Response prediction by FDG-PET after neoadjuvant radiochemotherapy and combined regional hyperthermia of rectal cancer:: correlation with endorectal ultrasound and histopathology

Accurate response assessment after neoadjuvant therapy is essential in patients with rectal cancer. The aim of this study was to assess the value of fluorine-18 fluorodeoxyglucose positron emission tomography (FDG-PET) in predicting response of locally advanced rectal cancer to preoperative multimodal treatment. Twenty-two consecutive patients with locally advanced (uT3/4) primary rectal cancer were entered in this prospective pilot study. FDG-PET was performed before and after neoadjuvant radiochemotherapy (RCT) with combined regional hyperthermia (RHT). Treatment consisted of external-beam radiotherapy (45 Gy), chemotherapy (folinic acid and 5-fluorouracil) and regional pelvic hyperthermia followed by curative tumour resection 6-8 weeks later. Semi-quantitative measurements (SUV) of tumour FDG uptake were made before and 2-4 weeks after completion of neoadjuvant treatment. Two patients who did not receive post-therapeutic restaging by FDG-PET were excluded from the analysis. Results were correlated with findings on endorectal ultrasound (EUS, n=17 patients) and histopathology. Histopathological evaluation of the resected tumour revealed complete response in one patient, partial response in 12 and stable disease in seven. SUV reduction in tumours was significantly greater in responders than in non-responders (60% (+/-15%) vs 30% (+/-18%), P=0.003, CI=95%). Using a minimum post-therapeutic SUV reduction of 36% to define response, FDG-PET revealed a sensitivity of 100% (EUS: 33%) and a specificity of 86% (EUS: 80%) in response prediction; the corresponding positive and negative predictive values were 93% (EUS: 80%) and 100% (EUS: 33%), respectively. FDG-PET results were statistically significant (P<0.001, CI=95%). FDG-PET has great potential in the assessment of tumour response to neoadjuvant RCT in combination with RHT and is superior to EUS for this purpose.