Introduction

Lipomas are neoplastic masses of mesenchymal origin composed of well-differentiated adipocytes. Subcutaneous lipomas are usually solitary, fluctuant to firm, benign masses in all domestic animals.¹ Complete excision is the mainstay of treatment.² Subcutaneous lipomas of the horse can be classified as encapsulated or infiltrative.^3,4 Infiltrative lipomas, while less common in horses, can show extensive involvement of the soft tissues, including adjacent subcutaneous adipose, muscle, nerve, and synovium. Subcutaneous lipomas are relatively rare and primarily found most commonly in young horses ≤ 2 years of age, which is in stark contrast to the more common mesenteric lipoma, which is found in older horses.^4–9 Subcutaneous lipomas typically occur on the limbs, thorax, and abdominal wall. Equine lipomas have also been reported in other areas, such as the pericardium, myocardium, meninges, and maxillary sinus.^3,10–12 While subcutaneous lipomas are undesirable in cosmetic appearance, they are often asymptomatic and clinically unremarkable. However, clinical signs may develop due to their insidious growth and location causing deleterious effects.^13,14

There are limited case descriptions and treatments of equine subcutaneous lipomas. Three cases were euthanized prior to or during surgery because of the difficulty or consequences of resecting infiltrative lipomas of the stifle joint, abdominal wall, and caudal antebrachium.^5–7 Three cases resolved without recurrence following surgical resection, including 2 completely resected encapsulated thoracic masses and an incompletely resected infiltrative mass of the dorsal carpus that involved the common digital extensor tendon sheath.^5,7 Recurrence was seen postoperatively in 2 cases of infiltrative lipomas, both following resection of thoracic masses; in 1 case recurrence was observed after a second more aggressive surgery, after which the owner decided not to pursue further treatment, while in the other case the second surgical procedure resulted in complete resolution.^8,9 Recurrence is also seen in other species, as postoperative recurrence of infiltrative lipomas in dogs and humans is reported as 36% to 50% and 3% to 62%, respectively.^15,16 In both species, complete excision of an infiltrative lipoma is often limited by the dimensions of the lipoma, which also appears to be the case in horses. These reports illustrate the difficulty in resecting infiltrative subcutaneous lipomas. Our case series serves to both consolidate the previous literature and to discuss our treatment of both encapsulated and infiltrative lipomas. Further, we illustrate the use of chemotherapeutics and active drains to assist in the treatment of these masses.

Methods

Results

Case 1

A yearling Warmblood gelding presented with a large mass on the left flank. No prior treatment had been attempted. Ultrasound was performed and showed that the mass extended from the subcutaneous tissue, infiltrating the external abdominal oblique muscle. The internal abdominal oblique and transversus abdominus muscles were unaffected. The mass was hypoechogenic and of similar echogenicity to the surrounding adipose tissue. The mass was resected by use of a 40-cm-long linear incision with the horse in right lateral recumbency and under general anesthesia. The skin overlying the mass was not removed in its entirety due to the size of the mass and desire to close the site primarily. The epidermis was dissected from the subcutaneous tissue by use of blunt and sharp dissection with Metzenbaum scissors. Differentiation of the mass from the surrounding normal subcutaneous tissue was not possible grossly, as the mass did not appear encapsulated. Upon removal, the mass measured 40 X 20 X 8 cm (Figure 1). A penrose drain was placed and exited the abdomen at the cranioventral border of the incision. The drain was removed 5 days after surgery.

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Gross appearance of the resected mass in case 1.

Citation: Journal of the American Veterinary Medical Association 2024; 10.2460/javma.24.02.0146

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Six weeks after surgery, a mass was noted at the exit site of the previously located drain, approximately 10 cm cranioventral to the incision (Figure 2). This mass was treated with 1 mg/cm³ injectable cisplatin mixed with sterile sesame oil on 3 occasions, at 3-week intervals. The lipoma continued to grow in the face of local chemotherapy, and subsequently the horse returned to surgery. The second mass was resected with 3-cm gross margins, at which time an active suction drain (Jackson-Pratt drain with grenade canister) was placed. The removed mass measured 12 X 5 X 2 cm. After the second surgery, the horse returned to normal athleticism as a performance horse. No recurrence was reported at 24 months after surgery.

