Ultrasound is the first-line imaging modality for evaluating a breast lump in women under 30 because it effectively differentiates between cystic and solid masses without radiation exposure, which is crucial in younger patients with denser breast tissue. Ultrasound provides real-time imaging, is safe, accessible, and cost-effective, and allows for guided biopsies if needed. Mammography is less sensitive in younger women due to dense breast tissue and involves ionizing radiation, which is best avoided unless necessary. CT and MRI are not first-line due to cost, availability, and specificity for initial evaluation.

Reference: Breast Imaging, D'Orsi et al., 5th Edition.