**4. Conclusions**

A patient with an initial diagnosis of asymptomatic SMM was monitored by means of dynamic and static 18F-FDG PET/CT during the course of the disease. Upon SMM diagnosis, the patient had no pathological signs on PET/CT. The transition from SMM to symptomatic MM was not accompanied by the typical signs of myeloma involvement on PET/CT, i.e., focal, hypermetabolic lesions. However, a diffusely increased 18F-FDG uptake in the bone marrow was observed, while at the same time, a marked increase of both semi-quantitative (SUV values) and quantitative, pharmacokinetic parameters was demonstrated. Following treatment, the diffuse uptake in the bone marrow as well as the semi-quantitative and quantitative parameters showed a pronounced remission. This response was also confirmed by the long-term, clinical follow-up of the patient. Altogether, the here-presented findings suggest, firstly, that in MM a diffuse 18F-FDG uptake in the bone marrow may reflect an actual bone marrow infiltration by plasma cells. Secondly, SUV values not only from myeloma lesions—as highlighted by previous studies—but also from random bone marrow samples, may be used for MM monitoring; this could be particularly helpful in the follow-up of myeloma patients negative for 18F-FDG-avid focal lesions. Finally, several pharmacokinetic parameters, derived from dynamic PET/CT, can

be used to increase the diagnostic certainty and provide valuable information on dedicated parameters of the tracer's metabolism.

**Author Contributions:** C.S. performed the PET/CT studies and evaluations, analyzed the data and drafted the manuscript; M.T. analyzed the data and revised the manuscript; A.D.-S. designed the study, performed the PET/CT evaluations and revised the manuscript. All authors have read and agreed to the published version of the manuscript.

**Funding:** The study was part of a special research area project (SFB TRR 79) funded by the German Research Foundation and the Dietmar-Hopp-Foundation.

**Institutional Review Board Statement:** The present case report is part of a study, which was conducted according to the guidelines of the Declaration of Helsinki, and was approved (10.05.2010) by the ethical committee of the University of Heidelberg (S-076/2010) and the Federal Agency of Radiation Protection in Germany ("Bundesamt für Strahlenschutz").

**Informed Consent Statement:** Written informed consent, including consent for publication, was obtained from the patient.

**Data Availability Statement:** The authors confirm that the data supporting the findings of this case report are available within the article.

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.
