Marc J. Gollub
Professor of Radiology Memorial Sloan Kettering Cancer Center
Mukesh G. Harisinghani
Professor of Radiology Massachusetts General Hospital; Genitourinary Imaging Section Editor, AJR
Rectal cancer comprises approximately one-third of all cases of colorectal cancer—the third leading cause of cancer death in the United States. In the last two decades, the role of imaging has become crucial to patient care. Until recently, staging of rectal cancer patients was mostly done by surgeons using three office procedures: digital rectal exam, endorectal ultrasound (ERUS), and endoscopy. A more recent shift away from ERUS towards pelvic MRI has now positioned MR imaging front and center for staging rectal tumors. We have noted this growing percentage at our own institution, Memorial Sloan Kettering Cancer Center.
Owing to direct, point-of-contact proximity and the intrinsic high-resolution, ERUS is superior to MRI in staging lower T-category tumors (T1 and T2); however, ERUS is limited by its restricted FOV and the lack of visualization of the radial (circumferential) tumor margin and the proximal margin. As a result, the mesorectal fascia (i.e., the intended circumferential resection margin for an ideally performed standard total mesorectal excision surgery), pelvic sidewall lymph nodes, upper pelvic lymph nodes, and high rectal tumors cannot be reliably imaged with ERUS. Pelvic MRI can overcome these limitations, thus allowing accurate surgical staging by providing critical information needed for successful surgery, as well as for initial treatment planning, which is needed by medical and radiation oncologists.
If one combines this current shift to pelvic MR imaging with the relative lower incidence of rectal cancer compared with prostate and gynecologic cancers, as reported by Cancer Facts and Figures 2019 (e.g., rectum: 44,180 projected cases; prostate: 174,650 projected cases; 109,000 combined gynecologic projected cases), the relative lack of exposure and inexperience by general and even abdominal-specialized radiologists can be understood. Because of this increased referral pattern to pelvic MRI, and particularly in view of the latest trends indicating a disturbing increased incidence of rectal cancer among persons 54 years and younger, there is a heightened interest and determination among radiologists to become competent in rectal cancer staging. Currently, the complexity of pelvic MRI acquisition and interpretation, combined with the lack of radiologists with relevant expertise, puts some patients at risk for inaccurate diagnosis and suboptimal outcomes.
This interactive, case-based course at the 2020 ARRS Annual Meeting will highlight pearls and pitfalls in using rectal MR for staging and follow-up evaluation.
Although locally advanced rectal cancer is common, achieving cure requires complex tri-modality care with major morbidity. Tri-modality therapy consisting of radiation, surgery, and chemotherapy is the standard of care for locally advanced rectal cancer and achieves cure for the majority of patients. These treatments improve local control and induce tumor downstaging in approximately 50–60% of patients and complete pathological response in 15–38%. Radiation is used to decrease local recurrence rates and increase the potential for rectal sphincter-sparing surgery, but radiation causes significant long-term toxicity, including fecal incontinence, impotence, and vaginal stenosis. High rates of distant recurrence and poor postoperative chemotherapy adherence have led to increasing use of chemotherapy preoperatively. Recent clinical trials have challenged the tri-modality paradigm—testing whether high cure rates can be achieved with two instead of three modalities, including only chemoradiotherapy and systemic chemotherapy (so-called non-operative management or “watch and wait”) or only systemic chemotherapy and surgery-avoiding radiation. From these varying and significant quality-of-life altering treatment options, it can be seen that the staging of rectal cancer is complex.
Fortunately, performance of pelvic MRI and knowledge of pelvic anatomy is familiar to many body radiologists from their experience with prostate and gynecologic cancers, so minimal protocol variation is needed to perform good studies. Retraining to look at the intestinal tract presents perhaps a greater challenge but is not insurmountable. Standards for rectal MRI performance are well-described by both American and European societies, and extensive training in the form of hands-on workshops is underway worldwide. Although there are remaining challenges to achieving uniform rectal MRI protocol performance around the world, larger and more specialized centers and practices perform satisfactory studies most of the time. A greater administrative challenge now faces academic and large-volume medical practices: task assignment.
Recognizing a need for expertise among radiologists, surgeons, and pathologists for this important cancer subtype, the Committee on Cancer—a quality program of the American College of Surgeons, working with the College of American Pathologists and American College of Radiology (ACR)—has created the first version of their standards, entitled National Accreditation Program for Rectal Cancer (NAPRC). The objective of NAPRC is to establish centers of accredited expertise to promote safe, standard, and excellent quality of care for patients with rectal cancer. The authors are among a group of radiologists who have created the teaching module required of radiologists on the ACR website and who continue to work with surgeons to update requirements to accommodate the varying practice patterns that exist in already recognized centers of medical excellence.
Similar to staging for other primary cancers, rectal cancer staging by MRI is well-suited to a reporting style called “structured reporting” or even “synoptic reports.” Initially used by surgeons and pathologists to allow searchable fields for data-mining and quality assurance, such report templates should now be used by radiologists to ensure not only standardized communication of important staging features that could otherwise be lost in a long free-text report, but also as a teaching tool and quality indicator for which features are critical to evaluate and report. This is especially useful for practices where a great number of interpreting radiologists are also responsible for everything from pediatric bone age radiographs to whole body MRI studies for myeloma. Rather than trying to memorize what is important for each tumor or type of imaging study, synoptic reports with pre-populated pick lists guarantee a higher-quality report. As for other types of radiologic exams, the incremental benefit of these structured reports is well documented in the literature.
Pelvic MRI has also begun to be used for restaging rectal cancer after preoperative therapy. This is because several options now exist for further treatment, where previously these were not considered. A good response to chemoradiotherapy will likely result in planned surgery and the need for confirmation of margins and any lymph nodes that may not be in the standard surgical field. An excellent response, particularly for a low tumor requiring removal of the anal sphincter and thus a permanent colostomy, may now prompt a “watch and wait” strategy of prolonged follow-up, anticipating a complete response rate potential of about 25% on average. A poor response, if not noted clinically mid-treatment, can be detected at MRI and can suggest the need for pre- or postoperative intensification of therapy. Restaging MRI is less widely practiced, due to poor overall sensitivity to detection of residual tumor. Fortunately, the ability of MRI to use functional information, such as diffusion weighted imaging, allows increased sensitivity with minimally decreased specificity, according to a widely cited meta-analysis. However, unlike solid organ tumors, the method of quantitating response is very challenging, cannot use established methods such as Response Evaluation Criteria in Solid Tumors and continues to undergo extensive investigation. Metrics and nomograms are desperately needed to standardize the treatment approach on the one hand and allow more personalized treatment—according to individual patient co-morbidities and lifestyle needs—on the other hand. Beyond the scope of this essay, the reader is referred to reviews on the topic of response assessment. Additionally, the importance of standard imaging features on rectal MRI, including T and N categories and the mesorectal fascia, may be shifting as newer imaging markers with emerging prognostic importance are discovered, including extramural venous invasion, tumor deposits (so-called “N1c”), and lateral pelvic side wall lymph nodes. As such, there is an ever-changing, dynamic, and academically demanding body of knowledge that accompanies the field of rectal MRI; much remains to be learned about how to best stage rectal cancer, including the potential to use machine learning to identify response.
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