What kind of breast lump (found by gp along with swollen lymph node) would not show up on us or mammo? scans showed i have very dense breast tissue.

Dense Breasts: Or cystic breasts are more common in younger women, or women under 40. Therefore it is also common to feel lumps in this type of breast tissue. Diagnostic mammograms usually give a clearer picture of what is going on, but if you are still concerned, consider a MRI of the breast, or a biopsy of the palpated lesion.

Neither: mammography nor ultrasound are 100% accurate for detecting cancer. In particular, the sensitivity of mammo can drop to lower than 50% in dense breasts. A small percentage of cancers are found only on physical exam, and not seen on mammo or US. However, the chance of a lump being cancer in the setting of a negative mammo and ultrasound is low, approx. 0.5%-3%.