PET combined with CT might be one of the most powerful imaging technologies available, but how effective it is in evaluating head and neck tumors and helping with treatment is not a cut-and-dried proposition, said Jonas Johnson, MD, professor and chair of otolaryngology at the University of Pittsburgh in his State of the Art Lecture.
Dr. Johnson gave the presentation at the 115th Annual Meeting of the Triological Society, held here on April 20 as part of the Combined Otolaryngology Spring Meetings.
He said that while those in his group at the University of Pittsburgh were “early adopters” of PET/CT in head and neck tumor treatment, he acknowledged that it’s a costly technology that requires understanding of the subtleties.
“It’s very sensitive and phenomenally expensive,” Dr. Johnson said. “The real problem is that while PET/CT is very reliable in terms of its negative predictive value, it is plagued by false positives.”
His address didn’t include any discussion of PET only, since by itself it gives “little or no anatomic detail.” “PET/CT is vastly better than PET alone,” he said.
He issued the caveat that neither the National Comprehensive Cancer Network (NCCN) Guidelines nor the Radiation Therapy Oncology Group (RTOG) includes any suggestions for the use of PET/CT. “If you use PET/CT, you’re out there a little in front of the rest,” he said.
Varying Benefits.
A main theme of Dr. Johnson’s remarks was that the benefits of PET/CT can be blurry. The value of PET/CT varies according to the size of the tumor, he said, discussing the case of a 4-mm metastasis.
“This is a very tiny microscopic metastasis,” he said. “We all understand that you cannot identify this volume of tumor with any modality currently available, other than by accident.”
As far as determining tumor staging, Dr. Johnson said the value of PET/CT might be dubious. “PET/CT has improved sensitivity, and the findings may influence therapeutic decisions, but frankly there’s no good cost-effective data,” he said. “We don’t have any yet and we need it…. PET/CT is inadequate to identify occult metastases in either the neck or distant (metastases).” But, he said, “the best available neck node data is from histology.”
Studies have shown that PET/CT detects about half of occult neck nodes and cannot replace the accuracy of a neck dissection, Dr. Johnson said. For identifying unknown primary tumors, studies have found that PET/CT has detected as many as 87 percent of them—which seems like “a lot,” Dr. Johnson said—down to 38 percent. The best meta-analysis suggests PET will identify one-third of these primary tumors, he said. “PET/CT, our most sensitive imaging modality, will not identify microscopic disease,” he added.
Chemoradiation Considerations
The use of PET/CT for surveillance in patients who’ve received chemoradiation treatment is one of the most difficult issues the technology presents, Dr. Johnson said. Generally, he said patients should get a PET/CT sometime around eight to 12 weeks after treatment.