Advances in Imaging to Assess Response in Rectal Cancer

Bowel cancer is the 4th most common cancer in the UK and accounts for 11% of all new cancer diagnoses, a third of these cancers are located in the rectum. Approximately 732, 000 rectal cancers are diagnosed in Europe per year.

Radiotherapy (RT) is a well-established treatment for locally advanced rectal cancer (LARC). It can be used pre operatively in margin threatening disease to increase the likelihood of R0 resection margins; reduce the rate of loco regional recurrence; and to improve loco regional control for patients unsuitable for surgery. There are two treatment regimes used internationally, with strong evidence from numerous randomised clinical trials. These are long course chemoradiotherpy (LCRT) with a dose of 45-60Gy delivered in 25 fractions and short course radiotherapy (SCRT) with a dose of 25Gy in 5 fractions. No long-term superiority of either approach has been found thus far.

RT indications for LARC are defined by clinic-radiological MRI defined criteria. Accurate and precise delineation of the target volume is fundamental to the success of RT. CT planning scans are traditionally used for target delineation in RT, have poor soft tissue contrast and on this dataset have shown overestimation of tumour volumes. Given that magnetic resonance images (MRI) brings superior visualisation of soft tissue in the diagnostic phase of LARC, there has been a move towards routine use of MRI within the delivery of RT.

MRI imaging is considered the gold standard in the staging and response assessment of rectal cancers. The rectum lends well to the exquisite soft tissue definition MRI offers, and additional functional information can be used to further characterise tumour. Improved tumour localisation up front and assessment of responses during RT has the potential to improve patient outcomes in numerous ways.

The use of MRI in RT planning of rectal cancer is relatively new. A survey of UK practice identified specifically 64% of centres use a diagnostic MRI alongside to aid delineation, 28% register MRI images with the diagnostic scan and only 3% use a registered scan in the RT position. National rectal cancer guidelines recommend MRI to be used if possible to aid delineation of the gross tumour volume (GTV). A report published by IPEM gives clear guidance for the optimal use in RT planning for rectal cancer.

There is a general acceptance that the use of MRI in rectal cancer RT planning is superior. In the literature there are a number of studies looking at different aspects of MRI application in the rectal cancer RT pathway. However, there is great disparity in the focus of these research papers.

One of the main challenges in gaining optimal benefit from this technology is the many uncertainties in the planning and delivery of RT to rectal cancers. Largely due to the target being a soft tissue structure, which moves independently due to gas and faecal filling. The surrounding elective volume includes the mesorectum and regional lymph nodes, resulting in targets with a differential motion. This brings more uncertainty in reproducibility, the foundation of accurate and precise RT. MRI with improved soft tissue visualisation could potentially enable voluming of only the tumour and not the full rectal lumen. This could be practice changing and requires careful consideration in implementation. For these reasons correct application of MRI in rectal cancer RT planning is crucial to ensure the optimal benefit from this technology is gained. In the setting of serial imaging reproducibility is more important, where repeatability and reliability of data matters more.