PET-CT after radiofrequency ablation of colorectal liver metastases: Suggestions for timing and image interpretation
Introduction
Colorectal cancer is the second most common malignancy in the Western world. At diagnosis of the primary tumor, metastases are already present in 20–25% of patients and another 20–25% of patients will develop metachronous metastases. Of these, 90% will become apparent within 3 years [1].
Although surgical resection of colorectal liver metastases remains the treatment of choice with a 5-year survival of up to 50% with additional chemotherapy, the majority of patients are ineligible for surgery due to location, size or number of metastases, or due to co-morbidity [2], [3].
For patients with irresectable colorectal liver metastases, several alternative treatment options that can achieve complete local tumor control have emerged, of which radiofrequency ablation (RFA) is the most extensively studied and most widely available. RFA can result in complete tumor clearance and recent literature suggests that 5-year survival rates following RFA have increased from 18% in early data [4] to 36% in recent years [5], with rates as high as 48% in patients treated for a solitary lesion [6].
The main area of concern following RFA treatment is the risk of developing a local site recurrence (LSR), which occurs in 3.6–27% of cases mostly depending on the size of the treated lesion [5], [7]. Prompt diagnosis of a local site recurrence is important because repeated treatment can lead to complete tumor clearance, especially when recurrences are still of limited size [8]. Currently, computed tomography (CT) and magnetic resonance imaging (MRI) are the most commonly used imaging methods to monitor post-ablative lesions for remnant or recurrent disease. One shortcoming of follow-up using these imaging modalities is the presence of post-ablation effects; for instance, in contrast enhanced computed tomography (ceCT), reactive tissue can present as a hypodense area around the ablated lesion. This can often be indistinguishable from viable tumor tissue, without proof of lesion growth on consecutive scans [9].
Recent literature suggests that fluorine-18 deoxyglucose positron emission tomography (FDG-PET) could play an important role in assessing the presence of residual tumor following RFA [9], [10], [11], [12], [13]. Unlike traditional anatomic imaging, FDG-PET visualizes glucose metabolism. Because glucose uptake is enhanced in tumor cells, FDG-PET has proven to be able to largely overcome the drawback of post ablation effect [14], [15]. Using PET-CT, PET images are combined with CT data to provide accurately fused functional and morphological data sets in a single session [16]. Several studies have shown the superiority of PET-CT over morphologic imaging alone in the follow-up after ablation of colorectal liver metastases with a sensitivity and specificity of PET-CT (92% and 100%) compared to ceCT (83% and 100%) regarding the detection of local tumor progression [9], [10], [11], [12].
Despite of these good results, no standardized PET-CT regime has yet been proposed in the literature and diagnostic criteria with respect to PET-CT image interpretation are lacking. Both qualitative and semi-quantitative criteria have been used in image interpretation [9], [10], [12], [17], [18], [19] and there is a growing need for standardization [11]. In the present study, we analyzed PET data in order to assess the time-point at which a local site recurrence is visible on PET-CT. We also intended to explore the normal post-ablation effects in the FDG uptake in treated lesions with the aim to describe image interpretation criteria for local failure.
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Patients
This observational study was conducted with the approval of the institutional Science Commission and following the Code of Ethics of the World Medical Association (Declaration of Helsinki). All included patients were treated between January 2005 and January 2011 for colorectal liver metastases with RFA alone or in combination with resection, and they all underwent at least one PET (or PET-CT) and ceCT in the first year after treatment. Patients who received their first PET scan beyond the
Results
Between January 2005 and January 2011, 79 consecutive patients with 246 lesions underwent RFA therapy or RFA combined with resection, and were screened within one year after treatment using at least one PET. A total of 179 lesions were ablated and 67 lesions were resected. The majority of the patients had 1–3 lesions treated with RFA (68), but this could increase up to 9 lesions. Baseline characteristics are summarized in Table 1. Twelve patients were referred to a medical oncologist following
Discussion
The majority of the PET-CT scans described in this study were obtained within the first six months after RFA. Over 95% of all local site recurrences became apparent within one year of RFA treatment, and only one patient was diagnosed on the third scan after fifteen months. No local site recurrence was detected during follow-up after 15 months. Local site recurrences are rare in treated lesions <2 cm. Our results show that an initial negative PET-CT does not rule out a local site recurrence and a
Conclusion
It is well-established that PET-CT shows a high sensitivity and specificity in the detection of a LSR after RFA of colorectal liver metastases. Focal FDG uptake at the ablation zone is clinically considered prove in this regard. However, in literature there is little attention for timing and image interpretation of the PET-CT.
Our study has shown that almost all LSR are detectable within one year. Increased FDG uptake around the rim of an ablated area can disappear spontaneously within 5 months
Conflict of interest
All authors declare no conflict of interest.
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