34th Annual Scientific Meeting proceedings

Lung Tumours
Most primary pulmonary tumours are carcinomas. Stage and margin status have both been identified to be prognostic. As surgeons, we can choose which patients to treat; however, we cannot alter stage. Margin status, on the other hand may be within our control. Effective staging, including contrast-enhanced CT, can minimise the risk of taking patients to surgery with excessively advanced disease. Some aggressive features may be difficult to assess, including adhesion to adjacent structures, early nodal metastasis, and carcinomatosis. Pleural effusion should be sampled as a neoplastic effusion alone would confer stage 4 designation.

Determination of resectability of extensive or equivocal lesions should be made by a surgeon with substantial experience in thoracic surgery. Decision-making regarding surgery should incorporate anticipated biologic behaviour. For example, a lung tumor with invasion into the body wall may be resectable, but the risk of early progressive disease is likely high. Similarly, patient status must be considered.

Evidence-based recommendations regarding gross surgical margins for pulmonary carcinomas are lacking in dogs and cats, although anatomic pulmonary resection or gross lateral margins of ≥ 2 cm for sublobar resection have been recommended for people with non-small cell lung cancer. While effective local control can be achieved in many dogs with a simple lobectomy, very large or hilar lesions may require a more involved approach, such as concurrent resection of multiple lung lobes (e.g. right caudal and accessory lobes) or pneumonectomy.

Pneumonectomy involves resection of an entire lung. Functional outcomes are typically good for dogs, with an expectation of largely normal activity as a typical pet; however, dogs with an expectation for a high level of performance may not be able to reach these standards and patients with underlying respiratory or cardiac compromise are likely at higher risk of acute respiratory insufficiency or pulmonary oedema. In experimental dogs, improvement can be seen over time, with remodelling of the remaining lung and associated improvement in gas exchange. Patients must, however, be able to live long enough to benefit from remodelling. Importantly, reported oncologic outcomes are not uncommonly poor, emphasising the importance of pre-operative evaluation and communication. Radiotherapy may provide a lower risk, non-invasive alternative. Median survival in the order of just under 1 year has been reported, with influence of stage yet to be well characterised.

Body Wall Resection
Osteosarcoma, chondrosarcoma, and soft tissue sarcoma comprise most primary body wall tumours in dogs and cats. Clinical outcomes are influenced by tumour type, stage, and grade (if applicable); however, if disease is localised at initial staging, surgery via body wall resection may present the opportunity of local control for quality of life and/or extended survival. Imaging and effective interpretation are critical to determining a reasonable surgical dose and implementation. Surgical dose, in this context, is often tied to options for reconstruction, due to the necessity of single stage restoration of body wall function for many patients. Simplification of reconstruction, in particular, is essential to mitigate risk, as many dogs with deep body wall tumours may not need resection of skin beyond previous biopsy tracts or defined anatomic corridors. Similarly, dogs with superficial masses requiring chest wall resection may not always need margins to be as extensive deep as the superficial margins, depending on sites of involvement and potential tissue barriers.

Use of autogenous tissues has been associated with a lower risk of complications than use of prosthetic mesh. Nearby muscles for use in reconstruction include the latissimus dorsi, external/internal abdominal oblique, and pectoral muscles, while manipulation of the diaphragm can be important in both a supporting role and, less commonly, a primary reconstructive role. Combinations of mesh and muscle can also be used to increase reconstruction strength and diminish the reliance on foreign material; however, the costs of additional dissection and tissue mobilisation should be considered if the surgeon is already committing to use of mesh. Specific sites, such as sternal resection have been associated with increased risk of complications in dogs; however, literature does not support a consistently increased risk in people.

Mediastinal Masses
The most common mediastinal neoplasms in dogs include thymic epithelial tumours (including thymoma and thymic carcinoma), lymphoma, and thyroid tumours. Sporadic sarcomas may also arise within the mediastinum or adjacent tissues. Staging, including cross-sectional imaging, is essential to evaluate the extent of local disease and to assess for regional and distant metastasis. Aspiration of mediastinal masses +/- flow cytometry can often provide a sufficient diagnosis, with a CD4+/CD8+ phenotype characteristic of thymocytes. Patients with thymic epithelial tumours should be evaluated for myasthenia gravis (MG). If MG is diagnosed, pre-surgical treatment has been recommended in people to mitigate peri-operative risk. Benefits of therapeutic strategies such as plasma exchange are unknown in dogs.

Surgery, if feasible, tends to be the mainstay of management of thymic epithelial tumours in both small animals and people. In humans, systemic therapy is recommended prior to resection of a thymoma if an R0 resection cannot be confidently achieved; however, in small animals systemic therapy has not yet been substantiated for thymic epithelial tumours. While radiotherapy may be used in the neo-adjuvant setting to decrease tumour size, the risks and benefits of this approach have not been explored and patient selection criteria have not been established.

Masaoka-Koga stage has been associated with outcome and synthesises pre-operative, intra-operative, and histological data. Importantly, size of the primary tumour is not a part of staging of thymic epithelial tumours. Instead, invasion into the thymic capsule, pericardium, mediastinal pleura, or surrounding anatomic structures are the key components. As such, surgeons should carefully examine pre-operative imaging for evidence of gross vascular invasion and irregularities suggestive of compromise of adjacent tissues and/or the thymic capsule.

At surgery, surgeons should evaluate pleural surfaces for evidence of seeding and resect foci of disease, where possible, to achieve gross microscopic disease; however, intralesional resection should be avoided whenever possible. A phrenic nerve can be sacrificed unilaterally but bilateral phrenic nerve compromise should be avoided, if at all possible, due to frequent morbidity +/- mortality. Haemoclips can be placed at the intrathoracic surgical site to guide adjuvant radiotherapy, if indicated. Approaches vary amongst veterinary radiation oncologists, with some willing to irradiate in the microscopic setting and others inclined to wait for gross recurrence, given the challenges of avoiding dose to normal structures, such as lung and myocardium. Evidence-based recommendations regarding adjuvant therapy for thymic epithelial tumours are lacking. For unresectable or high-risk surgical candidates, radiotherapy can result in marked responses; however, median survival times are typically shorter than those associated with surgery.