Chapter 7 132 Discussion The main finding of this study is that a CMR on PET/CT adequately predicts R0 resection in 96 percent of patients. In addition, this study established an association between the visual PET/CT response evaluation and the histopathological response. Although this connection is not strong enough to make definitive clinical decisions, it proves to be an alternative for the direct visualization of rectal cancer and is a complementary technique to aiddecision-making inmultidisciplinary tumour boardmeetings. Abetter responsewas noted in radiotherapy naive patients with regard to R0 resection rate and better Mandard scores, but this groupwas too small for further correlation analysis. Thestudy is thefirst to investigatethecorrelationbetweenthePET/CTresponseevaluation andthehistopathological outcomes inpatientswithLRRC. AlthoughPET/CTisanaccepted imagingtechniquetodetect thesuspectedrecurrenceofcolorectaldisease,usingPET/CTto monitor themetabolic response toneoadjuvant therapies inLRRCismorecontroversial.16,17 To the best of our knowledge, no previous studies have described neoadjuvant therapy monitoring by PET/CT in LRRC. Neoadjuvant therapy monitoring by PET/ CT has been explored to some depth in locally advanced primary rectal cancer, where studies showed an added benefit for PET/CT in addition to the current imagingmodalities, MRI and CT.18,19 Recent studieshaveassessedtherelationshipbetweenthePETmetabolicresponseandthe histopathological response in patients with locally advanced rectal cancer. Maffione et al. reported both visual response assessment and quantitation (i.e., maximum standardized uptake value [SUVmax], delta metabolic tumour volume [deltaMTV] and delta total lesion glycolysis [deltaTLG]) predictive of the histopathological response.20 Leccisotti et al. performed PET/CT both early and late in the course of neoadjuvant therapy.21 They confirmed that the PET/CT predicted the deltaSUVmax for early response assessment at the end of the second week of chemoradiotherapy, but not for the late assessment prior to surgery. Avallone et al. found that the deltaTLG showed the best accuracy in predicting pCR.22 In recent years, emphasis has been placed on 3D textural analyses of the primary tumour rather than its response toneoadjuvant therapy. Inthesestudies, arelationshipwas found between the metabolic volume of the primary tumour, some of its textural features and the histopathological response as well as disease- free survival.23,24 Unfortunately, a quantitative, full metabolic analysis was only possible in 23 patients in our study and thereforewas not included. The issues that may be responsible for these discrepant findings need to be discussed. The most important factor impeding a good correlation between the PET/CT and histopathological responses is time. The metabolic processes in a tumour change over
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