Mediastinal Diseases and Masses / Thymoma

The mediastinum consists of most parts of the chest that are not taken up by the lungs or the heart and its associated large blood vessels. It is an area that in healthy individuals is filled with fatty tissue, connective tissue, lymph node tissue, and an organ called the thymus.

The mediastinum is generally considered to include three distinct regions: the anterior (or anterosuperior mediastinum), the middle mediastinum, and the posterior mediastinum. The anterior mediastinum contains the thymus gland and thus is the usual location for thymomas (tumors of the thymus). Other common tumors of the anterior mediastinum are lymphomas (tumors of the lymph node system) and germ cell tumors (tumors originating in cells similar to testicular or ovarian cells but which are located abnormally in the chest). Masses in the middle mediastinum most commonly represent lymph nodes that are enlarged by a malignant, infectious, or inflammatory process. Masses in the posterior mediastinum are usually benign tumors or cysts originating from either the nerves that are present in this area (neurogenic tumors) or from the esophagus (foregut duplication cysts).

Thymomas are malignant tumors that arise in the thymus gland, typically in the anterior mediastinum.  Although considered malignant, most thymomas grow slowly and tend to spread only locally, by "seeding" cells into surrounding tissues and spaces such as the pleural space (the space surrounding each lung). There is therefore often a very good chance of curing thymomas by either surgery alone or surgery followed by radiation. We often proceed to remove these localized tumors without even the need for a preliminary biopsy.

More difficult thymic tumors that consist of frank carcinoma cells, and other thymic tumors that appear to invade surrounding structures on preoperative radiographic studies, are often best treated by administering preoperative chemotherapy or chemotherapy and radiation before surgical removal. These cases are discussed at our multidisciplinary Thoracic Tumor Board which consists of specialists from oncology, radiation oncology, radiology, and pathology in addition to thoracic surgeons. Here, the optimal treatment plan can be designed with the combined input from all the appropriate experts. These tumors require a preoperative needle biopsy or surgical biopsy to confirm the diagnosis before giving preoperative therapies. Surgery is then generally carried out if it is felt to be possible and there has been at least some shrinkage in response to the preoperative treatment.

The most common surgical incision through which to remove a thymoma is a median sternotomy. This is a vertical incision through the breast bone that provides excellent exposure of the anterior mediastinum. Since no chest wall muscles are cut with this incision, postoperative pain is modest, and recovery is not prolonged. Smaller thymomas may be treated with VATS (video-assisted thoracoscopic surgery and robotic thymectomy – see Lung Cancer) thymectomy, but this is an emerging area that we are moving into cautiously, with patient-safety at the forefront of our minds.  We are also among the most experienced centers in transcervical thymectomy for myasthenia gravis, but we believe this approach should be reserved primarily for patients without thymoma – it should certainly not be used for more than the smallest thymomas (less than 2 cm in size) given the risk of spillage of tumor cells.

There are several staging systems for thymoma, but the most widely used are the Masaoka system and the World Health Organization (WHO) system.  The Masaoka system is based on degree of invasion of the capsule around the tumor and of surrounding structures. The WHO system is based upon the appearance of the cells that make up the tumor under the microscope. Combining the two systems allows us to provide the best estimate of prognosis for each individual tumor, as well as allowing us to recommend when preoperative and postoperative treatments might be advantageous.

Masaoka System

I - Completely encapsulated tumor, without tumor invasion into capsule
II - Tumor invasion into fatty/thymic tissue surrounding the tumor or microscopic invasion into the capsule
III - Tumor invasion into organs surrounding the thymus
IVA - Tumor implants in the pleura or pericardium (the linings of the lungs or heart)
IVB - Spread to lymph nodes or via bloodstream to distant organs

World Health Organization (WHO) System

A
AB
B1, B2, B3
C Thymic Carcinoma

Cure rates for Masaoka stage I tumors are 90-95%; stage II 85-90%; stage III 50-70%; stage IV 20-50%. WHO type A and AB tumors within each of these Masaoka stages appear to have cure rates at the higher end of the range, while those with type B tumors will fall at the lower end of the cure range. Type C tumors – thymic carcinoma – have the worst prognosis.

