Desley van Zoggel

PET/CT response after induction chemotherapy 121 CHAPTER 7 Introduction Themost important prognostic factor for oncological outcomes in the curative approach for locally recurrent rectal cancer (LRRC) is surgery with negative resection margins (R0 resection). Owing to the removal of the fascial resection planes during surgery for the primary tumour, LRRC is usually not well defined and often invades multiple planes and structures. In order to achieve R0 resection, major extended procedures are required, whichare associatedwithhighmorbidity and loss of function. The rationale for intensified neoadjuvant strategies, including induction chemotherapy and chemo(re) irradiation, is to downsize and downstage the tumour to facilitate an R0 resection.1,2 In locally advanced primary rectal cancer, a watch and wait policy is warranted in a subset of patientswho achieve a clinical complete response.3,4 Such an approach has not been developed for LRRCpatients yet, due to difficulties in assessing a clinical complete response. In LRRC patients assessment relies heavily on imaging techniques as inmost patients local recurrences cannot be assessed with endoscopy. However, pathological complete response (pCR) rates close to 20 percent have been described in LRRC.1,5 The presence of postoperative changes and fibrosis mixed with tumour- bearing tissue complicates the assessment of the clinical response. After a complete response, the anatomy will not return to normal, and the remaining fibrosis can be challenging to differentiate from vital tumour tissue on MRI. The gold standard for assessing the response to neoadjuvant treatment is histopathological analysis. However, this method only confirms the response postoperatively. An objective imaging tool that could accurately identify patients with a pCR would allow an R0 resection to be predicted and could help to develop research programmes in which surgery is postponed or avoided. Alternatively, a poor response might guide treatment intensification or deferral from curative strategies. Traditionally,MRI andCThavebeenused tomonitor theresponse to therapyandexclude distantmetastases prior to surgery. MRI is particularly suitable for assessing anatomical changes in patients with primary rectal cancer.6 However, in LRRC, anatomical changes might be more subtle and areas of the vital tumour may have been replaced by fibrotic tissue. In addition, PET/CT can be used to monitor the metabolic changes that occur following neoadjuvant therapy and to predict the histopathological response. A decrease in fluorodeoxyglucose (FDG) uptake after chemotherapy and/or radiotherapy has been correlatedwith the histopathological response in several tumour types.7–9 The response on PET/CT can be scored visually, according to accepted guidelines, and/or quantitatively using full metabolic analysis.10,11 The main objective of this retrospective