PET/MRI Valuable in Diagnosis of Pelvic Malignancies

Pam Harrison

March 16, 2015

VIENNA — Studies evaluating the performance of hybrid imaging using simultaneous PET/MRI for the staging of both primary and recurrent pelvic malignancies indicate that the integrated approach could become a valuable diagnostic modality in both settings, new research shows.

"MRI is known to be the most accurate technique we have to determine local tumor extent of cervical cancers, and also the most accurate in identifying if the tumor has infiltrated adjacent pelvic organs, which you have to determine in order to arrive at the proper treatment strategy," Johannes Grueneisen, MD, University Hospital, Essen, Germany, told Medscape Medical News.

"We know that MRI imaging is a very accurate way to stage certain cancers, while the PET component is very useful to identify metastases, so by combining these two techniques, we hope we can determine the local extent of the tumor in the female pelvis and increase the [imaging] sensitivity for identification of metastatic spread," Dr Grueneisen said.

"We wanted to see if we could increase the sensitivity and specificity of the identification of primary tumors of the uterine cervix and found that PET/MRI correctly identified 100% of the primary tumors in our study, and it correctly staged the majority of tumors as well," he said.

For the primary staging study, Dr Grueneisen and colleagues evaluated 27 patients (mean age, 45 years) with confirmed primary cancer of the cervix.

All patients had a full-body PET/MRI examination prior to definitive [surgical] treatment using an integrated scanner (Biograph mMR, Siemens AG), Dr Grueneisen noted.

Imaging results confirmed that 22 patients had squamous cell carcinoma; the remaining five had adenocarcinoma, he added.

Compared with the reference standard, the German group also found that PET/MRI identified the correct T stage (T1b through to T4a) in 23 of the 27 patients, for an 85% accuracy rate.

Tumor infiltration into the vagina was detected in two patients on surgical staging; an additional tumor had invaded the parametrium in one patient.

"PET/MRI ratings resulted in only one false positive finding for tumor invasion into each of the vagina and the parametrium," Dr Grueneisen reported.

The modality was also assessed for its ability to identify metastatic spread. In this group of patients, 11 were known to be lymph node positive, and the remaining 16 were node negative.

Again, PET/MRI was found to have a sensitivity of 91% and a specificity of 94%, for an overall diagnostic accuracy of 93% for the identification of node positive patients.

In a second study, Dr Grueneisen and colleagues compared a new "FAST" protocol of PET/MRI with standard PET/CT for the identification and staging of recurrent pelvic malignancies in a group of 24 patients with suspected pelvic recurrences.

As Dr Grueneisen explained, both modalities are known to be very accurate for the identification of tumor recurrence and of metastatic spread, but PET/CT requires a high level of radiation exposure, and standard protocols using PET/MRI take a long time before all imaging can be completed.

So they used a new FAST protocol for PET/MRI, which reduces the time taken to complete whole-body staging to 26.5 minutes, compared with the 44 minutes taken with the standard protocol PET/ MRI.

The PET/CT procedure is even quicker, at 18 minutes, but this has a higher dose of radiation than the protocols that use MRI.

"The dose of radiation from PET/CT is around 20 mSv, while with PET/MRI, it is only about 5 mSv, so that's breaking it down to only one third of the radiation needed for PET/CT," lead investigator Lale Umutlu, MD, consultant radiologist, University Hospital, in Essen, Germany, told Medscape Medical News.

For the new FAST protocol for PET/MRI, the researchers took only three specific MRT sequences: a transversal DWI (diffusion-weighted imaging) echo-planar imaging (EPI) sequence; a transversal 2-dimensional HASTE (half-Fourier acquisition single-shot turbo spin-echo) sequence, and a transversal 3-dimennsional postcontrast fat-saturated VIBE (volumetric interpolated breath-hold examination) sequence.

"Both PET/CT and PET/MRI examinations were successfully completed in all 24 patients," Dr Grueneisen noted.

On reference standard, the tumor had recurred in 21 of the 24 patients; the remaining three patients had no evidence of recurrence.

PET/CT correctly identified tumor recurrence in 20 of the 21 patients and in all 3 patients in whom the tumor had not recurred, Dr Grueneisen noted.

Similarly, the FAST PET/MRI protocol identified 20 of the 21 patients in whom the tumor had recurred and two of the three patients in whom it had not recurred.

The reference standard also found that 81 recurrent lesions were malignant; 23 lesions were benign.

Again, PET/CT and the FAST PET/MRI protocol were equally accurate, at 84% and 86%, respectively, in terms of their ability to differentiate malignant from benign lesions.

Lesion conspicuity was similarly equally high with the two modalities for both malignant and benign lesions, indicating that both imaging modalities were equally good at the identification and delineation of these lesions, the researchers noted.

Clinical Studies

Asked by Medscape Medical News to comment on the potential value of simultaneous PET/MRI, Jon McConathy, MD, assistant professor of radiology, Mallinckrodt Institute of Radiology, St. Louis, Missouri, said that they, too, use simultaneous PET/MRI for clinical studies, because the PET component allows them to measure biochemical processes that in general are not readily measurable with MRI or CT.

Using 18F-fluorodeoxyglucose (FDG) PET, for example, "we can image the whole body and be fairly sensitive for most cancers and for detecting distant metastases as well as evaluating how lesions are after therapy to see if they responded to treatment based on metabolism and not just based on size criteria," he said.

Indeed, sometimes they can detect lymph nodes that do not meet size criteria on MRI or CT but which have increased FDG uptake, he said.

"With this, we can be fairly certain that they are malignant even if anatomic imaging is not conclusive," Dr McConathy said.

"The findings from the German group are in line with our own experience, and we also think that PET/MRI is very valuable for certain cancers," he said.

"And it's good to see groups generating data that give us a more quantitative and more rigorous sense that confirm this is an effective modality."

European Congress of Radiology (ECR) 2015. Abstracts B-0029 and B-0031. Presented March 4, 2015.


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