Nodular Dermatofibrosis and Renal Cystadenocarcinoma: Recognizing a Skin Lesion That Points Inward
Pathology Deep Dive · Canine · Dermatopathology, Oncology
When a German Shepherd Dog presents with multiple firm cutaneous nodules, the differential list typically runs toward common diagnoses — follicular cysts, lipomas, soft tissue sarcomas, mast cell tumors. Nodular dermatofibrosis is unlikely to lead that list unless the clinician already knows to look for it. That is precisely the problem. Nodular dermatofibrosis is a cutaneous marker for an internal malignancy, and in affected dogs, the skin lesions typically precede or accompany a renal tumor that will determine the patient's prognosis. The biopsy that comes back as "dermatofibrosis" is not the end of the diagnostic workup. It is the beginning.
The syndrome and its genetic basis
Nodular dermatofibrosis-renal cystadenocarcinoma syndrome (ND-RCAS) is a hereditary condition in German Shepherd Dogs caused by a germline mutation in the FLCN gene, which encodes folliculin — a tumor suppressor protein involved in the mTOR signaling pathway. The mutation follows an autosomal dominant inheritance pattern with incomplete penetrance, meaning not every dog carrying the mutation will develop clinical disease, but affected dogs are at significantly elevated risk. The syndrome has also been reported in a small number of other breeds, though German Shepherd Dogs account for the overwhelming majority of cases in the published literature.
Folliculin functions as a negative regulator of mTOR complex 1 (mTORC1). Loss of folliculin function leads to dysregulated mTOR signaling, which promotes cellular proliferation and survival. In the kidney, this manifests as multifocal cyst formation and, in a proportion of affected dogs, progression to bilateral renal cystadenocarcinoma. In the skin, folliculin loss drives fibroblast proliferation and collagen deposition, producing the characteristic firm dermal nodules of nodular dermatofibrosis. The cutaneous lesions are not malignant — they do not metastasize and are not directly life-threatening — but they are a visible and biopsy-accessible indicator that the underlying genetic defect is expressed and that renal surveillance is warranted.
The same FLCN mutation is responsible for Birt-Hogg-Dubé syndrome in humans, a condition characterized by fibrofolliculomas of the skin, pulmonary cysts, and an elevated risk of renal cell carcinoma. The parallels between the human and canine syndromes are striking and have made the German Shepherd Dog a naturally occurring animal model for this rare human condition.
Cutaneous lesions: what you see and what to biopsy
The skin lesions of nodular dermatofibrosis are firm, well-circumscribed dermal to subcutaneous nodules that typically range from a few millimeters to several centimeters in diameter. They are most commonly found on the limbs — particularly the distal extremities — and on the head, though truncal distribution occurs. Lesions are usually multiple and may number in the dozens in severely affected dogs. The overlying epidermis is generally intact unless lesions have been traumatized. The nodules are not painful on palpation in most cases and do not ulcerate spontaneously.
The clinical appearance is not pathognomonic — these nodules look like many other things. What makes nodular dermatofibrosis a consideration is the combination of signalment (German Shepherd Dog, typically middle-aged to older), distribution (multiple lesions, distal limb predominance), and character (firm, dermal, non-painful). In a German Shepherd Dog with multiple firm cutaneous nodules and no obvious inflammatory cause, nodular dermatofibrosis belongs on the differential list and biopsy is indicated.
For biopsy, excisional removal of a representative nodule is preferred over punch biopsy alone, as the architectural relationship of the lesion to the surrounding dermis and subcutis is diagnostically useful. Submit the entire nodule if size permits. If multiple lesions are present, sampling two to three from different anatomic locations is reasonable to confirm histologic consistency. The clinical history — including breed, number of lesions, distribution, and duration — should be included on the submission form, as this context materially affects interpretation.
Histopathologic findings: what the report will say
The histopathologic appearance of nodular dermatofibrosis is a well-demarcated dermal nodule composed of dense, haphazardly arranged bundles of collagen with interspersed fibroblasts. The collagen is often coarse and hypocellular in the center of the lesion, with a more cellular periphery. The overlying epidermis is usually unremarkable. There is no significant inflammatory infiltrate, no necrosis, and no cytologic atypia — these are benign fibrocollagenous proliferations.
