Herewith we report the utility of both pre- and post-CRT PET parameters for outcome prediction in a cohort of AC patients. The patients were uniformly treated according to mandated contouring and dosing guidelines on a prospective multi-centre trial with FDG-PET/CT scans performed at the time of staging, and at 12 weeks post-completion of CRT.
Due to the small sample size, no definitive conclusions can be made regarding any particular parameter, but our findings provide several valuable insights. Firstly, post-CRT PET imaging appears to better predict for local recurrence than does pre-CRT imaging. Secondly, while a volumetric parameter (MTV) was more predictive of recurrence on pre-CRT PET imaging (in keeping with some previous studies), SUV parameters were more predictive on post-CRT imaging. Thirdly, PET parameters extracted from the primary tumour of post-CRT images better predict local than any recurrence, as may be expected, however this was not true for the pre-CRT images.
Quantitative PET imaging
SUVmax is the most commonly used PET parameter in clinical practice. It is simple and quick to measure but is subject to noise because its value is taken from a single voxel. Alternatively, SUV mean, median or standard deviation within a VOI bounded by a threshold SUV may be more robust [
17]. SUV thresholds for bounding VOI may be absolute (e.g. 3, 5, or 10) or relative to the SUVmax of the tumour. Another PET parameter in use is the peak SUV which is the average SUV contained within a small VOI approximating 1 cm
3 that encompasses the voxels surrounding the SUVmax.
Volumetric PET parameters include the MTV, a measure of metabolic tumour volume above a threshold SUV, and total lesion glycolysis (TLG), which is the product of the mean SUV and MTV. Many feel that MTV and TLG better reflect metabolic tumour burden [
17] and studies that have correlated MTV and TLG with prognosis [
18‐
20] have included both cervical and head and neck cancer treated with CRT [
21,
22].
Pre-treatment PET imaging in anal cancer
FDG-PET/CT is recommended for AC staging and radiotherapy planning [
2,
4]. Retrospective studies have correlated staging PET parameters with outcome in AC. Kidd et al. reported that a higher SUVmax was associated with an increased risk of lymph node metastases and worse disease-free survival [
10]. In a smaller study, Deantonio et al. also showed that higher SUVmax predicted for more advanced stage but were unable to demonstrate any correlation with survival [
11].
Three other studies were unable to correlate SUVmax with outcome [
12‐
14]. They did however correlate MTV with outcome, although at different cut-off values. This is likely due to varying techniques used to calculate the MTV. Bazan et al. used a threshold of 50% SUVmax, included the primary and any nodal disease, and found a cut-point of 26 ml correlated with PFS [
12]. Gauthe and colleagues also used a threshold of 50% SUVmax but only included the primary tumour. They found a cut-off of 7 ml correlated with overall survival [
13]. Shali et al. included all metabolically active disease (primary, lymph nodes, and distant metastases) and used a threshold to define the volume of 30% of the SUVmax [
14]. Not surprisingly, the MTV cut-off was higher (45 ml) than the others.
While these studies suffered from a retrospective design and heterogenous treatments, we too found that a volumetric parameter on the pre-treatment PET better correlated with recurrence than an SUV parameter. Our volume was bounded by 41% of the SUVmax and surrounded the primary tumour only. We chose to focus on the primary tumour as this is the most common site of recurrence [
23‐
25] and is more practical to measure in routine clinical practice. We also pre-determined which parameters to examine prior to performing our analysis in order to reduce the impact of multiple statistical testing.
Post-treatment PET imaging in anal cancer
Retrospective studies have investigated the predictive value of post-treatment PET scans in AC [
5‐
9]. While most have reported that a complete response on PET correlated with improved survival, all suffered from heterogeneous treatment, variable timing of PET imaging, and visual assessment only. Following definitive treatment, the timing of PET imaging in particular can have a significant impact on the sensitivity and specificity.
This was highlighted in a thorough prospective study by Mistrangelo et al. who performed PET imaging and anal biopsies at 1- and 3-months post completion of CRT on 53 patients [
26]. Again, treatment techniques varied and imaging assessment was qualitative, but compared to 1 month, PET imaging at 3 months was found to have both improved sensitivity (66.6 vs. 100%) and specificity (92.5 vs. 97.4%) for detection of persistent disease.
The timing of post treatment PET imaging in the published retrospective studies ranged from 0.9–5.4 months [
5], 2-7 months [
6], 1-8 months [
7], 1-8 months [
8], and 1-6 months [
9]. This significantly undermines the ability to draw robust conclusions from their data and limits its application to clinical practice.
Cardenas et al. is the only published study to perform quantitative analysis of post-treatment PET imaging [
27]. Treatment was non-standardised, the only parameter assessed was SUVmax and, unfortunately, they fail to report a range for the timing of PET imaging. In addition, their follow-up ranged from 3.6–94.1 months, which is less than the median time to PET imaging (3.8 months). As such, our analysis is both novel and lacks some of the weaknesses present in the current literature.
While our study has many strengths, its major limitation is the small sample size which reflects the scarcity of AC. We also acknowledge that extracting seven PET parameters at six SUV thresholds generated a large amount of data which increased the likelihood of finding significant outcomes. However, such an exploratory analysis is useful for identifying signals which warrant further investigation in larger patient cohorts, which was the aim of this preliminary study.
That FDG-PET/CT imaging was performed on three different PET/CT systems is both a strength and weakness. While it introduces more variables, our results are likely more robust, they reflect real world practice, and are hence more applicable in the community. Future studies could consider including all metabolically active disease (primary and nodes) which may (or may not) better predict for any recurrence.
Finally, the emergence of digital PET/CT with enhanced image quality, greater sensitivity, increased signal to noise ratio, faster time-of-flight resolution, and improved quantitative accuracy means that PET/CT will represent an increasingly attractive biomarker in the future [
28].