Mucinous Tumours: — Why PET Often Misses Them

For most cancers, a negative PET is reassuring. For mucinous tumours, it is not. The scan measures glucose uptake, and mucin absorbs almost none — so a negative result can occur even when active cancer is present. Your team should confirm whether your pathology report described the tumour as mucinous before the PET result is treated as clearance. If it did, staging will be based on CT and MRI rather than on the PET alone.

Mucin is a thick, gel-like substance that some cancer cells produce in large quantities. It is mostly water and carbohydrates — not metabolically active tissue. PET works by tracking glucose absorption; active cancer cells absorb a great deal, but mucin absorbs almost none. When a tumour is mostly mucin with only scattered cancer cells inside it, the overall glucose uptake is too low for PET to flag clearly.

Not for all — only for mucinous subtypes. For the more common non-mucinous colorectal cancers, PET plays an important role in staging and response assessment. The recommendation depends on what your pathology report says about the tumour type. If yours is non-mucinous, PET remains part of the standard workup. If it is mucinous, MRI with diffusion-weighted imaging gives more reliable information for staging.

Low FDG uptake is consistent with mucinous tumours but does not diagnose one. Mucinous histology is confirmed from your biopsy pathology report, not from the scan. Several other things can cause low uptake on PET — low-grade tumours, very early disease, or small deposits below the scan's detection threshold. Ask your oncologist whether the pathology was reviewed for mucinous features, and what the CT and MRI findings showed alongside the PET.

Possibly, yes. Even with mucinous cancers, PET may still be ordered to assess sites where it remains informative — lymph nodes or distant organs that are not mucinous in nature. The two scans answer different questions: MRI gives detail about the primary tumour and its local spread, and PET gives a whole-body overview of metabolically active disease. Your oncologist will decide which combination gives the most useful picture for your specific situation.

Mucinous histology does affect some treatment decisions, and your oncologist will explain how it applies to your specific cancer and stage. In colorectal cancer, mucinous tumours respond differently to some chemotherapy regimens, and this is taken into account when planning treatment. We do not yet have consistent evidence about how mucinous histology affects long-term outcomes across all cancer types — this is an active area of research. Ask your oncologist what it means specifically for you.