**4. Discussion**

PNS are rare (less than 1%) manifestation of malignancy. However, its significance is greater, because of diagnostic difficulties and demanding therapeutic process. Due to the increased availability of onconeural antibody tests, there is a progress in both early diagnosis and the implementation of effective targeted therapy.

According to published guidelines for PNS diagnosis [8] in all cases tumor removing is indicated, increasing chance for proper control of PNS symptoms. In patient (No. 4), the neurological symptoms including involuntary movements and psychotic symptoms, were refractory to therapy with steroids, intravenous immunoglobulins and monoclonal anti-CD20 antibody. Only identification and resection of the papillary thyroid cancer led to remission, however PET/CT examination revealed dissemination.

Other risk factors of neurological autoimmune disorders such as coexisting autoimmune disease or a family history of autoimmunity may mislead the initial diagnosis, but in all cases, exclusion of malignancy is mandatory. The evaluation of serum cancer markers such as CA-125, CA-15.3, prostate-specific antigen, and carcinoembryonic antigen with subsequent CSF assessment are usually the first steps in diagnostic workup. In our cohort, only one patient (No. 10) had an elevated carcinoma antigen 15-3 (Ca 15-3), suggesting an underlying paraneoplastic syndrome; however, the [18F]FDG PET/CT result was negative.

In many patients, CSF analysis demonstrates abnormalities such as elevated protein, pleocytosis, the presence of oligoclonal bands or mixed abnormalities, but normal CSF analysis results do not exclude the possibility of PNS [9]. Patients No. 2, 4 and 14 had normal CSF findings, although other elements of the clinical picture indicated the possibility of PNS. Finally, only in the patient with cerebellar syndrome (No. 2) did the PET/CT scan show metabolically active lymph nodes in the mediastinum and chest, suggesting myeloproliferative syndrome.

Onconeuronal antibodies may act as markers for the specific tumors, e.g., anti-Yo for breast or reproductive system tumors, anti-Hu for small-cell lung cancer, anti-Ri for breast or lung or anti-Ma2 for testicle malignancy. In these cases, targeted tumor seeking by CT, mammography, pelvic or testicular ultrasound is essential. However, in cases with high risk of malignancy established by the presence of antibodies directed against intracellular neuronal proteins and adequate clinical presentation, PET/CT with [18F]FDG may bring additional benefits due to the high sensitivity of PET/CT in the detection of small, aggressive tumors. Furthermore, many types of PNS and onconeuronal antibodies can appear in several types of tumors, such as Anti-Yo antibodies which were found in patients with cerebellar syndrome (No. 1, 2), primary CNS vasculitis (No. 14) and motor neuron disease (No. 15). In this context, in cases with positive, unspecific for particular malignancy, antibodies, PET/CT may have additional value, enabling whole-body screening during a single examination.

The meta-analysis evaluating the suitability of [18F]FDG PET/CT in PNS demonstrated a pooled sensitivity of 0.81 and specificity of 0.88 in patients with suspicion of paraneoplastic syndrome [10]. However, that high pooled sensitivity results from data analysis of all patients with PNS having both positive and inconclusive/negative results of prior conventional imaging. In our study, the efficacy of [18F]FDG PET/CT was 53%, but only patients with negative previous radiological and endoscopic exams were evaluated in whom PET/CT had a real clinical benefit.

In the clinical context, it is worth considering if and when PET/CT should be repeated in case of a negative initial study. The guidelines recommend regular oncological surveillance every 6 months for up to 4 years [11] for initially negative imaging results; however, prolonged time of observation may be connected with unfavorable outcomes in some patients.

The worsening of a known autoimmune syndrome or the occurrence of a new one may suggest dissemination or recurrence of the malignancy. In two patients (No. 3 and 8), the PET/CT exam was repeated (after 12 and 3 months, respectively) due to progressing clinical symptoms. In patient 8 the second exam revealed lung malignancy.

Several studies have shown that whole-body [18F]FDG-PET and [18F]FDG PET/CT are very useful in the screening of patients with suspected paraneoplastic syndrome and positive paraneoplastic antibodies [12]. However, a recent study conducted by another group suggested that the presence of paraneoplastic antibody should not be an indispensable factor for performing [18F]FDG PET/CT [13,14]. In our study, the effectiveness of [18F]FDG PET/CT in the detection of pathology in patients with the presence and absence of onconeuronal antibodies was confirmed. In three PNS patients with negative onconeural antibodies and clinical presentation highly suggesting malignancy, PET/CT scans were positive. Furthermore, in total, there were 53% of positive [18F]FDG PET/CT results (50% and 57% of patients with non-classical and classical PNS, respectively).

The results of our study indicate the potential benefits of [18F]FDG PET/CT in a routine neurological practice in selected patients with PNS, regardless of their onconeural antibody status and type of PNS.

#### **5. Highlights**

[18F]FDG PET/CT may be effective in malignancy detection in a large group of PNS patients with negative results of prior diagnostic procedures.

In a routine neurological practice, selected patients with PNS may benefit from [18F]FDG PET/CT regardless of their onconeural antibody status and type of PNS.

Repeating [18F]FDG PET/CT may have additional value, but the optimal timing for the second examination remains uncertain.

**Author Contributions:** M.O.: Data collection, imaging review, data analysis, manuscript drafting, manuscript editing and approval, study coordination. A.S.-S.: Data collection, imaging review, data analysis, manuscript drafting, manuscript editing and approval. K.W.: Data collection, imaging review, manuscript drafting, manuscript editing and approval. A.S.: Analysis and interpretation of data, manuscript editing and approval. J.J.: Analysis and interpretation of data, manuscript editing and approval. A.H.-D.: Imaging review, data analysis, analysis and interpretation of data manuscript, editing and approval. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** Research was conducted ethically in accordance with the World Medical Association Declaration of Helsinki. The study protocol was approved by the Local Ethics Committee of the Jagiellonian University in Krakow (approval no 1072.6120.162.2021).

**Informed Consent Statement:** Written informed consent for participation was not required for this study in accordance with the national legislation and the institutional requirements.

**Data Availability Statement:** All data generated or analyzed during this study are included in this article. Further enquiries can be directed to the corresponding author.

**Conflicts of Interest:** The authors declare no conflict of interest.

### **References**

