CRANIAL mediastinal tumors

BACKGROUND

Tumors of the cranial mediastinum are rare in both cats and dogs. The two most common cranial mediastinal tumors are lymphoma and thymoma, although ectopic thyroid carcinomas and neuroendocrine carcinomas are also reported. Thymomas are usually benign and are classified as either non-invasive or invasive into adjacent structures such as blood vessels, pericardium, or lung lobes. Thymomas can be associated with paraneoplastic syndromes such as hypercalcemia (34%), second non-thymic tumors (27% at the time of diagnosis and a further 14% at a later date after surgery), myasthenia gravis (17%), and immune-mediated diseases (7%) including anemia, polymyositis, and, in cats, exfoliative dermatitis.

DIAGNOSIS and CLINICAL STAGING

Thoracic radiographs or CT scans are required for diagnosis of a cranial mediastinal mass, however there are no characteristics which help differentiate lymphoma from thymoma on CT scan. In contrast, sonographic appearance of cranial mediastinal masses may provide some indication of tumor type with thymomas more likely to have internal cystic structures and a heterogenous appearance, whereas lymphomas are more likely to be solid and either hypoechoic or heterogenous echogenicities. Regardless, ultrasound-guided fine-needle aspirates or needle-core biopsies are recommended to differentiate lymphoma from thymoma and other cranial mediastinal tumors, because cranial mediastinal lymphomas are treated medically whereas other cranial mediastinal tumors are treated either surgically or with radiation therapy. Thoracic CT scans can also provide information for planning surgical excision or radiation therapy.

TREATMENT

Thymomas can be treated with either surgical excision or radiation therapy. Surgical exploration is recommended to determine whether the thymoma is non-invasive or invasive. Non-invasive and even most invasive thymomas are amenable to surgical resection. Radiation therapy is recommended for unresectable invasive thymomas.

Lymphomas are treated with chemotherapy and occasionally radiation therapy, but surgery is rarely required or indicated.

PROGNOSIS

Dogs - Thymoma

The prognosis for dogs with thymoma is excellent with the majority of dogs with non-invasive thymoma cured with surgical excision.  However, in one clinical study of 80 dogs with surgically excised thymomas, the perioperative mortality rate was 20% because of either unresectable tumors or complications. The local recurrence rate following surgical excision is 9%-17%, but re-excision of the locally recurrent thymoma is often curative. The median survival time following surgical excision is 617-790 days with 1-, 2-, and 4-year survival rates of 55%-88%, 42%, and 44%, respectively. In one study of 80 dogs with thymomas, 27 dogs died of tumor-related reasons, including local recurrence, failure of myasthenia gravis to resolve, failure of pleural effusion to resolve, metastasis, and surgical complications.

Prognostic factors for survival include treatment type, presence of preoperative paraneoplastic syndromes, second non-thymic tumor at diagnosis of thymoma, clinical stage, completeness of histologic excision, and percentage of lymphocytes within the thymoma. Dogs treated surgically had a median survival time of 635 days which was significantly longer than the median survival time of 76 days for untreated dogs. Dogs with preoperative paraneoplastic syndromes were 5.8-times more likely to die of tumor-related reasons. Dogs with a concurrent non-thymic tumor at the time of diagnosis of thymoma had a median survival time of 282 days, which was significantly shorter than the median survival time of 568 days for dogs without concurrent tumors. The Masaoka-Koga staging system was used in one clinical studyof 84 dogs with surgically excised thymomas where stage I is a histologically encapsulated tumor; stage II (1) is a thymoma with microscopic trans-capsular invasion; stage II (2) is a thymoma with macroscopic invasion into thymic or adjacent adopise tissue or adherent to the mediastinal pleura or pericardium; stage III is a thymoma with macroscopic invasion into an adjacent organ; stage IVa is a thymoma with pleural or pericardial dissemination; and stage IVb is a thymoma with lymphatic or hematogenous metastasis. Dogs with stage I, II (1), or II (2) thymoma had a median survival time of 1045 days, which was significantly longer than the median survival time of 224 days for dogs with stage III, IVa, and IVb thymomas. Dogs with incomplete excision of their thymoma had a 6.1-times greater risk of tumor-related death, but this was more related to failure of tumor-related pleural effusion to resolve in dogs with residual disease. Dogs with thymomas with an increased lymphocyte count had a significantly improved survival time such that every 10% increase in lymphocyte count results in a 35% reduction in the mortality rate. Hypercalcemia, myasthenia graves, megaesophagus, and the development of postoperative non-thymic tumors are not prognostic. 

In one study of 17 dogs with invasive thymomas treated with radiation therapy, the response rate was 65% with 12% of dogs having a complete response and 53% of dogs having a partial response. The median survival time for these 17 dogs was 248 days. 

Dogs - Ectopic Thyroid Carcinoma

There is a paucity of information on the prognosis for dogs with ectopic cranial mediastinal thymomas. The only information is a 2008 clinical study of nine dogs. In this study, the overall median survival time was 243 days. There were no prognostic factors identified, but the median survival time for dogs with no evidence of pleural effusion, metastasis, or invasive disease was 512 days compared to 243 days for dogs with pleural effusion, metastasis, or local invasion at the time of diagnosis.

Cats - Thymoma

The prognosis for cats with thymoma is also very good following treatment with either surgery or radiation therapy. The majority of thymomas in cats are non-invasive, so they are more amenable to surgical excision. However, in one clinical study of 32 cats with surgically excised thymomas, the perioperative mortality rate was 22% because of either unresectable tumors or complications. The local recurrence rate following surgical excision is 9%-22%, but re-excision of the locally recurrent thymoma is often curative. The median survival time of1354-1825 days and 1-, 2-, and 4-year survival rates of 70%-89%, 74%, and 47%, respectively. In one study of 32 cats with thymomas, five cats died of tumor-related reasons, including local recurrence, metastasis, and surgical complications.

In one study of seven cats with invasive thymomas treated with radiation therapy, the response rate was 57% with 29% of cats each having a complete response and a partial response. The median survival time for these seven cats was 720 days.

Last updated on 6th March 2017