Surgical approach to cutaneous and subcutaneous masses: staging and surgical margins

Cutaneous and subcutaneous masses are a common presenting complaint in first opinion practice. Such masses may also be encountered incidentally during routine examinations. For many cutaneous and subcutaneous neoplasms, surgical excision provides a good chance of a complete cure, provided diagnosis and intervention are timely and appropriate. Despite this, decision making and owner communication can present significant challenges. 

Differentials

Patients may present with visible or palpable masses belonging to a broad range of differentials with varying prognosis and treatment options (Table 1). While this article pertains to cutaneous and subcutaneous neoplasms, clinicians should be aware other lesions such as mammary masses or lymphoma may present with palpable superficial masses.

Cytology and histology

When planning the biopsy, ensure the entire biopsy tract can be readily excised along with the mass. This will prevent seeding of neoplastic cells to tissue that is not being excised at the time of curative intent surgery

Owners need to be aware that while cytology is an important first step, non-diagnostic samples and misrepresentative samples can and do occur, and further investigations are frequently required. Neoplasms cannot be graded on the basis of cytology; histopathology is required for grading. If histological diagnosis would alter the treatment plan, it is important to obtain a biopsy for histopathology before attempting excision. When planning the biopsy, ensure the entire biopsy tract can be readily excised along with the mass. This will prevent seeding of neoplastic cells to tissue that is not being excised at the time of curative intent surgery.

Staging

Tumour staging is a way of describing the size of a tumour, how far it has spread and its malignancy. The most widely used system is the tumour, node, metastasis (TNM) staging system (Table 2). Staging is an essential part of the diagnostic process and should take place before any treatment is initiated. In many cases, staging procedures can take place in a general practice setting. Understanding the tumour’s biological behaviour and whether it has spread beyond the initial location are essential to surgical planning and discussions around adjunctive therapies that may be indicated. Consultation with a veterinary oncologist is recommended in the case of unfamiliar or complex neoplasms.

Sentinel lymph node (SNL) mapping is steadily becoming more prevalent in the veterinary field and may offer a more accurate approach to assessing for lymphatic spread in many cases

Tumour biology can alter the approach to staging between different tumour types, because the propensity for lymphatic spread and likely sites for metastasis will vary between tumour types.

Sampling of enlarged lymph nodes should be performed regardless of tumour type; however, in tumours that readily spread through the lymphatic system, such as melanoma (Polton et al., 2024) or mast cell tumours (Oliveira et al., 2020), sampling of any regional lymph nodes even if not enlarged is advised. Sentinel lymph node (SNL) mapping is steadily becoming more prevalent in the veterinary field and may offer a more accurate approach to assessing for lymphatic spread in many cases. SNL mapping involves the administration of a contrast agent around the tumour, followed by computed tomography (CT) or ultrasonography to visualise the exact lymph nodes that drain the patient’s tumour. These SNLs can then be sampled in a targeted manner to check for metastasis.

Thoracic imaging is advised for tumours that readily result in pulmonary metastases, such as grade III soft tissue sarcomas. While CT is considered the gold standard for identifying pulmonary metastasis, three-view inflated plain radiographs should be used in cases where availability or finances preclude the use of CT. 

If visceral metastasis is a concern, imaging and/or sampling of abdominal organs may be advised. For example, mast cell tumours (MCT) are unlikely to result in pulmonary metastases; however, abdominal ultrasonography and cytological sampling of the liver and spleen should be considered, particularly where high-grade MCT is suspected or confirmed. 

Surgical considerations

The first surgery gives the best chance of complete tumour removal; therefore, careful surgical planning is essential. The surgical goal will depend on a variety of factors:

• Diagnosis (cytological/histological)
• Prognosis
• Ability to achieve adequate surgical margins and associated morbidity
• Co-morbidities
• Evidence of paraneoplastic syndrome
• Owner factors:
  • Desire to treatPersonal experiences

Surgical dose 

In oncological surgery, “surgical dose” refers to the extent or amount of tissue removal performed during surgery to treat cancer. The concept of surgical dose emphasises precision in achieving cancer control while being mindful of the patient’s long-term outcome, function and quality of life. The following surgical doses could be considered:

• Debulking: incomplete resection of a tumour with residual gross disease. Rarely recommended as unlikely to provide significant benefit to the patient even with adjunctive therapy
• Marginal resection: sometimes referred to as “shelling out”; removes the gross mass around the level of the pseudocapsule. It can be a useful approach for benign neoplasms such as lipomas, but is not appropriate for malignant masses unless planned in conjunction with adjunctive therapy. Marginal resection of malignant masses will likely result in residual neoplastic cells and complicate further surgical intervention
• Wide resection: curative intent treatment of choice for many malignant neoplasms as both the gross and microscopic tumour cells are removed
• Radical resection: removal of the entire compartment affected by the neoplasm; the most likely example for an appendicular cutaneous/subcutaneous mass would be limb amputation. While likely to be effective in removing all neoplastic tissue, radical resections can be debilitating or offer limited advantage over other surgical approaches (Farese et al., 2017)

