Jim Kling

June 20, 2013

VANCOUVER, British Columbia — An 11C-choline PET/MR protocol for the restaging of prostate cancer was well tolerated by patients and detected significantly more recurrences than PET/CT, according to new research. PET/MR was particularly good at detecting small local recurrences.

In theory, magnetic resonance imaging is more beneficial than computed tomography for prostate cancer restaging because it provides higher soft-tissue contrast. "In the analysis of recurrent disease, it would potentially have a higher detection rate," said lead investigator Matthias Eiber, MD, a radiologist from the Technical University Munich in Germany.

Dr. Eiber presented the research here at the Society of Nuclear Medicine and Molecular Imaging 2013 Annual Meeting.

To compare the techniques, investigators evaluated 31 patients who underwent a single-injection, dual-imaging protocol with PET/CT (Siemens Biograph 64) 5 minutes after injection, followed by PET/MR (Siemens Biograph mMR) 51 minutes after injection.

The PET/MR scan involved 3 to 4 bed positions (4 minutes per position) and covered 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.

Investigators obtained axial diffusion-weighted images (b-values 0, 400, and 800 s/mm²) 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.

The researchers used a 3-point scale to classify suspicious lesions, with 1 indicating definite metastasis, 2 indicating probable metastasis, and 3 indicating indeterminate status.

Imaging time was longer with PET/MR than with PET/CT (41 vs 23 minutes), but patients tolerated the longer procedure. PET/MR outperformed PET/CT, particularly with respect to local recurrences.

Table. Detection Rates for the PET/MR and PET/CT Protocols

Detection Rate Regions, n Patients, n Mean Rating
PET/MR      
   Local recurrence 17 12 1.12
   Lymph node metastases 42 1.38
   Bone metastases 17 5 1.17
PET/CT      
   Local recurrence 12 8 1.40
   Lymph node metastases 39 1.45
   Bone metastases 14 4 1.15

 

Because PET/MR machines are expensive, they are currently very rare; there are about 20 installed worldwide, according to Dr. Eiber. It remains to be seen if clinical applications will justify the cost.

In the case of prostate cancer recurrence, PET/MR has clear value, he noted. 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 Dr. Eiber.

Another advantage is that the radiation dose delivered by PET/MR is about 80% lower than that delivered by PET/CT, according to the researchers' calculations. This potential dose reduction could be relevant for patients who undergo many exams over several years, because radiation can accumulate, Dr. Eiber explained, although he noted that many prostate cancer patients are elderly.

These results clearly demonstrate the value of PET/MR, Sandi Kwee, MD, told Medscape Medical News. "MR provides better tissue contrast and some additional features that are beyond just the structural information that the CT portion provides," said Dr. Kwee, who is program director for PET research at The Queen's Medical Center in Honolulu, Hawaii.

Nevertheless, the expense of PET/MR demands a high bar for clinical utility. "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," said Dr. Kwee. He noted that most major medical centers have both PET and MRI scanners, and doing consecutive scans can often provide the same information, albeit with potential imaging artifacts because of position changes between scans.

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," said Dr. Kwee. "We're still in the early days of PET/MR."

This study was funded by the German Research Foundation DFG. Dr. Eiber is on the speaker's bureau for Siemens AG, which manufactures PET/MR instruments. Dr. Kwee has disclosed no relevant financial relationships.

Society of Nuclear Medicine and Molecular Imaging (SNMMI) 2013 Annual Meeting: Abstract 343. Presented June 11, 2013.

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