It's not that they are less accurate, it's more a question of being unnecessary. The CT component of a PET/CT will show pretty much the same type of images that a standalone CT will show, but the PET scan will light up where there are areas of sugar uptake, which can indicate cancer, inflammation or infection but can't really distinguish between them. An increase in uptake (known as SUV) from one PET to another does not necessarily indicate progression. The best way to determine if cancer is progressing is to compare CT scans over time to see if a tumor is getting larger; PET scans aren't good at discerning size and shape of tumors, and the faculty here have often expressed the opinion that decisions about progression and change of treatment should not be based solely on increases in SUV, and they do not favor them for assessing response to treatment.
You can read more about the differences in the scan types here:
Hi Katie, I just want to share our experience with PET/CT for surveillance. In my understanding and opinion, there are two disadvantages other than the higher cost wastage. The CT portion of PET/CT is said to be of lower resolution, and the Radiologist has the tendency to focus on the SUV readings and neglect the changes and measurements in sizes and shapes. In hindsight, I would rather have CT for regular follow up.
These are all important factors. It's not that a PET/CT doesn't have value, but it needs to be the right tool for the job. It's very good for initial staging, but for surveillance of response, the changing SUV findings can be misleading, especially in the face of minimal or no progression on the basis of size. njliu is also right to point out that the CT on a PET/CT is not of the same quality as a "dedicated CT" of the chest, which is higher resolution. So wasteful cost is an issue, but I would summarize that the changing metabolic values (SUV, or standard uptake value) on a PET can mislead very often, and I and most other specialists in this field definitely prioritize clear changes on a CT as being the most important arbiter of disease progression.
Reply # - February 3, 2014, 04:50 PM
Reply To: Difference between CT /PET and a CT
Hi katief,
It's not that they are less accurate, it's more a question of being unnecessary. The CT component of a PET/CT will show pretty much the same type of images that a standalone CT will show, but the PET scan will light up where there are areas of sugar uptake, which can indicate cancer, inflammation or infection but can't really distinguish between them. An increase in uptake (known as SUV) from one PET to another does not necessarily indicate progression. The best way to determine if cancer is progressing is to compare CT scans over time to see if a tumor is getting larger; PET scans aren't good at discerning size and shape of tumors, and the faculty here have often expressed the opinion that decisions about progression and change of treatment should not be based solely on increases in SUV, and they do not favor them for assessing response to treatment.
You can read more about the differences in the scan types here:
http://cancergrace.org/cancer-101/2007/01/09/pet-scanning-intro/
http://cancergrace.org/lung/2007/11/03/pet-scans-for-surveillance/
JimC
Forum moderator
Reply # - February 3, 2014, 06:53 PM
Reply To: Difference between CT /PET and a CT
Hi Katie, I just want to share our experience with PET/CT for surveillance. In my understanding and opinion, there are two disadvantages other than the higher cost wastage. The CT portion of PET/CT is said to be of lower resolution, and the Radiologist has the tendency to focus on the SUV readings and neglect the changes and measurements in sizes and shapes. In hindsight, I would rather have CT for regular follow up.
Reply # - February 3, 2014, 08:47 PM
Reply To: Difference between CT /PET and a CT
These are all important factors. It's not that a PET/CT doesn't have value, but it needs to be the right tool for the job. It's very good for initial staging, but for surveillance of response, the changing SUV findings can be misleading, especially in the face of minimal or no progression on the basis of size. njliu is also right to point out that the CT on a PET/CT is not of the same quality as a "dedicated CT" of the chest, which is higher resolution. So wasteful cost is an issue, but I would summarize that the changing metabolic values (SUV, or standard uptake value) on a PET can mislead very often, and I and most other specialists in this field definitely prioritize clear changes on a CT as being the most important arbiter of disease progression.
-Dr. West