A standard therapeutic approach for advanced malignant thymoma has yet to be defined given the rarity of this condition. We present a patient with advanced thymoma, evaluated as inoperable at diagnosis due to multiple serosal metastases. The strong constitution and determination of the patient allowed treatment with six distinct and subsequent chemotherapy regimens, all administered on an outpatient basis. A survival of 64 months from diagnosis was achieved. A favorable clinical response was obtained after the first three treatment lines, with the disappearance of all lesions on both computed tomography and positron emission tomography (PET) images. However, this result was not confirmed by surgical exploration of the thorax, undertaken with the aim of radical excision of possible residual disease. The presence of multiple pleural nodules, not evident on the imaging techniques, prevented even limited tumor debulking. The chemotherapy lines administered following detection of the lessions, stabilized the disease for a further 2 years, while a satisfactory quality of life was maintained. Only in the last months did the tumor progress and signs of cardiotoxicity appear, with the latter constituting the eventual cause of death. This case is important since the medical literature does not indicate non-cross-resistant regimens for advanced thymoma following second-line chemotherapy, and the sequence of regimens presented in this case study may serve as a feasible outline program. Moreover, we highlight the known possibility of false-negative PET studies, which can occur despite the claimed glucose avidity of thymoma tissue.