What is the best diagnostic step after an initial ultrasound for a woman with a right breast mass that has remained unchanged through three menstrual cycles?

The best diagnostic step after the initial ultrasound for a woman with a right breast mass that has remained unchanged through three menstrual cycles is aspiration of the dominant mass. In cases where a breast mass is stable over multiple menstrual cycles, it generally suggests that the mass is likely benign. Aspiration allows for the evaluation of the mass without an invasive procedure and provides immediate information.

If the mass is a cyst, aspiration can relieve any discomfort and confirm the diagnosis by yielding fluid. In the case of a solid mass, the fluid aspiration can help differentiate between benign and malignant processes based on cytology. This approach is less invasive and can often be performed in an office setting, allowing for prompt management.

Mammography might be considered depending on the age of the patient and the specifics of the mass. However, after an initial ultrasound in this context, performing aspiration directly addresses the benign-appearing nature of the mass and assists in determining the next steps based on cytological findings.

Excisional biopsy, while definitive for diagnosis, is a more invasive and unnecessary step if the mass does not present concerning features, especially given its unchanged status over time. Similarly, repeat ultrasound may not provide new insights if the initial imaging was non-concerning and may delay appropriate management.