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Gross appearance of the recurrence of the mass in case 1. The area of recurrence is shown with a circle.

Citation: Journal of the American Veterinary Medical Association 2024; 10.2460/javma.24.02.0146

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Case 3

A yearling Thoroughbred filly presented with a mass of the dorsolateral tarsus of the right hindlimb that appeared on ultrasound to be adherent to the joint capsule of the tarsocrural joint (Figure 3). The joint capsule appeared smooth, and the mass did not appear to be invading the synovial space. The mass extended from the level of the joint capsule dorsally and wrapped around the long digital extensor tendon. It did not appear encapsulated on ultrasound. The filly was sound. The mass was incompletely resected by use of a 15-cm-long incision over the dorsal aspect of the joint with the horse in dorsal recumbency and under general anesthesia. The mass measured 15 X 8 X 2 cm. The joint capsule was not resected, and as seen grossly during surgery, there was adipose adhered to the capsule. The priority in this case was for the filly to remain sound and to not compromise the joint capsule and extensor tendon rather than achieve complete resection. In the 24-month follow-up period, the mass did not grow further and the horse remained sound (Figure 4) and was used as a broodmare.

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Transverse ultrasonographic image of the mass surrounding the common digital extensor tendon (arrowhead) in case 3.

Citation: Journal of the American Veterinary Medical Association 2024; 10.2460/javma.24.02.0146

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Gross appearance of the tarsocrural mass 4 weeks after resection in case 3.

Citation: Journal of the American Veterinary Medical Association 2024; 10.2460/javma.24.02.0146

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Discussion

Subcutaneous lipomas occur in young horses, most commonly ≤ 2 years of age, and appear to be hyperplasia of normal adipose tissue, with no tendency to metastasize.¹⁷ Despite being classified as histologically benign, subcutaneous lipomas can grow insidiously to large sizes and be invasive to local tissues. Gross identification of tumor margin is extremely difficult in locations with significant subcutaneous adipose.

The behavior of subcutaneous lipomas in horses differs from those in other species. These tumors are consistently found in younger horses in comparison to humans and dogs, in which they appear later in life.^2,18 Further, the tumors appear to stop growing as the horse enters adolescence, as seen following the incomplete resection in case 3 and in previous literature.⁷ In 2 cases of incomplete surgical removal, including case 3 of this study, the tumors remained but did not continue to grow.⁷ The pathogenesis for the early occurrence of lipomas in horses and their lack of growth after adolescence is unknown. However, it may be suggested that infiltrative lipomas present at birth or early in life may be congenital in origin, compared to lipomas with a later age of onset such as in humans and dogs, which may represent postdifferentiation genetic aberration of the normal adipocyte’s karyotype.⁸

Previous literature has described 3 cases in which passive drainage (capillary, fenestrated, and penrose drains) was utilized following the resection of lipomas in horses; in all 3 instances, the lipoma was infiltrative.^4,7,9 In 2 of these cases, the infiltrative lipoma was located on the thorax, and in both animals the lipoma recurred.^8,9 Passive drainage was also employed in the resection of a lipoma of the carpus that infiltrated the common digital extensor tendon sheath.⁷ There was no recurrence following excision of the tumor and synovial lining.

The thoracic mass reported in this case series (case 1) was also an infiltrative lipoma. A penrose drain was utilized, and the mass was observed to recur 3 months postoperatively at the location of the exit site of the drain. An active suction drain was used at the time of the second resection, after which no recurrence was observed. Subsequently, an active suction drain was used in the resection of a preputial lipoma (case 2) and no recurrence was noted.

