Leiomyosarcoma of the Prostate after Prior Adenocarcinoma: A Rare and Aggressive Malignancy on FDG PET/CT

INTRODUCTION

Sarcomas of the prostate are exceedingly rare neoplasms arising from the non-epithelial mesenchymal components of the prostate and account for less than 0.1% of all primary prostatic malignancies.^[1–3]

Leiomyosarcoma is the most frequently encountered histological subtype. It may occur with or without a history of previously treated prostate adenocarcinoma and can affect both paediatric and adult male patients. This report presents fluorodeoxyglucose (FDG) positron emission tomography computed tomography (PET/CT) findings of a rare case of prostatic leiomyosarcoma in a male patient who had been treated for prostate adenocarcinoma several years earlier.

CASE REPORT

An 81-year-old male patient, who was diagnosed with prostate adenocarcinoma 13 years ago and had undergone palliative transurethral resections of the prostate (TUR-P) and radiotherapy, was hospitalised due to urgency and macroscopic haematuria. A repeat TUR-P was performed, and histopathological examination revealed a high-grade pleomorphic leiomyosarcoma. Immunohistochemical staining was positive for vimentin, calponin, smooth muscle actin (SMA), and desmin, while negative for prostate specific antigen (PSA), S100, CD34, CD117, and progesterone receptor. The patient's PSA level was 0.01 μg/L. FDG PET/CT imaging revealed an intensely hypermetabolic mass (arrow) with standardised uptake value (SUV) max: 15.2 originating from the prostate gland, extending toward the penile root and corpora, and infiltrating the bladder and rectum. The lesion demonstrated cystic and necrotic components. Additionally, mildly hypermetabolic inguinal lymph nodes (arrowhead) with SUVmax: 3.1, suspicious for malignancy, and left hydroureteronephrosis were identified [Fig 1]. No additional pathological lesions were identified elsewhere in the body.

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Fig 1: (A) Anterior and (B) sagittal maximum intensity projection, (C and D) axial, and (E) sagittal fused fluorodeoxyglucose positron emission tomography/computed tomography and computed tomography images demonstrate an intensely hypermetabolic prostatic mass (thin arrow in A-E) with a maximum standardised uptake value of 15.2, extending toward the penile corpora with cystic and necrotic components. Additionally, mildly hypermetabolic inguinal lymph nodes (arrowhead in C), suspicious for malignancy, with a maximum standardised uptake value of 3.1, and left hydroureteronephrosis (thick arrow in A and B) are also observed.

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DISCUSSION

Prostatic leiomyosarcoma is a rare and aggressive malignant neoplasm derived from the smooth muscle cells of the prostate.^[3] It accounts for approximately 38%–52% of all primary sarcomas of the prostate^.[4]

Typically, affected individuals are between 41 and 78 years of age, with a mean age of 61 years.^[2,5] Clinical presentation often includes symptoms of lower urinary tract obstruction. Other symptoms may include perineal or rectal pain, haematuria, painful ejaculation, difficulty with defecation, and systemic signs such as weight loss.^[6] Diagnosis is frequently delayed owing to nonspecific symptoms, and approximately 25% of cases present with metastatic disease at the time of initial evaluation. The most common site of distant metastasis is the lungs, followed by the liver and bones.^[4] The tumour spreads through both lymphatic and hematogenous routes, contributing to extensive regional and systemic dissemination.^[7] Serum PSA levels typically remain within normal limits, as the tumour originates from non-epithelial tissues. Histologically, most cases of prostatic leiomyosarcoma are high-grade neoplasms characterised by necrosis, cystic changes, marked nuclear atypia, and high mitotic activity.^[8] Low-grade variants are rare and typically exhibit moderate atypia, fewer mitoses, and a focal infiltrative pattern around benign prostatic tissue. Immunohistochemically, tumour cells often express markers such as vimentin, SMA, desmin, and occasionally cytokeratin or progesterone receptors. Markers such as PSA, S-100, CD117, and CD34 are typically negative.^[9] Compared to prostatic adenocarcinoma, leiomyosarcoma carries a significantly poorer prognosis, with median survival reported between 15 and 18 months. Prognostic factors include tumour size, histological grade, complete surgical resection, and the absence of metastasis at diagnosis. Complete surgical excision remains key to improved outcomes, often requiring a multimodal treatment approach.^[10] Although Ga-68 prostate-specific membrane antigen (PSMA) PET/CT is well established for imaging prostate adenocarcinoma, this report describes the first documented case of FDG PET/CT findings in prostate leiomyosarcoma.

CONCLUSION

In conclusion, careful vigilance is warranted for the possibility of rare and aggressive tumour transformations during prostate cancer follow-up. In such scenarios, FDG PET/CT plays a key role in accurate disease assessment and management.