Benign laryngeal tumours (e.g. rhabdomyomas or oncocytomas) may be resected with preservation of normal laryngeal function, and have a good prognosis. In most malignant laryngeal cancers (Figure 14.1), total laryngectomy and permanent tracheostomy are required in the attempt to remove all local disease. One dog undergoing total laryngectomy and permanent tracheostomy for a laryngeal rhabdomyosarcoma had no evidence of disease 18 months after surgery (Block et al 1995). Pharyngeal dysphagia, local/distant tumour recurrence, care of a permanent tracheostomy wound, short- and long-term morbidity to the animal are factors for the client and clinician to consider carefully.

If the tumour has a known responsiveness to chemotherapy or radiation therapy (e.g. if the tumour is of lymphoid origin), then this may be the best treatment option. However, it is important to obtain a definitive histopathological diagnosis prior to treatment. An incisional biopsy may also be therapeutic if it assists in the relief of upper respiratory tract obstruction. However, in cases of severe obstruction, an emergency debulking surgery may be required. This may allow initial palliation of clinical signs and provides tissue for diagnosis. It may be followed with adjuvant chemotherapy or radiotherapy, if considered beneficial.

Many tracheal tumours are benign and have a good prognosis with complete excision (Figure 14.2).

Surgery generally involves the resection of affected trachea and end-to-end anastomosis of tracheal rings. Tension is more of a problem in puppies than in adult dogs. Tension-free anastomosis is achievable if no more than 25–50% of the tracheal length (five or six tracheal rings) is removed in adult dogs and 20–25% in puppies (Nelson 2003). A simple continuous suture pattern is recommended (Demetriou et al 2006).

Tracheal wall replacement with latissimus dorsi (deficit <30% of ventral tracheal wall, <50% of dorsal tracheal wall and limited to five tracheal rings or less) has been successful (Fujita et al 1987). If fewer than five rings or >50% of the trachea is removed, reconstruction becomes more problematic. Rigid structures such as pedicled costal cartilage and rib, free cartilage, polypropylene mesh, collagen grafted mesh, mucous membrane grafted mesh, silicone rubber or non-porous silastic tubing can be used (Eckersberger et al 1987, Goldstein et al 1987, Kim et al 2004, Klopper 1969, Okumura et al 1994, Suh et al 2001, Yamato 1992).

Polytetrafluoroethylene grafts did not work in one study (Cull et al 1990) and jejunal autografts, with or without metal stenting, are sometimes successful but mostly unrewarding (Letang et al 1990, Ma et al 1990, Nakayama 1990, Szántó et al 2001).

These include radiation, chemotherapy, endoscopic removal/debulking and photodynamic therapy. Chemotherapy is beneficial for tracheal LSA.

However, a number of other tumours and structures can also be seen, e.g.

• thymic carcinoma

• branchial cysts

• ectopic thyroid tissue

• chemodectoma

• metastatic disease

• other rare tumours, e.g. HSA, mesotheliomas

• local extension of rib or sternal tumours into the mediastinum.

These can be similar for both tumours; however, PU/PD is more common with LSA, and regurgitation secondary to megaoesophagus is seen more frequently with thymomas.

As a mass in the cranial mediastinum, thymoma and lymphoma can be indistinguishable. The presence of other enlarged lymph nodes or abnormalities with other organ systems may indicate the origin of the mediastinal mass.

Fifty per cent of dogs with mediastinal LSA are hypercalcaemic, whereas hypercalcaemia is less likely with thymoma. However, ultimately a tissue sample is required as paraneoplastic syndromes are not exclusive to one tumour type.

Ultrasound assists not only in delineating the size and echogenicity of a mass, but will also determine whether there is one or more masses in the mediastinum, i.e. the presence of metastatic disease within the mediastinal lymph nodes. It also facilitates diagnosis as fine needle aspirates (FNA) or tru-cut biopsies under ultrasound guidance can often be carried out.

Ultrasound can have difficulty in determining the full extent of a tumour regarding its invasiveness. Thymomas are often of mixed echogenicity and cavitated/cystic (compared to more homogeneous thymic LSA). Because they are cystic, thymomas may need a wedge biopsy for diagnosis.

A CT or MRI scan can be very useful in determining the full extent and invasive nature of a tumour and assist the surgeon in presurgical planning. Wherever possible, a presurgical diagnosis is desired and this is usually attempted either by cytology or tru-cut biopsy.

Surgery is the treatment of choice for non-invasive thymomas, and can be curative (Figure 14.4).

For invasive thymomas, venous grafts can be performed for invasion of the cranial vena cava. For non-invasive thymomas, careful sharp/blunt dissection, with care of nerves and vessels, often allows removal. Both require thoracotomy via median sternotomy.

Resectability cannot be predicted or assessed until surgery. Size is not an indicator of resectability, and non-invasive thymomas can reach a very large size. Invasive thymomas will be adherent to surrounding tissues, including major vessels, nerves, trachea and oesophagus. Debulking an invasive tumour may alleviate symptoms and enhance the effectiveness of adjuvant treatments.

There are few documented cases of radiation treatments in patients with incompletely resected thymomas. Thymomas are moderately radiation sensitive and with appropriate planning there is a role for radiation. Palliative radiotherapy would be an option in patients that were seen as poor surgical candidates due to other medical problems or PNS. For patients with large tumours, presurgical radiotherapy will reduce the size of the tumour very quickly, thus reducing respiratory distress associated with a large-space occupying lesion in the mediastinum. Typically, a single fraction of 800 cGy is sufficient and surgery can then be scheduled within 5–7 days of treatment. Radiotherapy is applicable for those tumours deemed unresectable by surgery.

Adjuvant radiation (combined with incomplete resection) has an increased survival time compared with incomplete resection alone. Complications include pneumonitis and pericarditis.

Although rarely considered in veterinary medicine, chemotherapy may be beneficial in patients that are not good surgical candidates and where radiation is not available.

Prednisolone may provide symptomatic relief by reducing the percentage of small lymphocytes in lymphocyte-rich thymomas. In humans, combination protocols utilizing doxorubicin/cisplatin/cyclophosphamide have been used. A definitive diagnosis of thymoma versus LSA versus other cancer should be sought before starting chemotherapy.

Cats have an excellent prognosis when treated with surgery or radiation therapy. The median survival time with surgery is 21 months, and with radiation therapy is 24 months (Kaser-Hotz et al 2001, Smith et al 2001).

For dogs, the prognosis is excellent for non-invasive thymomas without megaoesophagus. When treated with surgery, these dogs have an 83% 1-year survival and often die of unrelated disease. The prognosis is guarded for invasive thymomas or if megaoesophagus is present (median survival time (MST) 4 days; with myasthenia gravis 67% died within 1 week) (Atwater et al 1994).

Dogs have a higher proportion of invasive thymomas compared to cats, which reflects on the overall improved survival of cats treated with surgery (MST approaching 2 years) (Gores et al 1994). If the tumour is not resectable, then prognosis is poor but survival times for these patients would undoubtedly be improved if they were considered for other treatment options, particularly palliative radiation. The MST for dogs treated with radiation therapy was 248 days (Smith et al 2001).

A recent study (Zitz et al 2008) reported even longer survival times in both cats and dogs treated with surgery alone (MST for cats 1825 days and for dogs 790 days).