'Sub-Centimetre Node' — and Why Size Matters on PET

Sub-centimetre describes a lymph node measuring less than one centimetre at its widest point. On a PET scan, nodes this size cannot have their metabolic activity accurately measured because of a physics constraint called the partial volume effect. The term is a size description, not a diagnosis. Your oncologist will explain what the finding means in the context of your full scan results — the word alone, without that context, is not something you can interpret yourself.

Yes — this is the limitation the radiologist is naming when they flag a sub-centimetre node. The partial volume effect means disease in a small node can produce a diluted signal, making the node look less active than it truly is. Your oncologist accounts for this and does not rely on PET alone for very small structures. The full picture includes CT size measurement, your clinical examination, and any other relevant findings from your workup.

Partial volume effect is what happens when a scanner tries to measure something smaller than its reliable range. Instead of reading only the node, the scanner averages the signal from the node and the surrounding tissue together, diluting the result. The node's true activity level is hidden inside a weaker combined reading. This is not a machine fault — it is a physical property of how PET imaging works, and every radiologist reporting sub-centimetre structures is working within this constraint.

Probably not immediately. Nodes under one centimetre are generally too small to sample reliably with standard biopsy techniques — locating the node and obtaining enough tissue both become technically difficult at this size. Your team is more likely to monitor the node on your next follow-up scan and look for any change in size or behaviour. If the node grows or becomes clearly active on a later scan, the decision about biopsy will be reassessed at that point.

Your radiologist compares each scan to the one before it. The node's size and location are recorded in the report so the next reader can see whether anything has changed. You will not usually be able to track this yourself between appointments. If you notice a new lump, swelling or tenderness near where the node was reported before your next scheduled scan, tell your team rather than waiting. Any new finding in the interval is worth reporting early.

Neither size automatically means more or less to worry about — that question requires your oncologist's answer for your specific case. They are looking at the node's location, FDG activity level, comparison to previous scans, and where it sits within your overall cancer picture, not size alone. A large non-avid node in a low-risk location is a different finding from a small avid node near confirmed disease. Bring this question to your next appointment with your scan report in hand.