How do doctors determine that PRRT should be the treatment of choice for a neuroendocrine tumour?
PRRT or Peptide Receptor Radionuclide Therapy is a molecular therapy that is one of the accepted modes of treatment for Neuroendocrine Tumours (NETs).
Before we go on to discuss the above topic at hand, let me explain PRRT. In PRRT, a radiopeptide is created by combining a small amount of radioactive material or radionuclide (Yttrium-90 or Y-90 and Lutetium-177 or Lu-77) with a cell-targeting protein. This radiopeptide, when injected into a neuroendocrine tumour patient’s bloodstream, travels and binds itself to the neuroendocrine tumour cells to deliver a high dose of radiation that targets only the cancer cells.
Doctors recommend PRRT for patients diagnosed with Advanced Metastatic or Progressive Neuroendocrine Tumours (NETs); also for NET patients who are ineligible for surgery; and also for those NET patients who are unresponsive to other modes of treatment. While, PRRT is a palliative treatment, it does help in alleviating the symptoms, slow or in some cases even stop the tumour progression thereby improving not only the quality of life, but also the survival rate.
Now, the questions that most NET patients ask the doctors is –
“How do you determine whether PRRT is the recommended mode of treatment for my condition?”
The doctor’s decision to treat an NET patient with PRRT depends on the following parameters:
- The origin of the tumour
- The aggressiveness and proliferation of the tumour
- The progression or stability of the tumour – how aggressive it is growing?
- And, the most important point of consideration for the doctors is whether the tumour is expressing the receptors that is used to target PRRT.
Let’s address these parameters one-by-one.
1. What is the Origin of the Neuroendocrine Tumour?
A neuroendocrine tumour or NET has its origin in the specialised cells that have the traits of both hormone-producing endocrine cells and nerve cells, of the body’s neuroendocrine system. These cells can be found throughout the body and are responsible for controlling many of the body’s functions. While the neuroendocrine tumour can grow from anywhere, doctors recommend PRRT for neuroendocrine tumour patients who have the origin of the tumour from either the thymus – a specialized primary lymphoid of organ of the immune system; or from the lungs; or stomach; or the upper part of the intestines (i.e. small intestines – the jejunum or ileum); or from the lower part of the intestines (the rectum); or the pancreas.
The neuroendocrine tumours originating from the lungs and the pancreas are often the more aggressive ones as compared to the neuroendocrine tumours originating from the small intestine and the rectum. The tumour originating from the intestine and the rectum has a tendency to spread to the liver much more than the tumour originating from the lungs. The neuroendocrine tumour originating from the lungs has a more indolent (lazy) progression.
2. How aggressive is the tumour? What is the proliferation of the tumour?
To understand this aspect of the neuroendocrine tumours, doctors look at the patient’s Histopathology Report. In the report, there is something called the Ki-67 or a Mib Index. This index tells the doctors how aggressive is the growth of the tumour.
Ki-67 or Mib Index in the Histopathology Report:
- If Ki-67 is less than 2%, the neuroendocrine tumour is classified as a Grade I tumour;
- If Ki-67 is between 2-20%, then it is classified as a Grade II tumour; and
- If Ki-67 is above 20%, then it is classified as a Grade III tumour.
Doctors typically recommend PRRT if the neuroendocrine tumour is classified as a Grade II tumour.
3. How aggressive is the growth of the tumour?
This is where the doctors understand the progression or the stability of the tumour. This is an important parameter that doctors need to understand. If the tumour is stable and the patient is fairly asymptomatic, doctors recommend to wait and watch. And if the tumour is progressing and is spreading to other sites, doctors recommend PRRT as the recommended mode of treatment.
To determine whether the tumour cells are showing a good receptor expressions that can be used for targeting PRRT, doctors ask for a Gallium 68 Dotanoc Scan or an Octreotide PET CT. A Gallium 68 Dotanoc whole body scan is a type of PET CT that is specifically done for neuroendocrine tumours. Apart from this scan, doctors might also look at a Technetium Hynic-TOC Scan that also looks at the receptors.
Further, doctors also ask for a regular FDG PET CT so as to determine the aggressiveness of the tumour. This FDG PET CT also helps doctors on the prognosis of the condition – if the neuroendocrine tumour shows positive on the FDG PET CT scan, then doctors know that the condition of the patient is less likely to benefit from PRRT.