Chapter 7 134 Another imagingmodality often used to provide information about LRRC isMRI, which is also used to evaluate tumour regression. The correlation with R0 resection was a littleworse than after PET/CT (P = 0.042 and P = 0.002, respectively). As the agreement between PET/CT andMRI is only fair27, both imaging modalities seem complementary. PET/CT reflects the metabolic activity in vital tumour cells, whereas MRI focuses more on fibrosis surrounding the tumour cells. In selected patients there may be reasons to delay surgery after complete response. In these patients, PET/CT may be complementary to MRI to differentiate between good and poor responders and can therefore aid in treatment decision-making. One of the major limitations of this study is that PET/CT evaluations were not performed according to a standard protocol. Regular PET/CT evaluations before, during and after neoadjuvant treatment were implemented during the entire inclusion period. This led to different time intervals, also caused by the fact that a substantial proportion of the included patients were referrals from other hospitals. The suboptimal timing of response assessment with PET/CT in some of the included patients negatively influenced the reliability of PET/CT. Another limitation is the relatively short interval of median 31 days between the end of radiotherapy and response evaluation PET/CT. The reason for this is that, after prolonging preoperative treatment with the implementation of induction chemotherapy, early re-evaluation was performed in order to avoid patients who were progressive and subsequently would forfeit the chance of resection as a consequence of this. However, no patients were identified as progressive and could not undergo surgery. It would be better to prolong the interval after radiotherapy and shorten the interval between PET/CT and surgery. Furthermore, in the early years of the inclusion period, PET/ CT was performed only in highly complex cases. In addition, the number of patients with LRRC receiving induction chemotherapy is still low; therefore, the subset analysis is underpowered and should be considered with care. Conclusion Metabolic PET/CT response evaluation after neoadjuvant treatment proves to be a complementary diagnostic tool to standardMRI in assessing tumour response andmay play a role in treatment planning in LRRC patients. In order to be able to perform an R0 resection the anatomical and topographical information fromanMRI remains the cornerstone of the preoperative imaging in LRRC. Comparable to primary rectal cancer, response evaluation in LRRC may become more important for decision-making.
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