The authors hypothesize that best practice would be to utilize active suction drainage in the resection of large lipomas where dead space is created and cannot otherwise be mitigated. The use of a drain following excision of neoplasia may allow for the passage of abnormal cells to a previously unaffected area of tissue. In the human literature, surgical drain site metastasis is rare but possible, even following cases of microscopically and macroscopically complete excision.^19,20 The authors consider the use of an active suction drain best practice when a large amount of dead space is created when compared to the alternative of a seroma forming.

Cisplatin was utilized in case 1, despite which the recurrent mass continued to grow. Local chemotherapy was performed to attempt to decrease the size of the tumor and prevent a subsequent, more aggressive resection. Cisplatin was utilized due to its common use in the treatment of cutaneous tumors in the horse.²¹ There is no report of the use of cisplatin in the literature to treat lipomas of any species or of other chemotherapeutics used to treat lipomas in horses. In humans, chemotherapeutic agents have not been utilized to treat lipomas to the authors’ knowledge. There is 1 report²² of a dog having partial response to systemic doxorubicin administration within 6 weeks. There is no strong evidence to suggest the use of chemotherapeutics in equine patients that can be extrapolated from other species, likely due to the low mitotic rate of lipomas rendering them ineffective. There is evidence to suggest irradiation (cobalt 60 radiation) can benefit dogs in long-term local tumor control of infiltrative lipomas.^15,23 For incompletely removed infiltrative lipomas in the dog, the average time to recurrence is 8 months following surgical excision, which increases to a mean of 46 months when combined with radiation therapy in dogs.^15,24 While irradiation has not been described to treat lipomas in equids, radiation therapy has been shown to be a very effective treatment modality for equine tumors, but logistical reasons limit its impact in equine oncology.²⁵

Infiltrative lipomas also pose a further diagnostic dilemma: they are indistinguishable from simple lipomas with either cytology or biopsy, as at a cellular and histological level they are identical. Even with histopathology, an infiltrative lipoma may be erroneously diagnosed as a simple lipoma, unless the tumor cells are appreciated to infiltrate the normal adjacent tissue.¹⁵

Imaging modalities can assist in differentiating infiltrative and encapsulated lipomas. Ultrasonography is a commonly used modality to assist in this determination. Infiltrative lipomas have been ultrasonographically described as either having poorly defined borders or simply infiltrative in appearance.^6–9 In contrast, simple lipomas have been described as well-encapsulated masses with hyperechoic margins and hypoechogenicity compared to the surrounding adipose tissue.²⁶ The best modality for imaging lipomas is CT, as it enables the visualization of the interdigitation of the fat attenuation of the mass in comparison to the soft tissue attenuation of muscle and the extension of the lesion through fascial planes.¹⁵ Magnetic resonance imaging is more sensitive at detecting microscopic fat in comparison to CT.²⁷ However, both CT and MRI pose limitations for the imaging of abdominal and thoracic masses in juvenile horses due to their disproportionate size in comparison to the gantry diameter but are still useful for the head and limbs. For cases 1 and 2, CT and MRI would not have been possible due to the anatomical locations of the lipomas (flank and prepuce).

Lipomas should be resected with 3-cm gross margins when possible if a capsule is not identified at surgery or with imaging modalities, as infiltrative lipomas may more appropriately be considered an aggressive tumor type, warranting the large excisional margins that are afforded to other aggressive tumor types.^9,28 Ultrasonography and gross examination may not be helpful in determining the margin of the mass. If necessary, due to large amounts of dead space, active suction drains should be utilized, as this may aid in the removal of neoplastic cells. Chemotherapy was not beneficial in the single case reported in this case series, and likely the lower rate of mitosis in lipomas would make these cells less amenable to chemotherapy.

Disclosures

The authors have nothing to disclose. No AI-assisted technologies were used in the generation of this manuscript.

Funding

The authors have nothing to disclose.