Impact of dual tracer study to differentiate malignant vs benign peritoneal diseases.

Introduction: F^18- FDG PET CT scan was the standard imaging modality for detecting peritoneal diseases . However, it is absolutely not tumor specific. To overcome this problem, Ga⁶⁸ labeled FAPI-46 (fibroblast activation protein inhibitor) was developed. Lilan Fu, Shun Huang et al. showed that the sensitivity for peritoneal detection is around 93.2% by FAPI vs 53.8% by FDG.

we aim to look for the utility of dual tracer study to differentiate benign from malignant pathologies.

Methods: All the oncological patients who had indeterminate or negative findings in F¹⁸ FDG PET CT scan underwent Ga ⁶⁸ FAPI-46 PET CT scan. Results were grouped into 4 categories based on quantification marker SUV (maximum standardized uptake value). Category I: likely benign (FDG positive & FAPI negative or both negative), II : Probably benign (both positive but the SUV of FAPI is ≤ 50% of SUV of FDG), III: indeterminate (both positive with SUV of FAPI ranging from 50-70 % of SUV of FDG), IV: likely malignant (both positive with SUV of FAPI >70% of SUV of FDG or only FAPI positive). Histopathology or follow-up imaging served as the standard for the final diagnosis

Results: We have included a total of 30 patients who had indeterminate findings in F¹⁸ FDG PET CT scan. Of these, 4 patients were being evaluated for residual/ recurrent disease.

Likely benign category: All the 6/30 patients were true negative for malignancy; hence FAPI-46 scan alone helped in preventing false positive findings in these patients.

Probably benign category: 4/30 patients in this category were true negative for malignancy; there by showing that dual tracer indeed additionally helped in reducing false positive rate.

Indeterminate category: 6/30 patients in this category, of which 3 (3/6) were malignant histopathologies including serous cyst-adenocarcinoma ovary and endometrial carcinoma and rest 3 (3/6 ) were benign (tubercular and inflammatory).

Likely malignant category: Remaining 14/30 patients were true positive for malignancy. In 2/14 patients who were histo-pathogically proven to have signet ring variant of adenocarcinoma stomach, alone could detect primary and metastatic disease ; hence reducing false negative rate. Additionally, FAPI-46 PET CT scan also helped in effectively detecting primary mitotic lesions in 4/14 patients with histopathology of adenocarcinoma colon, stomach and ovaries. Rest 8/14 patients had positive findings in both the scans, with 2 (2/8) patients positive for residual disease post-surgery, 2 (2/8) with proven recurrent peritoneal disease and remaining 4/(4/8 ) patients were treatment naive.

Conclusions: From the literature survey, we found neither FAPI -46 nor FDG were absolutely tumor specific. But it is already proven that Ga⁶⁸ FAPI-46 PET CT scan has definitely surpassed F ¹⁸FDG PET CT scan in case of sensitivity. In this study, we have found that using dual tracer helped in differentiating benign from malignant thereby, reducing false positive and negative rates in peritoneal diseases. Our major limitation however, was the low study population; thus further studies with larger population groups are required.

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