PET/CT response after induction chemotherapy 133 CHAPTER 7 time and are influenced by different neoadjuvant treatment modalities. Even after the end of neoadjuvant treatment, tumourmetabolismremains a dynamic process, and the tumour may either regress or progress. The timing of the post-treatment PET/CT and the interval to resection are important to predict the response. If the interval between the post- treatment PET/CT evaluation and resection is too long, local tumour regrowth might have occurred.25 Alternatively, if the interval between the post-treatment PET/ CT evaluation and resection is too short, there is no time for an ongoing response to develop. In this dataset, the median time between post-treatment PET/CT and surgery was 51 days. We found that the correlation between the PET/CT response and the histopathological outcome was significantly better at shorter intervals. The sensitivity to detect major histopathological responsewas as high as 86 percent, compared to only 53 percent for the longer interval. When the interval between post-treatment PET/ CT and surgery exceeded 51 days, the PET/CT response did not reliably predict the histopathological response. If this prediction is pivotal for treatment planning, the PET/ CT evaluation may need to be repeated. Regardless of the fact that the PET/CT evaluation is a snapshot of a dynamicmetabolic process, further factors can confound the correlation with the histopathological response. Mandard classification is a score based on tumour density, whereas the PET/ CT response also accounts for the tumour volume, hindering a direct comparison of the scoring systems.10,14 In some cases, the tumour volume may have decreased when the clinical responsewas recorded, but if the percentage of vital tumour cells in the residual tumour was above 50 percent the Mandard score would be 4 or 5, which is considered a poor response. It is also known that some histological types are less FDG- avid and persist after neoadjuvant therapy.26 In this dataset, no patients with signet ring cell differentiation were included. Mucinous histopathology did not significantly impact the predictability of the response. The tumour response can be underestimated by PET assessment when there is persistent inflammation at the time of PET/CT. Inflammation is known to occur after chemoradiotherapy. Usually, the inflammatory reaction is only temporary, but it can persist at recurrence locations such as the presacral area. For this area, the visual response assessment is probably advantageous over a quantitation because the visual assessment can appreciate the patterns of FDG uptake typical for inflammation (e.g., diffuse FDG uptake in the presacral area, FDG uptake of an even nature in the wall of a presacral cavity).