MRI Evaluation of Rectal Cancer Following Preoperative Chemoradiotherapy

https://doi.org/10.1053/j.ro.2020.07.009Get rights and content

Introduction

MR imaging is the modality of choice in patients with newly diagnosed rectal cancer for staging and determining the initial course of treatment. Neoadjuvant/adjuvant therapy is recommended by current National Comprehensive Cancer Network (NCCN) guidelines for patients with some stage II (T3-4, node-negative disease with tumor penetration through the muscularis propria) or stage III (node-positive disease without distant metastasis) given high risk of local recurrence (Fig. 1). Additionally, tumors with poor prognostic features such as a threatened circumferential resection margin (CRM), tumors with extramural vascular invasion (EMVI), or tumors in certain anatomic locations (eg, lower third of the rectum) may benefit from neoadjuvant therapy.1

Neoadjuvant chemoradiotherapy (nCRT) for rectal cancer most commonly consists of induction chemotherapy followed by chemoradiotherapy. According to current NCCN guidelines, most patients with stage II or III rectal cancer are recommended for fluoropyrimidine-based chemotherapy and ionizing radiation to the pelvis (chemoradiotherapy) as well as chemotherapy with the total perioperative therapy course not exceeding 6 months.2, 3, 4 However, the management of rectal cancer continues to evolve and there remain several different therapy sequences depending on tumor characteristics, response to initial therapy, and various pretreatment clinical characteristics.4,5 For example, nCRT may not be indicated in patients with early T-stage low rectal tumors.6 In patients who do undergo nCRT, however, MR evaluation plays a key role in reevaluating the tumor prior to surgery.

Section snippets

Surgical Management Postneoadjuvant Therapy

MR features influence surgical planning; thus, it is necessary to understand the most common surgical approaches for rectal tumors. While local excision is typically performed for tumors confined to the rectal wall, total mesorectal excision (TME) is the standard surgical treatment for high-risk patients following nCRT. TME involves en bloc resection of the rectum and the mesorectum, the fatty tissue envelope surrounding the rectum with all blood vessels and lymphatics included.7

In tumors that

Post-Treatment MR

Repeat MR is routinely performed prior to surgery for restaging. Commonly, the tumor has undergone interval nCRT. Locally advanced rectal cancers demonstrate variable response to nCRT, with only 4%-31% of patients achieving complete remission.16 This subset of patients have improved quality of life with post-nCRT options of local excision or active surveillance.17

Following nCRT and prior to surgery, repeat high-resolution pelvic MR may be useful to assess tumor response to treatment,

Primary Tumor Size

Prior to interpretation of post-treatment MR findings, it is important to review the initial staging MR to establish the baseline appearance of the primary tumor. Changes in tumor location or tumor bulk can thus be more accurately assessed. Interval decreased tumor height has been shown to portend a better prognosis.28

Further, a decrease in tumor size correlates with tumor regression; however, it is unclear how great of a reduction in size is necessary to impact prognosis. Tumor volume

What To Report

The following features should be included in a report for the restaging of rectal cancer following nCRT:

  • Primary tumor:

    • Presence or absence of remaining tumor

    • If present, remaining tumor length and morphologic growth pattern

    • Position of tumor (o'clock) and distance from tumor to anal verge and to anorectal junction

    • Signal characteristics of remaining tumor, including presence or absence of fibrosis and/or mucinous degeneration

    • yT stage and yT3 depth

  • Lymph nodes: yN stage, addressing total number of

Conclusion

MR represents the imaging modality of choice for rectal cancer both in initial staging and for restaging following nCRT. The primary tumor location, extent, and signal characteristics must be carefully assessed. The presence of fibrosis, restricted diffusion, and enhancement characteristics are key factors in evaluating response. Additional imaging features of CRM, presence of EMVI, and lymph node involvement must also be evaluated. These imaging features play a critical role in determining

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      When interpreting mucin response, it is imperative to review the pretreatment MRI tumor signal characteristics to avoid confusion with a primary mucinous rectal tumor on pretreatment MRI. Restaging primary mucinous adenocarcinomas is a unique challenge, again limited because of the similar imaging appearance of a persistent mucinous tumor and a favorable acellular mucin response (Fig 19).38,55 Features suggestive of a favorable response include decreasing signal heterogeneity and resolution of intermediate signal solid tumor components.

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