What are the differential diagnoses for a female of reproductive age presenting with a lower abdominal mass initially thought to be a leiomyoma?

Differential Diagnoses for Lower Abdominal Mass in Reproductive-Age Women

When a lower abdominal mass initially appears to be a leiomyoma in a woman of reproductive age, the differential diagnosis must include both gynecologic and non-gynecologic etiologies, with careful attention to distinguishing benign from malignant processes through systematic imaging evaluation.

Ovarian and Adnexal Masses

Benign Ovarian Neoplasms

• Benign cystic teratoma (dermoid cyst) is the most common ovarian neoplasm in women of reproductive age and can be diagnosed by ultrasound showing echogenic attenuating components or small horizontal interfaces 1
• Fibrothecoma tumors can present as solid adnexal masses and are usually diagnosable on ultrasound 1
• Primary ovarian leiomyoma is rare but must be considered when a solid mass is detected in the ovary, as it can be misdiagnosed preoperatively as uterine myoma 2

Functional and Benign Cystic Lesions

• Hemorrhagic cysts demonstrate spiderweb-appearing or retracting clot with peripheral vascularity on ultrasound 1
• Endometriomas show low-level internal echoes, mural echogenic foci, or nonvascular solid attenuating components 1
• Peritoneal inclusion cysts take the shape of underlying pelvic spaces and are located adjacent to or surrounding a functioning ovary 1

Tubal Pathology

• Hydrosalpinx appears as a tubular cystic mass with or without folds on ultrasound 1

Uterine Pathology Beyond Typical Leiomyoma

Pedunculated and Atypical Leiomyomas

• Pedunculated or broad ligament fibroids are the most common solid adnexal masses in the nonacute setting and can be mistaken for ovarian masses; identification of normal ovaries displaced by the mass and blood supply from uterine vessels helps avoid this error 1
• Giant leiomyoma with massive cystic hydropic degeneration can mimic aggressive neoplasms clinically and radiologically, requiring careful histopathological evaluation 3
• Lipoleiomyoma with extensive fatty and cystic degeneration can simulate malignancy on imaging, particularly when pedunculated and giant-sized 4
• Myxoid leiomyoma represents a degenerative variant that can present with atypical imaging features 5

Cervical Masses

• Cervical leiomyomas constitute 1-2% of total leiomyoma cases and can present as large abdominal masses, with supravaginal type being most common 5

Malignant Uterine Pathology

• Leiomyosarcoma comprises 1.7% of women undergoing hysterectomy for presumed fibroids in their seventh decade but is extremely rare in premenopausal women (0.1% of all uterine tumors) 1, 6
• Risk increases significantly with postmenopausal status and rapid tumor growth 6
• MRI with diffusion-weighted imaging is the most effective preoperative imaging modality for distinguishing leiomyosarcoma from benign leiomyoma 7, 6, 8

Critical Diagnostic Approach

Initial Imaging Strategy

• Transvaginal ultrasound with color Doppler is the essential first-line examination, with 90-99% sensitivity and 98% specificity for detecting fibroids 8, 9
• Color Doppler helps differentiate true solid components from debris within cysts but cannot definitively distinguish benign from malignant lesions 1, 8

Advanced Imaging Indications

• Pelvic MRI with diffusion-weighted imaging should be performed when ultrasound findings are suspicious, inconclusive, or when malignancy cannot be excluded 7, 8
• MRI provides superior soft-tissue characterization and can visualize the endometrium even with coexisting leiomyomas 7
• Diffusion-weighted sequences improve sensitivity and specificity for detecting malignancy, with up to 100% sensitivity and specificity for leiomyosarcomas 8

Key Distinguishing Features on Ultrasound

IOTA Simple Rules for Risk Stratification:

• Benign features (B features): unilocular cyst, solid components <7 mm, acoustic shadows, smooth multilocular tumor, largest diameter <100 mm, no blood flow 1
• Malignant features (M features): irregular solid tumor, ascites, at least four papillary structures, irregular multilocular-solid tumor, largest diameter ≥100 mm, very strong flow 1
• These rules achieve 93% sensitivity and 81% specificity for predicting malignancy 1

Critical Pitfalls to Avoid

• Do not rely on ultrasound alone to exclude leiomyosarcoma, as no pelvic imaging technique can reliably differentiate between benign leiomyoma and leiomyosarcoma 9
• Avoid morcellation procedures when malignancy cannot be excluded, as tumor spillage dramatically worsens prognosis if leiomyosarcoma is present 6
• Carefully identify both ovaries during ultrasound examination to avoid misdiagnosing pedunculated fibroids as ovarian masses 1
• Consider age and menopausal status as critical risk factors; postmenopausal women with growing masses warrant heightened suspicion for malignancy 6
• Recognize that simple cysts have <0.4% malignancy risk in premenopausal women, establishing a benign process in 98.7% of cases 1