Retroperitoneal tumors account for only 0.2% of all tumors. Among them, liposarcomas show the highest frequency of retroperitoneal tumors, at 14.7% [1]. In the WHO classification, liposarcomas are divided into five categories based on their clinicopathological and genetic features: atypical lipomatous tumor/well-differentiated liposarcoma, DDLPS, myxoid liposarcoma, pleomorphic liposarcoma, and mixed-type liposarcoma. DDLPS is a rare and aggressive disease with a six-fold increase in the mortality risk compared with differentiated liposarcomas [2, 3]. It was reported that 30% of dedifferentiated retroperitoneal liposarcomas develop distant metastases, and the lung is the most common distant metastatic site, being associated with decreased survival [4, 5]. Surgical resection is conducted for pathological diagnosis and curative purposes because effective systemic therapy has not been established for metastatic DDLPS.

FDG-PET/CT is useful as an adjunctive method for diagnosing DDLPS. Recurrent tumors of DDLPS tend to show high FDG uptake (SUV Max: 2.3–29.5), and using SUV Max 4 as a cut-off, the diagnostic sensitivity and specificity of DDLPS are 81.8 and 88.9%, respectively [6]. In this case, the left and right metastatic lesions appeared at the same time but showed completely different FDG uptake. DDLPS is heterogeneous, consisting of multiple components and various grades; thus, the heterogeneity of tumors generally explains the difference in FDG uptake. Differentiation was similar in the left and right tumors; therefore, we examined the Ki-67 labelling index because a correlation between FDG uptake and the Ki-67 labelling index was reported in soft tissue sarcomas [7]. The difference in the Ki-67 labelling index between left and right tumors was small, and so we consider that it could not explain the difference in FDG uptake. Although the doubling time of the tumor differed between the left and right nodules (left S3: 105 days, right S8: 207 days), we did not elucidate the reason for this difference in FDG-PET/CT from histopathological findings. The differences in tumor metabolism might underlie and affect glucose metabolism even though differentiation and tumor cell division were similar. Surgeons should note that pulmonary metastasis of DDLPS might appear as uncommon for metastatic nodules, and a lack of FDG uptake does not always rule out metastatic disease of DDLPS.