How likely is it that a "1.6 x 1.6 x 1.6 cm irregular hypoechoic nodule" in breast is cancer? ultrasound says it has spiculation and microobulation.

Breast biopsy/gold: Breast tissue biopsy by fine needle or excision is the best way to know what is cancer and what is not and is the "gold standard" for diagnosis. Unless the hypoechoic nodule turns out to be a cyst, then physicians prefer not to speculate on ultrasound findings about cancer presence.

Radiologists: quantify the probability of malignancy based on various mammo/US characteristics(BIRADS classification). This determines subsequent management. Irregular shape and spiculation/microlobulation are suspicious features. Your lesion is at least a BIRADS 4(suspicious), and probably BIRADS 4b, which implies a 10-50% chance of malignancy.US guided core biopsy would be the next step to see waht it is.