Simultaneous PET/MRI in the Evaluation of Breast and Prostate Cancer Using Combined Na[18F]F and [18F]FDG: a Focus on Skeletal Lesions

Abstract

“Don’t cross the creek for water” is a Danish proverb expressing the wisdom “don’t take unnecessary trouble,” which is what Sonni et al. appear to do in their recent article in Molecular Imaging and Biology [1]. In 74 patients (23 women and 51 men with breast and prostate cancer, respectively) referred for whole-body bone scintigraphy (WBBS) with [Tc]Tc-MDP, the results of this examination was compared with the findings by combined sodium [F]fluoride (Na[F]F)/2-deoxy-2-[F]fluoro-D-glucose ([F]FDG) positron emission tomography (PET)/magnetic resonance imaging (MRI) performed within less than 1 month after WBBS. Per patient, WBBS identified skeletal lesions in 37 and Na[F]F/[F]FDG PET/MRI in 45 patients, a surplus of 23 %; per lesion, WBBS identified a total of 81, whereas Na[F]F/[F]FDG PET/MRI detected 140 lesions (+ 73 %). Additionally, PET/MRI showed extraskeletal lesions in 19 patients, including lymph nodes (16), prostate (4), lung (3), and liver (2) lesions. The authors concluded that the ability of Na[F]F/[F]FDG PET/MRI to identify more skeletal lesions than WBBS and to additionally identify extra-skeletal disease “may be beneficial for patient care and represent an alternative to the single modalities performed separately” and that the combined PET/MRI procedure “is a promising approach for evaluation of skeletal and extra-skeletal lesions in a selected population of breast and prostate cancer patients.” In a way, one cannot disagree because like everyone needs water and should have it even if collected on the other side of the creek, every patient should benefit from being offered the very best method of demonstrating skeletal and extra-skeletal metastases even if it is cumbersome. However, combined Na[F]F/[F]FDG PET/MRI may be superior to WBBS but is overkill causing more confusion than clarity. The reason is that Na[F]F will identify bone matrix lesions that may or may not be secondary to active bone marrow metastases, which are the ones we need to diagnose and monitor and not bone matrix metastases, which are lateoccurring structural skeletal changes that may persist long after active metastases have disappeared [2–4]. Outside the skeleton, Na[F]F, which is a highly sensitive tracer, may identify cancer-like foci, which are not cancer but foci of microcalcification caused by many other conditions than malignancy [5, 6]. Therefore, to combine Na[F]F with [F]FDG for metastasis imaging is superfluous and may at times lead to wrong conclusions about the burden of skeletal and extra-skeletal malignancy. [F]FDG PET/CT, or PSMA PET/CT when it comes to prostate cancer, will suffice and be more accurate and clinically more useful since both tracers will give an impression of the degree of activity of the cancer, as is the case with [F]FDG [7]. Future studies will show if cumbersome whole-body PET/MRI is preferable to PET/CT with one of these tracers for demonstrating bone marrow and extra-skeletal metastases.