliable for tumors with a greater than 5 mm thickness and has the added advantage of visualizing extramural disease (venous or nodal spread).

  • MRI is a reliable method for predicting the tumor stage and the safety of CRM.

  • There is no ideal imaging modality for assessing lymph nodes, but emerging evidence suggests that, in the context of optimal total mesorectal excision surgery, pathological nodes can be resected without the need for radiotherapy with minimal impact on local recurrence rates.

  • mrEMVI is associated with a relatively chemoradio-resistant tumor and a poor prognosis.

  • Current trials report a 14% pCR rate following neoadjuvant therapy; this figure can exceed 30% when dual-agent chemotherapy or greater than 45 Gy is used.

  • The major limitation of pathological complete response as a measure of tumor regression is that the opportunity to allow organ preservation has passed.

  • mrTRG appears to be the most encouraging method for assessing tumor response to neoadjuvant therapy.

  • mrTRG 1–2 patients appear to behave in a prognostically equivalent fashion to pCR (5-year overall survival 72%). With frequent follow-up, it may be possible to use mrTRG as the key criterion for assessing complete response.

  • mrTRG 3–5 patients are poor responders and have a worse prognosis (5-year overall survival 27%). It may be possible to use additional preoperative pharmacotherapy to shift these patients into a better prognostic category.

Notes