Clinical application of multimodality imaging in radiotherapy treatment planning for rectal cancer

Abstract

Radiotherapy plays an important role in the treatment of rectal cancer. Three-dimensional conformal radiotherapy and intensity-modulated radiotherapy are mainstay techniques of radiotherapy for rectal cancer. However, the success of these techniques is heavily reliant on accurate target delineation and treatment planning. Computed tomography simulation is a cornerstone of rectal cancer radiotherapy, but there are limitations, such as poor soft-tissue contrast between pelvic structures and partial volume effects. Magnetic resonance imaging and positron emission tomography (PET) can overcome these limitations and provide additional information for rectal cancer treatment planning. PET can also reduce the interobserver variation in the definition of rectal tumor volume. However, there is a long way to go before these image modalities are routinely used in the clinical setting. This review summarizes the most promising studies on clinical applications of multimodality imaging in target delineation and treatment planning for rectal cancer radiotherapy.

FDG-PET/CT and MR imaging for target volume delineation in rectal cancer radiotherapy treatment planning: a systematic review

“…Several lines of evidence have established that PET/CT delineates smaller GTVs which practically approximate the true tumour. 13,16,26,28,43 A possible explanation for the larger PET/CT-CTV result from this study is likely related to the inclusion of proximal and/or metastatic lymph nodes missed on CT in the CTV. 28 Although both MRI findings are consistent with the literature, this present review found two studies that compared T2W-MRI with DW-MRI, whereas the earlier review reported three studies which compared MRI with CT and co-registered MRI-CT.…”

“…12 Currently, the mainstay of radiotherapy delivery technique for rectal cancer has been based on 3D conformal radiotherapy and advanced conformal techniques, namely, intensity-modulated radiotherapy (IMRT), volumetric-modulated arc therapy and imageguided radiotherapy (IGRT). 13 The advent of these techniques has demonstrated beneficial therapeutic ratio effects by maximising dose to the TV, reducing radiation toxicity to organs at risk (OAR), increasing the chance of local tumour control, and consequently, improving the quality of life. 14 However, the success of these techniques requires excellent imaging quality for accurate visualisation and delineation of the TV and OAR.…”

“…17,18 MRI in RTP is now well established to overcome the limitations of CT in rectal cancer RTP as it provides additional information that allows for detailed and accurate evaluation of the rectal wall and local tumour extent. 13 This could consequently result in smaller but accurate tumour contours thereby enabling dose constraint to the OAR and tumour dose optimisation. 19 MRI can provide functional information in addition to anatomical imaging.…”

Neoadjuvant PET and MRI-based intensity modulated radiotherapy leads to less toxicity and improved pathologic response rates in locally advanced rectal cancer

“…Accurate coverage of tumor volumes can be markedly improved with the use of PET and MRI (7,22). Prospective planning studies utilizing pre-treatment PET and MRI have led to changes in shape and size of GTVs in up to 72% of treatment plans and have identified new lesions in 15% of patients often leading to a change in treatment strategy (7,22).…”

Trimodality PET/CT/MRI and Radiotherapy: A Mini-Review

“…As a mobile organ, non-rigid registration between PET/CT and MRI shows good results, but this must be considered for the treatment planning ( 83 ). FDG PET/CT adds therefore information to MRI, with potentially a larger GTV in total when using the union of MRI and PET, and new or differently evaluated lesions in as many as 15% of the patients, potentially changing the treatment ( 84 ); further studies are necessary to well define the place of FDG PET in this indication ( 85 , 86 ).…”