The report will typically render a diagnosis of nodular dermatofibrosis or fibrocollagenous nodule consistent with nodular dermatofibrosis. A good report will also include a comment flagging the association with renal cystadenocarcinoma in German Shepherd Dogs and recommending renal imaging. If yours does not, that recommendation should still be acted upon. The histopathologic diagnosis is the cue to look at the kidneys — not a reassurance that the workup is complete.
The renal lesion: what to look for and when
Renal involvement in ND-RCAS ranges from bilateral multifocal renal cysts — which may be present years before malignancy develops — to bilateral renal cystadenocarcinoma, which carries a guarded to grave prognosis given its bilateral nature and the resulting progressive loss of functional renal parenchyma. The bilateral distribution is a critical feature: unlike sporadic unilateral renal carcinoma, which may be managed by nephrectomy with preservation of the contralateral kidney, bilateral cystadenocarcinoma forecloses that option in most cases.
Renal imaging should be performed in any German Shepherd Dog in whom nodular dermatofibrosis has been confirmed on biopsy. Abdominal ultrasound is the first-line modality and will detect cysts and mass lesions. CT provides more complete characterization of lesion extent, bilaterality, and regional lymph node involvement. The imaging findings should be interpreted in the context of the biopsy result: multifocal bilateral renal cysts in a German Shepherd with confirmed nodular dermatofibrosis are not an incidental finding — they are part of the syndrome and warrant serial monitoring even in the absence of overtly malignant features.
Intact female German Shepherd Dogs with ND-RCAS commonly develop uterine leiomyomas as a third component of the syndrome. In practice, this is an uncommon clinical finding in the United States given that the majority of pet dogs are spayed. For the intact female presenting with nodular dermatofibrosis, however, reproductive status should be documented and abdominal imaging should include evaluation of the uterus alongside renal assessment.
Prognosis and the role of the pathology report
The prognosis for dogs with ND-RCAS is largely determined by the renal disease. Dogs with cutaneous nodular dermatofibrosis and no renal involvement or early cystic disease may have a prolonged clinical course with appropriate monitoring. Dogs with bilateral renal cystadenocarcinoma typically develop progressive renal insufficiency, and the timeline from diagnosis to end-stage renal disease is variable but often measured in months to a few years. There is no curative treatment for bilateral renal cystadenocarcinoma; management is supportive and aimed at maintaining quality of life and renal function for as long as possible.
The pathology report for a dermatofibrosis nodule is the hinge point in this diagnostic sequence. A report that describes the histologic findings, renders the correct diagnosis, and explicitly flags the renal association gives the clinician a clear and actionable next step. A report that describes "dense fibrocollagenous tissue, benign" without contextual comment leaves the clinician without the information needed to act — and the renal tumor to develop undetected.
This is one of the clearest examples in veterinary dermatopathology of a skin biopsy that has consequences beyond the skin. The cutaneous lesion is benign. The diagnosis it points to is not.
What to do when the biopsy comes back
If the histopathology report confirms nodular dermatofibrosis in a German Shepherd Dog, the next steps are straightforward: renal imaging, uterine evaluation in intact females only, and a conversation with the owner about the hereditary nature of the syndrome and its implications for related dogs. A genetic test for the FLCN mutation is available through select veterinary genetics laboratories and can be used to screen relatives of affected dogs before clinical lesions develop.
Serial monitoring — abdominal ultrasound every six to twelve months in dogs with confirmed syndrome and early renal changes — allows detection of progression before the animal becomes azotemic. Baseline bloodwork including renal values and urinalysis should be obtained at the time of diagnosis and repeated at each monitoring interval. The goal is not to cure the underlying genetic defect but to identify renal disease at a stage where quality of life can be maintained for as long as possible.
Eric Snook, DVM, PhD, DACVP — Vetopathy. Dermatopathology submissions evaluated with breed-specific context and systemic syndrome flags included as standard practice.