Our study is the first to investigate the role of interim FDG PET-CT after ICT with TPF as a predictor of the efficacy of CCRT for HNC.
ICT before RT or CCRT is a treatment option for advanced HNC, particularly for tumors in the oropharynx, hypopharynx, and larynx, where organ preservation is an important therapeutic consideration [1]. Triple therapy with TPF is the standard ICT regimen [5,6,7,8]. Salvage surgery is recommended for patients who do not achieve CR after CCRT; however, the complication rate of salvage surgery after CCRT is higher than that of primary surgery, possibly because poor quality tissue contributes to less than optimal wound healing and postoperative function [21]. Thus, early prediction of the response to CCRT would allow clinicians to consider surgery following ICT for patients predicted to have a poor response to CCRT. Furthermore, although the ICT response rate is generally high, clinical outcomes are mixed and CCRT after ICT can cause severe toxicity [22].
We found that interim FDG PET-CT after ICT could be used to assign patients to treatment and predict the outcomes of CCRT. Patients showing deterioration after ICT might show decreased CR with subsequent CCRT. FDG PET-CT enables clinicians to evaluate the metabolic response to therapy and identify possible biomarkers of outcome and survival. Studies comparing FDG PET-CT with CT have demonstrated higher efficacy of FDG PET-CT in patients with advanced HNC [23, 24]. Furthermore, FDG PET-CT after ICT has been shown to be a better predictor of outcome than the endoscopic response or magnetic resonance imaging results in patients with HNC [25, 26].
A decrease in FDG uptake after therapy has been associated with prognosis and CR in other malignancies [27, 28]. In HNC, a significant decrease in SUVmax after one cycle of ICT indicates a better prognosis and local control [17, 29]. Changes in SUVmax after two or three cycles of ICT with S-1 and cisplatin are associated with CR after CCRT, PFS, and OS [20]. However, because TPF ICT administered in three cycles is the treatment of choice, it would be premature to evaluate the response to ICT after the first cycle, and few studies have investigated the role of interim FDG PET-CT in triple ICT for HNC. All of our patients received the standard triple regimen and completed three cycles of ICT. Moreover, we investigated the associations of MTV and CR with survival outcomes.
We found that patients who achieved CR after CCRT showed greater decreases in LN SUVmax (88.8% vs. 62.5%, respectively; p = 0.009) and total MTV (99.7% vs. 89.9%, respectively; p = 0.020) after ICT than did non-CR patients. Moreover, multiple regression analysis revealed that a ≥ 73% decrease in LN SUVmax and ≥78% decrease in total MTV predicted CR after CCRT. Thus, we cautiously suggest that interim FDG PET-CT after ICT can be used to assign patients to treatment including CCRT and surgery.
Notably, a decrease in the LN SUVmax, and not in the primary site, was a predictive factor. The primary lesion may involve more inflammation and physiological uptakes than the LN after ICT, so a decrease in the SUVmax of the primary lesion could be underestimated. There was no difference in the SUVmax of the primary lesion after induction chemotherapy in the non-CR and CR groups (3.6 vs. 2.2, respectively, Table 2), so it could not be a predictive factor. Moreover, the LN was usually smaller than the primary lesion, and the decrease in the LN SUVmax could be overestimated due to partial volume effects.
We examined the association of changes in LN SUVmax and total MTV after ICT with survival outcomes. Several previous studies have shown that SUVmax and MTV before CCRT, and changes in SUVmax and MTV after ICT, are associated with survival [14, 15, 17, 18]. Our multivariate analysis revealed that changes in total MTV and LN SUVmax were associated with PFS and OS. However, changes in total MTV were more strongly associated with survival than were changes in SUVmax. This difference may be explained by the fact that MTV is a volumetric and metabolic biomarker of the tumor; thus, unlike SUVmax, MTV can quantify the overall tumor burden.
Our study had several limitations. First, it was a retrospective analysis of a relatively small number of patients. First, this study was retrospective. The PET scan time from our protocol was not sufficient to reproducibly evaluate the FDG uptake in tumors. A prospective study is therefore necessary to measure metabolic parameters within a consistent range and compare them before and after therapy. Second, patients in this study were heterogeneous. We used the stage and cancer site for parameters to predict outcomes, but they were not significant. When we divided the patients by cancer sites, nasopharynx (n = 15) vs. other sites (n = 28), there was no significant marker for predicting outcomes in the nasopharyngeal cancer group. But in the other cancer site group, the LN SUVmax decrease and total MTV decrease remained as valid parameters for predicting outcomes (Supplementary Table 2). Because these subgroups were too small to be reliable, further studies involving more patients are necessary. Third, our cutoff values were based on data from a single center; thus, further studies are needed to validate our findings. Nevertheless, we found marked differences in clinical outcomes when patients were classified as good or poor responders according to cutoff values, and our findings were consistent with those of a previous study [20]. Fourth, suspected residual lesions were confirmed by pathological biopsy, and the responses were assessed by radiological and laryngoscopic examination. Moreover, we found differences in incidence of chemotherapy during CCRT, and method of radiation among patients. However, we found no differences in CR, PFS, or OS according to the type of chemotherapy or method of radiation (data not shown).
Our findings suggest that interim FDG PET-CT after three cycles of triplet ICT may predict the efficacy of CCRT in patients with HNC, and we cautiously suggest that metabolic parameters measured by FDG PET-CT after ICT can be used to further assign patients to treatment and predict the prognosis after CCRT. However, future prospective studies with large sample sizes are needed to validate our findings.