An evaluation of four CT-MRI co-registration techniques for radiotherapy treatment planning of prone rectal cancer patients

Objectives

MRI is the preferred staging modality for rectal carcinoma patients. This work assesses the CT–MRI co-registration accuracy of four commercial rigid-body techniques for external beam radiotherapy treatment planning for patients treated in the prone position without fiducial markers.

Methods

17 patients with biopsy-proven rectal carcinoma were scanned with CT and MRI in the prone position without the use of fiducial markers. A reference co-registration was performed by consensus of a radiologist and two physicists. This was compared with two automated and two manual techniques on two separate treatment planning systems. Accuracy and reproducibility were analysed using a measure of target registration error (TRE) that was based on the average distance of the mis-registration between vertices of the clinically relevant gross tumour volume as delineated on the CT image.

Results

An automated technique achieved the greatest accuracy, with a TRE of 2.3 mm. Both automated techniques demonstrated perfect reproducibility and were significantly faster than their manual counterparts. There was a significant difference in TRE between registrations performed on the two planning systems, but there were no significant differences between the manual and automated techniques.

Conclusion

For patients with rectal cancer, MRI acquired in the prone treatment position without fiducial markers can be accurately registered with planning CT. An automated registration technique offered a fast and accurate solution with associated uncertainties within acceptable treatment planning limits.Randomised trials have demonstrated that adjuvant radiotherapy (RT) in patients with resectable rectal cancer offers a statistically significant reduction in the risk of local recurrence compared with surgery alone 1]. Two meta-analyses and a systematic review have confirmed this finding 2-4]: cancer-specific survival was found to be improved when RT was delivered with biological equivalent doses of >30 Gy pre-operatively. Two further trials have confirmed similar benefit when short-course pre-operative RT was combined with total mesorectal excision (TME) 5,6].Three further randomised trials have evaluated the role of pre-operative adjuvant chemoradiotherapy (CRT) for patients with stage T3–T4 or node-positive disease. Two of these studies demonstrated a reduction in local recurrence when pre-operative adjuvant CRT was used rather than long-course adjuvant RT alone 7,8]. The third study demonstrated both reduced local recurrence and reduced acute and late toxicity for pre-operative CRT compared with post-operative CRT 9]. The Cochrane review 4] also examined CRT and concluded that CRT provided incremental benefit in local control, irrespective of the timing of the chemotherapy. These results have led to a significant increase in the use of pre-operative radiation for patients with rectal cancer.MRI offers increased soft-tissue contrast compared with other radiographic imaging modalities such as CT. This improvement allows the accurate identification of the gross tumour and the mesorectal fascia in at least three planes. A number of studies have recommended using high-resolution MRI as a tool to determine the relationship between the potential circumferential resection margin (CRM) and the tumour 10-15], with one recent large prospective multicentre study demonstrating a specificity of 92% in predicting an involved CRM 16]. These results are supported by a recent meta-analysis of nine studies (529 patients) that revealed an overall sensitivity of 94% and specificity of 85% 17]. The review article by Klessen et al 18] also suggests that MRI is the only imaging modality that enables such accurate evaluation. Consequently, MRI is the reference standard for the clinical assessment of patients with colorectal cancer and is recommended for routine use by the National Institute for Clinical Excellence (NICE) in the UK 19].Current clinical practice at St James’s Institute of Oncology is for patients to be selected for pre-operative CRT if the margins of resection are considered at risk by the colorectal multidisciplinary team. The patient is CT-scanned in the prone treatment position and the oncologist defines the gross tumour volume (GTV) on individual transaxial images, leading to the determination of the planning target volume (PTV). To assist in the definition of the GTV, the oncologist is aided by diagnostic CT and/or MRI and other clinical information. Where diagnostic MRI is available, it is usually acquired in a different patient orientation (e.g. supine position) or radiographic plane (e.g. orthogonal to the long axis of the tumour) and at a different time to the CT study, meaning that the anatomy may differ in appearance. Furthermore, the MRI may be on radiographic film or on a different computer system to the planning CT.Image co-registration is the process of finding the mathematical transformation that aligns several different radiographic studies 20]. Various rigid-body co-registration techniques are currently offered by commercial RT treatment planning systems (TPSs), including those that employ operator-defined corresponding landmarks, interactive drag-and-drop or full automation.There are obvious potential advantages of including image registration in the treatment planning process 21], but historically, technical issues associated with MRI distortion, artefacts and lack of electron density information, along with little evidence of its positive effect on patient outcome, are likely to have precluded its universal use in treatment planning 22]. Despite these issues, studies have demonstrated the feasibility of pelvic image registration for RT treatment planning 23-26].Furthermore, rectal cancer specifically has been identified as a disease for which co-registered MRI could enhance treatment planning 27,28]. There is considerable interest in RT in which the GTV is defined on MR imaging in the same frame of reference as the planning CT. Increased accuracy in defining the GTV and/or clinical target volume (CTV) would give a more accurate definition of the dose required to those volumes or allow dose escalation to the GTV in patients with locally advanced disease. Acquiring an MRI with the patient in the prone position, however, introduces a new set of potential challenges, including increased respiratory artefacts that affect registration accuracy.Several studies have investigated the accuracy of image registration in a RT context, but as concluded by Sharpe and Brock 29] in their review of image registration quality assurance, there is no consistent approach. One approach is to image phantoms with objects of known spatial location with both CT and MRI 30-33]. This has the advantage of enabling controlled measurement of errors throughout the entire process of image acquisition and image registration, with a ground truth established through the objects with known spatial location. Such phantoms can also be used to estimate the spatial distortion in the images. However, these studies are all based on the use of MRI to image the head and neck rather than the pelvis. The studies’ dependence on physical phantoms means that uncertainties that may arise from natural variations in shape, size and composition in the patient population are not measured. Furthermore, the two studies evaluating the performance of commercial registration algorithms using patient images are both based on the registration of images of the head 32,34].Several studies have employed CT-MRI co-registration for pelvic sites 23-26,35,36], but few data exist on measured image registration uncertainties using existing image registration functionality in commercial TPSs. Furthermore, no evidence could be found of image registration for patients imaged in the prone position.The primary aim of this study is to assess the co-registration accuracy of two fully automated and two hybrid manual-automatic techniques using two commercial TPSs. On the basis of these results, this study aims to determine the most appropriate clinical image registration process and whether manual adjustments after automatic image registration are necessary or even beneficial. It is expected to provide supporting information for RT centres that are considering utilising co-registered prone MRI in treatment planning for rectal cancer.