[en] Full text: (¹⁸F)FDG PET has been known to have insufficient sensitivity for the detection of low (¹⁸F)FDG concentrating tumors. We retrospectively evaluated the value of (¹⁸F)FDG PET for the detection of primary and metastatic low FDG concentrating tumors. Aim: 1. To validate the value of (¹⁸F)FDG for the detection of primary and metastatic disease in known low FDG concentrating tumors in comparison with radiological modality and its effect on diagnosis and treatment of the same. 2. To determine the % of new, concurrent and missed lesions in our study and to predict role of (¹⁸F)FDG in follow-up of these patients. Materials and Methods: Histologically proven cases of respective tumor pathology, who had undergone (¹⁸F)FDG PET scan at our Centre (RMC) from the year 2003 to 2009, were selected for the study. This comprised of Renal cell carcinoma (n=24), Hepatocellular carcinoma (n=5) and Maltoma (n=5). Patient-by-patient and lesion-by-lesion analysis of ¹⁸F-FDG PET scan findings was done in comparison with conventional imaging modalities in each group. Percentages of new, concurrent and missed lesions on PET were calculated in respect to the conventional imaging modalities. Where possible, the SUVmax values of lesions (corrected for body weight) were calculated and compared with the results of the previous studies. Results: ¹⁸F-FDG PET scan findings were compared with the conventional imaging modalities in each sub group of both categories. Patient by patient analysis and lesion-by-lesion analysis of the data was done. This was used for calculating percentages of new, concurrent and inactive lesions on PET as compared to the conventional imaging modalities. The observations were as follows- PET detected 39% new lesions in patients of renal cell carcinoma (RCC), 57% in Hepatocellular carcinoma (HCC) and 35% in Maltoma. PET confirmed 39% CT detected lesions in RCC, 28% in HCC and 35% in Maltoma. PET missed 21% lesions that were detected by CT in RCC, 14% in HCC, and 28% in Maltoma. SUVmax of lesions could be calculated for 4 cases of RCC and 2 cases of HCC. The SUVmax was found to be in the range of 2.4 -16.95. This was found to be well above the cut off limit of 1.5 gm/ml for low FDG concentrating lesions. Conclusion: This study confirms that ¹⁸F-FDG PET has a reasonable sensitivity in the detection of metastasis, although it is not sensitive enough to be the only modality for the evaluation of primary tumor and its metastasis. Our results suggest that it has a mutual complementary advantage, similar to the conclusion found previously in the detection of primary low FDG concentrating tumors. Although the SUV values remained well above the cutoff for the positive lesions, there was lack of sufficient data, including the factors affecting SUV, to substantiate it further including the lack of histopathological evidence of PET detected lesions for calculating true positive and true negative lesions