Retroperitoneal NeoplasmsRetroperitoneal SpaceLiposarcomaLymphangiomaGanglioneuromaRetroperitoneal FibrosisSolitary Fibrous TumorsErdheim-Chester DiseaseAbdominal NeoplasmsAdenosarcomaTomography, X-Ray ComputedLiposarcoma, MyxoidPelvic NeoplasmsMediastinal NeoplasmsPeplomycinHemangiopericytomaParagangliomaTesticular NeoplasmsPerivascular Epithelioid Cell NeoplasmsNeurilemmomaSeminomaNeoplasms, Germ Cell and EmbryonalGranuloma, Plasma CellGerminomaTeratomaSoft Tissue NeoplasmsSarcomaFatal OutcomePeritoneal NeoplasmsTreatment OutcomeNeoplasm Recurrence, LocalMagnetic Resonance ImagingCombined Modality TherapyImmunohistochemistryPrognosisRetrospective StudiesFollow-Up Studies