I very often have patients who ask me “is there a test that can detect cancer cells in the body?” or “shall we do PET CT and confirm the presence of cancer and then give next treatment?” Well, the answer is NO! The PET scan can only detect cancer when it is at least larger than 5mm in size in a given place. This is about 10 million cells sitting in one place. Its accuracy is better when the size is 10mm which is about 100 million cells. So, PET detects cancer nodules and not cells.
Over the recent years, PET/CT imaging has become a valuable tool in oncology or cancer treatment. Based on the higher sensitivity and specificity of staging compared to conventional imaging modalities such as CT or MRI alone, PET/CT is preferred in staging. In addition, PET/CT may also be used for identifying patients with
cancer who are at high risk of relapse and predicting benefit from treatment.
PET scan detects cancer by its high level of activity. The cancer cells multiply in number very rapidly and so consume a lot of glucose. PET CT detects this and shows cancer as an orange activity as shown in the image.
The role of PET CT is well defined in cancer care:
1. Diagnosis: Has very limited role in the diagnosis of cancer as many other diseases show activity and are mistaken for cancer. However, when we find cancer in that is already spread and are unable to locate the origin, we use PET CT to locate primary cancer.
2. Staging : After the biopsy confirms cancer, to assess the extent of disease before the start of treatment.
3. Response Evaluation: Assessment of response to treatment during or after therapy. This allows us to change treatment if it is not working.
4. Restaging: Assessment of the extent of the disease after treatment or after confirmed recurrence
5. Suspected Recurrence: When blood tests suspect recurrence in an old case of cancer, it can help us to locate it.
6. Follow-up or Surveillance: Often we request for yearly scans. Depending on the type of cancer we may ask for PET CT.
7. Radiotherapy Planning (RT): To plan the area that has to receive radiation. This allows for the normal area to be spared. Fewer side effects.
Is PET useful in all cancers?
Though most cancers are picked up on PET CT, there are a few which do not. The most important of these would be cancer of stomach (signet cell type). In such cases performing this test would be waste. However, there are cancers which are very sensitively detected which include lymphoma, GIST, etc. Your doctor will tell you when it is useful.