Colorectal PET/CT

Professor Jonathan Hill, Lancashire Teaching Hospitals NHS Foundation Trust

 Consultant in Radiology and Nuclear Medicine

Introduction:
The advent of new hybrid technologies combining multi-slice CT with high resolution crystal PET detectors has resulted in a new modality of PETCT imaging.

The CT component not only gives high anatomical resolution it is able to provide a body map which allows for the different densities of the body in order to compensate for photons arising from the FDG decay. This gives precise localisation of tumour.

The most common isotope in use at present is FDG. This is a positron emitter combined with glucose. Tumours over-express glucose receptors and due to their cellular metabolism trap glucose within the cells and hence ‘concentrate’ the tracer.

The examination takes an order of 30 minutes, which includes a CT protocol. Evolving technology means that it is possible to undertake a contrast study with the CT component and, for example, the Preston PETCT Centre scanner is a 64-slice model.

Indications

PETCT is indicated in a number of evidence-based situations:

• Evaluation of hepatic/intrapulmonary metastases. PETCT is of value for the confirmation of metastatic disease when identified by other imaging modalities, especially as it can detect additional non-visible anatomical pathology on MR and CT. This is particularly important in the planning of hepatic resection. Currently all patients in the north west are evaluated with PETCT prior to surgical resection. Ideally this should be combined with high-resolution MR as not all metastatic disease is FDG avid particularly low metabolic tumours such as adenocarcinoma which calcify.

PETCT will also demonstrate metastatic ‘M’ disease in such cases, which may affect decision-making regarding proceeding to intrapulmonary nodule or hepatic metastatectomy.

• The presence of pre-sacral thickening and distortion due to post-surgical change, infection, radiation or disease recurrence is a difficult radiological and clinical problem. FDG accumulates highly in tumour but less so in surgical scarring and post-radiotherapy change. Localised infection with abscess may show similar appearances to tumour but there are often supportive clinical features in infection. PETCT can assist in whether further intervention is required.
• The presence of a surveillance rising CEA level in the presence of a negative CT scan suggests occult disease. PETCT is of value for demonstrating activity in lesions that may be small by CT criteria and target therapy or biopsy.

Emerging indications include therapeutic response to chemo agents, e.g. staging PETCT to establish that the primary tumour and any metastases are FDG avid followed by a therapy-response scan after the first 1-2 cycles would show if the tumour is responding to the agent. If no response then an alternative agent could be considered without proceeding through a full chemo cycle with the attendant side effects and no attendant benefit.

Assessment of post-chemo treatment. This should take place 6 weeks after the last cycle to ensure that the images obtained are representing completion of therapy treatment rather than assessing whether the tumour has responded and therefore may be ‘stunned’ rather than ‘cured’.

The development of FDG intra-operative portable surgical probes which can detect the 511 KeV radiation characteristics could be useful for localising affected nodes in, for example, raised CEA levels. Resection limits could also be planned in the surgical theatre if the involvement of drainage nodes were known.

There is ongoing tracer development utilising carbon-based isotopes and further agents following processes of angiogenesis which will be arriving on the market in the next few years.

Published data shows that PETCT changes management in around 30% of cases by increasing the accuracy of localised disease. In some cases disease is down-staged. Caution is required in the case of low metabolic tumours and liver lesions thought to be cystic on CT and negative on PETCT should always be treated with suspicion for hepatoma in the correct clinical context.

The Royal College of Radiologists Making the Best Use of Clinical Radiology Services Referral Guidelines, 6th Edition (MBUR6) confirms PETCT in colorectal disease with Category B evidence at present in the main indications.

In a retrospective study of 45 patients with colon cancer, comparing [18F]FDG-PET/CT with [18F]FDG-PET alone, there was a decrease in the number of equivocal readings by 50% for [18F]FDG-PET/CT. In addition, lesion characterization increased by 30%, and accurate localization by 25% (Tables 3 and 4). Overall staging accuracy improved from 78% to 89% on a patient-by-patient analysis (Fig. 7) [, 11].

Table 3

Table 4

Figure 7

A 57-year-old woman presented with pain and constipation and colonoscopy revealed an obstructing rectal mass. A staging FDG-PET/CT demonstrated intense FDG avidity in a circumscribed mass-like thickening of the proximal rectum (arrows in top row images)

In another study of 65 patients with suspected or known colorectal carcinoma, sensitivity and specificity for PET/CT was 96% and 97% compared to 77% and 89% for PET alone. Similarly the sensitivity and specificity for metastases detection were 95% and 98% for PET/CT compared to 66% and 79% for PET alone [12–14].
11. Cohade C, Osman M, Leal J, Wahl R. Direct comparison of (18)F-FDG PET/CT in patients with colorectal carcinoma. J Nucl Med. 2003;44:1797–1803. [PubMed]
12. Shiepers C. PET/CT in colorectal cancer. J Nucl Med. 2003;44:1804–5. [PubMed]
13.Francis DL, Visvikis D, Bomanji JB, et al. The impact of FDG PET/CT in colorectal cancer: an outcome study (abstract). J Nucl Med. 2003;44:26P.
14. Berger I, Goerres GW, von Schulthess GK, et al. PET/CT: diagnostic improvement in recurrent colorectal carcinoma compared to PET alone. Radiology. 2002;225((suppl P)):242.