TARGET AUDIENCE
Radiologists with an interest in cancer imaging who would benefit from: · Understanding the role of high-resolution MRI reassessment following chemoradiotherapy. · How this information can improve outcomes by providing important prognostic and therapeutic information.
The selective use of preoperative chemoradiotherapy has been shown to be an effective method of enabling down staging and regression of poor prognostic features in patients presenting with advanced rectal cancer. Successful tumour regression results in reduction in the risk of recurrence as well as a reduction in the risk of distant metastatic disease. Accurate assessment of tumour response prior to definitive surgery has the following impacts on patient care: 1. To offer surveillance and potentially the permanent avoidance of surgery where tumour has significantly regressed. 2. In tumours that are not responding to therapy, to understand the biology of treatment resistance and innovate improvements in future therapies. 3. To plan surgery taking into account how surgical planes are affected by tumour and fibrosis. 4. To provide prognostic information based on the MRI assessment of persistence of risk factors that are known to be associated with local and distant recurrence. 5. To influence the ongoing use of adjuvant therapy 6. To provide information for the counselling and surveillance of patients after preoperative treatment has been completed. Traditionally, MRI has not been considered sufficiently accurate compared with pathology assessment of rectal cancers following chemoradiotherapy to alter patient management. However such comparisons do not take into account that pathological assessment of tumour response is highly inaccurate and subject to problems with: 1. Interobserver variability by pathologists 2. Errors in sampling due to inconsistencies in processing of pathology specimens 3. Inability to assess treated tumour since knowledge of the original tumour extent is missing. 4. Imperfect histopathological tumour regression grading systems with highly variable results in terms of ability to predict survival outcomes (overall survival, disease-free survival and local recurrence) 5. Complete eradication of tumour following completion of chemoradiotherapy is time-dependent and there is no known pathological technique that can predict the long-term viability of tumour when it has been irradiated. MRI on the contrary, does not suffer the problems of interobserver variability and is therefore superior to pathological tumour regression grading systems. Another advantage of imaging is that the whole tumour is assessed and can be compared with the pretreated tumour to derive a regression grade. The MRI tumour regression grading system compared with the various pathology systems is the only system that consistently predicts overall survival and disease-free survival outcomes. Serial imaging MRI enables the monitoring of treated tumour and is therefore a non-invasive method of assessing viability. Factors such as extramural vascular invasion, circumferential resection margin status are known to accurately predict outcomes. It is now been proven that MR EMVI status after treatment has a greater accuracy than histopathology in predicting disease-free survival. Other techniques such as volume measurement, RECIST and DWI have not improved the performance of high-resolution T2 weighted imaging and therefore cannot be advocated for the staging of patients with rectal cancer following chemoradiotherapy.
Prognostic features assessed by MRI after chemoradiotherapy
1. Extramural depth of tumour spread: at baseline the depth of tumour spread is a known prognostic feature with tumour spread beyond the muscularis propria of 5 mm or more associated with worsening cancer specific survival rates. When preoperative chemoradiotherapy achieves regression of extramural spread it is the final depth of spread that then governs prognosis. Thus a tumour that starts off with spread of more than 5 mm regressing to 0 mm takes on the survival characteristics of a tumour limited to the bowel wall (T2).
2. Circumferential resection margin status at baseline. Tumour extending to within a millimetre of the mesorectal fascia or the TME excision plane is associated with a significant risk of surgical margin involvement and subsequent local recurrence. If preoperative therapy is given to such tumours and regression away from the margin can be demonstrated on MRI prior to surgery this is associated with a significantly improved recurrence free survival rate. This means that if tumour still extends to the mesorectal margins following chemoradiotherapy, then beyond TME surgery, in the form of exenterative procedure is needed.
3.MRI detected extramural venous invasion. Tumour extension into extramural veins can be reliably detected using high-resolution MRI and is identified by the demonstration of tumour signal expanding and distorting the period rectal and mesorectal veins which are normally seen as linear signal void structures within the perirectal fat. This feature is present in up to 40% of patients on initial diagnosis and is associated with 3 to 4 fold increased risk of metastatic disease and local recurrence compared with patients who are EMVI negative. The use of preoperative chemoradiotherapy causes regression of vascular invasion in up to 50% of these patients and when this is achieved, the local recurrence and distant failure rates are the same as in patients who are initially EMVI negative on MRI.
· It is important that patients are imaged using a high-resolution T2W-FSE technique
· Dedicated specialist reporting should be undertaken utilising proforma reporting
· Information regarding the anatomic and prognostic features of treated tumour are consistently documented using proforma reporting templates and relayed to surgeons and oncologists to optimise treatment planning.
· The ongoing patient prognostic risks based on the image assessment of the treated tumour should be shared with patients so that they can seek the best onward care