PET/MR Identifies Prostate Cancer Recurrence

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A new study  presented at the Society of Nuclear Medicine and Molecular Imaging (SNMMI) 2013 Annual Meeting reveals a system of PET/MR was well received by patients and identified a higher number of prostate cancer recurrences compared to PET/CT. PET/MR was highly effective at identifying small local recurrences.petmr

It is usually thought that magnetic resonance imaging is more efficient than computed tomography for prostate cancer recurrence because it supplies higher soft-tissue comparison.

"In the analysis of recurrent disease, it would potentially have a higher detection rate," said a radiologist from the Technical University Munich in Germany and lead investigator, Matthias Eiber, MD.

To assess the two different approaches, researchers examined 31 patients who were subjected to a single-injection , dual-imaging protocol with PET/CT five minutes after injection, followed by PET/MR 51 minutes after injection.

The PER/MR scans included three to four bed positions, taking approximately four minutes for each position, and examined the area from the chest to the pelvis. It involved a coronal Dixon-VIBE scan for attenuation correction, a coronal T1-weighted spin-echo sequence, and an axial fat-saturation T2-weighted sequence.

Researchers acquired axial diffusion-weighted images (b-values 0, 400, and 800 s/mm˛) and axial T1-weighted dynamic contrast-enhanced sequences during an additional 20-minute PET scan over the pelvis for T2-weighted axial and coronal planes. They used axial contrast-enhanced fat-saturated T1-weighted gradient recalled-echo sequences for the trunk.

Researchers then employed a 3-point-scale to categorize chary abrasions, with 1 identifying definite metastasis, 2 identifying a possible metastasis, and 3 identifying an undefined metastasis.

Imaging time was longer with PET/MR than with PET/CT, 41 compared to 23 minutes, but patients endured the longer procedure. PET/MR outdid PET/CT, chiefly in local recurrences.
Yet because PET/MR systems are so expensive they are somewhat of a rarity with only 20 installations worldwide, it remains to be seen if their medical importance will outweigh the costs.

“In the case of prostate cancer recurrence, PET/MR has clear value. Patients are monitored for levels of prostate-specific antigen, and when the levels rise, physicians suspect a recurrence. But the recurrence could be local, in the lymph nodes, or a bone metastasis. The site of recurrence tremendously influences the choice of therapy," said Eiber.

One other advantage the researchers note is that radiation dosage emitted from PET/MR is significantly lower than that emitted by PET/CT (80%). This probable dose diminution can be applied and beneficial for patients who experience many exams over several years, because radiation tends to accumulate over time.

"MR provides better tissue contrast and some additional features that are beyond just the structural information that the CT portion provides," said program director for PET research at The Queen's Medical Center in Honolulu, Hawaii, Sandi Kwee, MD.

However, the high cost of PET/MR remains a key factor and limitation for medical use.

"It has to be looked at on a more global population scale to see whether the overall benefit is worthwhile for medical centers to adopt that technology. It must be shown that PET/MR has an impact on clinical practice. It has to be more influential in the outcome of the patient. These are preliminary studies. We're still in the early days of PET/MR," said Kwee.

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