Bluegrass Regional Imaging

CLINICAL Value of PET

PET is the most advanced medical imaging technique available today. PET is based on imaging metabolic processes in the body. Since metabolic changes generally precede changes in anatomy, PET can often detect cancer sooner, or identify the full extent of disease more accurately, than CT, MR or other anatomic imaging approaches. PET is extremely sensitive and can identify disease long before it has grown to a detectable size for CT or MR.

PET vs. MR/CT/US

Anatomic (Other Modalities)

• Localization or Mass
• Difficult to differentiate scar tissue from recurrence
• Difficult whole body study
• Slow to assess response to therapy, must wait for anatomic change
• Requires concentrations of contrast agents that alter metabolic process

Metabolic (PET)

• Identification of change in cell biochemistry
• Straightforward delineation of disease from scar tissue
• 30 minute whole body assessment
• Fast assessment of therapy effectiveness due to change in metabolism
• Trace quantities of FDG do not change metabolism

Because FDG highlights the uniqueness of the cancerous tissues, most of the clinical application for PET is focused in Oncology. At a typical PET practice, the vast majority of the studies performed are in Oncology.

Percent of Studies
Oncology: 60-90%	Cardiology: 10-40%	Neurology: 0-10%

The clinical value of PET in Oncology includes:

• Early Detection of Disease
• Precise Staging of Disease
• Response to Treatment

Colorectal Cancer Example

Colorectal cancer is a great example of the importance of PET. The only cure for colorectal cancer is surgery, and so nearly all colorectal patients have surgery early in their course of disease. Since one third of all colorectal cancer patient experience recurrent disease within 2 years, it is common for radiologists to have difficulty interpreting follow-up restaging CT scans, due to the presence of scar tissue.

Alternatively, PET can easily detect highly metabolic tumor cells, particularly when surrounded by scar tissue which has almost no FDG uptake at all. The literature shows PET to have a much higher accuracy in the detection of colorectal recurrence. PET is greater than 90% accurate, compared to 50-60% with CT.

Recurrent Colorectal Cancer

• 1/3 of all Colorectal Cancers recur within 2 years.
• Detection of local colorectal recurrence:

	Accuracy	Sensitivity	Specificity
PET	92%	96%	96%
CT	25-73%	71%	96%
Barium Enema	80%	49%	85%
CEA Level	--	59%	84%

• Despite a negative CT, 25-50% of patients will have non-resectable lesions at time of laparotomy.
• PET has been shown to detect unsuspected lesions in 28% of studies.

Lung Cancer Example

Lung cancer is an important disease when it comes to accurate staging, and a great example of the effectiveness of PET. Nearly 180,000 people present each year with an abnormal chest Xray. Of these, 30% have benign lesions based off the X-ray and are sent home. Of the remaining 130,000, approximately 20-30% are also benign, but CT alone is not able to differentiate them from a malignancy. In the absence of PET, many of these patients would go on to expensive, invasive, unnecessary surgeries.

Studies have shown that for 1 cm nodules in the lung, CT performs poorly (missed 92% of malignant lesions.) Even over 2cm in sizes, CT alone was only 60% accurate. When PET was also used on these patients, 32% of patients were either up-staged or down-staged based on more accurate PET results.

Lung Cancer

• Second leading cause of death behind heart disease
  • 130,000 new cases each year
• 20-30% lesions are benign, but CT cannot differentiate these from the malignancies
• Resected Lymph Nodes
  • 50% are benign, surgery was unnecessary

Size of metastatic disease
1 cm	8% found with CT (92%missed!)
1-2 cm	30% found
> 2 cm	60% found

• PET frequently changes the TNM stage
  • PET increased correctly the TNM in 14%
  • PET decreased correctly the TNM in 18%
  • Source: Rigo e.a. EANM communication 1997

In a direct head to head comparison of PET and CT in staging of the mediastinum, PET was shown to be 82% accurate versus 52% for CT. In this study, PET changed patient management in 29% of patients.

Note:

• PET is frequently credited with finding unsuspected disease because it is a whole-body scan.
• Melanoma in particular requires a full head to toe PET study, since it is much more likely to spread anywhere.

Lung Cancer and Staging

• Staging the mediastinum, nodal disease
  • Determine surgical candidacy
  • Eliminate unnecessary thoracotomy

	Accuracy	Sensitivity	Specificity
PET	82%	88%	93%
CT	52%	63%	80%

• Staging whole body*
  • PET found more malignant lesions and changed patient management in 29% of cases
  • * "Cancer Management: A Multidisciplinary Approach"; 1996 PRR Huntington NY

PET vs. Conventional Nuclear Medicine

PET uses the same basic concept as traditional Nuclear Medicine of injecting a radioactive isotope into the human body and using a scanner to detect the energy and providing algorithms to create an image where higher metabolic activity shows up darker than the surrounding areas. However, there are very unique differences in PET…starting from the higher energy, positron-emitting isotopes used to the detectors and algorithms used to create an image. This makes PET ideal for oncology applications where precision provides insight to the treatment.

PET vs. Nuclear Medicine

Conventional Nuclear Medicine

• Isotopes decay by releasing energy (gamma rays or photons)
• Lower energy range (80-400keV)
• Radiation has to be collimated (focused) for imaging
• Lower energy causes attenuation (tissue scatter) degrading image quality

PET

• Isotopes decay by emitting a positron (positively charged particle)
• High energy (511 keV)
• Collision with electron results in two emitting gamma rays (photons) at 180 degrees of each other
• Less tissue scatter, better image quality

So the value of PET is clearly in the power of metabolic imaging versus anatomic imaging. The ability to identify disease prior to visible anatomic changes allows for early detection. It also alleviates the uncertainty that anatomical imaging techniques face in the presence of scar tissue.

In addition, for therapy monitoring questions, anatomic techniques can only give information once a structural change in the tumor has occurred. PET can give a much earlier indication of the state of the cancer cells thus demonstrating the response (or non-response) of a cancer to treatment. PET can clearly delineate a reduction in the metabolic rate of a dying tumor using quantitative measurement of FDG uptake.