Stage I - Surgical resection alone
Stage II - Surgical resection, sometimes followed by radiation
Stage III - Chemotherapy or chemoradiation followed by surgical resection; or surgical resection followed by radiation or chemoradiation
Stage IV - Chemoradiation followed in some cases by by surgical resection

Myasthenia gravis is a disease of the junctions where nerves that control muscle function meet the muscles themselves (the neuromuscular junction). In myasthenia gravis patients, antibodies form within the neuromuscular junction that make the transmission of nerve impulses from nerve to muscle less efficient. This creates a variety of problems which may include weakness of the peripheral muscles (usually worsening with repetitive contraction of the muscles), droopy eyelids (ptosis), double vision, and swallowing or breathing difficulties.

Drug therapy, which usually is initiated with pyridostigmine (mestinon) alone, often helps these symptoms of myasthenia gravis. Other immunosuppressive drugs, such as prednisone or azathioprine, are added in some cases.

Surgical removal of the thymus gland (thymectomy) has had a role in the management of patients with thymoma for over 70 years. It has now been established in a large, international, randomized clinical trial that patients who undergo thymectomy have an improved course for their myasthenia gravis than patients treated with drug therapy alone. While very few patients have a complete remission of the disease without thymectomy, 30-40% of patients will have a complete remission after surgery.

At Stanford, we are able to perform thymectomy for most myasthenia gravis patients through a single, 5 cm long incision at the base of the neck. This operation, called transcervical thymectomy can be performed with very little risk or discomfort. Patients return home the day of surgery or the following day, and they generally return to their usual activities within a week. The alternative procedure – thymectomy through a median sternotomy – has a substantially increased complication rate in patients with myasthenia gravis, who are susceptible to lung and other complications due to their muscle weakness.

Before coming to Stanford, Division Chief Dr. Joseph Shrager published the world's largest series of extended transcervical thymectomy. This study demonstrated that the operation provides very similar remission rates to those achieved by thymectomy by median sternotomy, with far less complications and faster recovery. We are among the centers that has the greatest experience with this operation, and we are pleased to be able to offer it to myasthenia gravis patients in the greater San Francisco Bay Area as well as those from around the country.

A variety of types of lymphomas (tumors of the lymphatic, or lymph node, system) may arise primarily in the mediastinum. The most common of these is Hodgkin’s Lymphoma and B-cell Lymphomas. These are most often located in the anterior mediastinum, but they may also involve the middle mediastinum. 

The treatment of lymphoma is primarily chemotherapy and/or radiation therapy and not surgical removal. However, prompt and accurate pathologic diagnosis by surgical biopsy is the essence of successful therapy, and Stanford surgeons are experienced in all of the techniques available to biopsy mediastinal masses. These minimally invasive procedures may include anterior mediastinotomy (Chamberlain Procedure), mediastinoscopy, and occasionally video-assisted thorascopic surgery (VATS). The availability at Stanford of the video mediastinoscope – useful from either the cervical or anterior approaches – permits accurate tissue sampling with minimal morbidity. Stanford’s world-renowned lymphoma oncologists can then plan the appropriate therapy.

Nearly all tumors that occur in the posterior mediastinum are benign. These tumors are thus ideally suited to minimally invasive, video-assisted thoracoscopic surgical (VATS) approaches (see Lung Cancer section of this website). This approach can be used to resect all but the largest neurogenic tumors, bronchogenic cysts, and foregut duplication cysts. VATS permits far less discomfort and faster recovery than the traditional approach via thoracotomy, which requires a larger skin incision, more cutting of muscle, and spreading the ribs.