Surgical margins

Tumours are three-dimensional structures, so deep margins are just as important as lateral margins; there is no benefit to a wide lateral excision if deep margins cannot be obtained. In some cases, it may be beneficial to shrink the tumour prior to resection; however, surgeons should be aware that controversy exists as to where the cut margins should then be measured from. Margins may be based on the original tumour size; the area of tumour is marked before shrinking and owners asked to maintain the mark during the process. More recently it has been suggested that in the case of specific tumours, it may be appropriate to measure margins after tumour shrinking (Ciammaichella et al., 2024).

Tumours are three-dimensional structures, so deep margins are just as important as lateral margins; there is no benefit to a wide lateral excision if deep margins cannot be obtained

Deep margins frequently present the greatest challenge, as natural barriers such as fascia, cartilage or bone will likely dictate the extent of the deep margin. Subcutaneous tissue, fat and muscle provide poor barriers to tumour cell invasion and should not be considered adequate tumour barriers. In the case of tumours located on areas other than the trunk or in the case of very large tumours, this may present a significant surgical challenge, both in terms of obtaining sufficient margins and closing a large deficit. It is important not to enter the tumour capsule, if present, and to remove any biopsy tract as part of the en bloc excision. Remember to strictly adhere to basic surgical principles, handle the tumour carefully and change gloves and instruments before closure.

The entirety of excised tissue should be submitted for histopathology to confirm the diagnosis, grade the tumour and provide information on surgical margins. Labelling the excised tissue with coloured surgical dye (Figure 1) will help the pathologist to identify which margins are “dirty”. This is of particular importance if revision surgery is a consideration or if complications are encountered with healing of the surgical site.

Specific tumour recommendations

Recent findings indicate that wide resection margins alone do not eliminate the risk of local recurrence

Soft tissue sarcomas: 2 to 3cm lateral margins and one fascial plane are typically recommended for soft tissue sarcomas (Baker-Gabb et al., 2003) (Figure 2). Low-grade soft tissue sarcomas on distal limbs are commonly seen in older dogs. Curative intent surgery can be challenging as deep margins are difficult to achieve due to limited soft tissue coverage and wide lateral margins are of little use without deep margins. As the recurrence rate of low-grade soft tissue sarcomas is low, marginal resection may be appropriate provided owners are appropriately counselled and grading has been obtained on the basis of incisional biopsy and histopathology (Bray, 2017).

Low-, intermediate- and high-grade mast cell tumours: low- and intermediate-grade MCTs are usually excised with 2cm lateral margins and 1 deep fascial plane, while 3cm margins are advised for known high-grade tumours (Selmic and Ruple, 2020). Where Kiupel and Patnaik grades differ, the highest histological grade should be used to plan margins. If the mass is less than 2cm diameter, it may be appropriate to use narrower margins equivalent to the diameter of the mass (proportional margins) (Saunders et al., 2021). 

Feline injection site sarcomas: feline injection site sarcomas (FISS) are associated with high mortality and morbidity making them among the most challenging skin tumours to manage. Due to the locally aggressive nature of these masses, 5cm lateral margins from the palpable borders of the mass and two fascial planes are considered the required margins for curative intent surgery (Phelps et al., 2011). These margins are likely to involve significant underlying structures such as vertebrae or the scapula. As a result, surgical planning is key; CT and 3D reconstructions are particularly helpful in this regard.

Closure

It is important to plan closure to avoid tension and eliminate dead space (Figure 3). Avoid the use of surgical drains where possible to reduce the risk of seeding neoplastic cells. Complete closure should be performed where possible, although the location and required margins will determine the difficulty. Challenging tumours may require advanced techniques such as axial pattern flaps or skin stretching techniques, for which referral may be indicated. Consider initial open wound management, pending histology, in cases where margins are in doubt or significant reconstruction (eg axial pattern flaps or skin grafts) is required. Partial closure can be used in select cases where curative intent surgery offers significant benefit over a more conservative approach, but complete closure cannot be achieved. Preoperative owner counselling regarding ongoing management, timelines and associated costs is key.

What if my margins are incomplete?

Despite our best efforts, there will always be occasions where histopathological margins are reported as incomplete. This is frustrating both for the clinician and for the owner. Considerations for when this occurs and how to proceed are:

• What is the histological diagnosis?
• What is the tumour biology indicating regarding speed of growth?
• How old is the patient and do they have other co-morbidities?
• Are adjunctive treatments an option for the tumour type or the patient/owner circumstances?
• If planning a revision surgery, ensure the entire scar-line is resected as part of the planned excision