Thymomas and thymic carcinomas are rare diseases in which malignant tumors form on the outside of the thymus. The differences between the two thymus-related tumors have important treatment and prognosis implications.
“They are both rare — maybe less than 2,000 cases a year in this country — so there’s not a lot of familiarity with treating these tumors,” said Stevenson. “Thymomas have a better overall prognosis than thymic carcinomas, and tend to be smaller and more resectable than thymic carcinomas, which have more of a tendency to be aggressive and metastatic.” Another factor that distinguishes thymomas from thymic carcinoma is that thymomas often present with autoimmune diseases, particularly myasthenia gravis, the chronic autoimmune neuromuscular disease that causes weakness in the skeletal muscles. According to a recent review, approximately half of patients with cortical thymoma develop myasthenia gravis, while 15% of myasthenia gravis patients have thymomas.
Therefore, said Stevenson, when a patient presents with symptoms associated with autoimmunity, clinicians should consider the possibility of an underlying thymoma. Other patients may have symptoms related to the tumor itself, said Stevenson. “The [tumors] could be large and cause chest pressure, pain, and shortness of breath. And then there are patients who are probably asymptomatic and [whose tumors] are found incidentally on a CT scan done for other reasons.”
Whether or not a tumor is considered resectable will be key in determining treatment options, said Stevenson. For patients with resectable cancers, surgical removal of the tumor is the primary treatment for thymoma and thymic carcinoma, and offers the best chances for longer-term survival.
“One thing we have to keep in mind when treating thymomas if they are more advanced is understanding that these people can have more prolonged survival,” Stevenson added. “We want treatments that aren’t going to be too toxic and have too many long-term side effects that can readily affect quality of life for someone who is going to be living for more than 5 years with an incurable malignancy. We want to treat these thymomas, but we want to keep prognosis and quality of life in mind.”
The Research Behind it
While at Georgetown Lombardi Comprehensive Cancer Center in Washington, D.C., Giuseppe Giaccone, MD, PhD, led a study looking at pembrolizumab in patients with recurrent thymic carcinoma. “We found pembrolizumab is active in thymic carcinoma,” said Giaccone, now associate director for clinical research at the Sandra and Edward Meyer Cancer Center of Weill-Cornell Medicine in New York City. The response rate among the 40 patients in the study was 22.5%.
A study from South Korea showed similar results, he noted. Giaccone also said that while pembrolizumab was well tolerated in general, 15% of patients had serious immune-related adverse events. “So the drug is active, the treatment is definitely a good option, and the duration of response is much longer than anything else I’ve seen before,” said Giaccone. “However, patients just have to be monitored very carefully, because of the potential to develop autoimmune disorders.”
As for other immunotherapies, Giaccone pointed to a Japanese study that looked at nivolumab for unresectable or recurrent thymic carcinoma. This phase II trial failed to show any tumor shrinkage in 15 patients treated with nivolumab, leading the researchers to terminate patient accrual and conclude that “further development of nivolumab is not recommended in previously treated unresectable or recurrent
Those results seem to suggest that there are differences between these drugs, “even though they have the same target,” Giaccone said. As for immunotherapy and thymomas, Giaccone noted that the South Korean study included seven patients with thymomas, five of whom developed severe autoimmune disorders.
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