Hard lump

  I am 25. Had undescened testicle repaired as infant. I recently found a small hard lump at tip of left testicle. It is near teste and epiditimys. Can't tell if it is connected to either. No pain. I had it checked, but doctor couldn't find it. It is in delicate area so it is hard to get a grip on. I had an ultra sound that showed nothing, radiologist said if it was attached to testi, it would show so even if it was missed in u.s, it wouldn't be cancerous. I KNOW it is there, size of a bb. Is this harmless or could it be cancer missed by ultra sound? I have appointment with urologist in two weeks. Can ultra sound miss a tumor??
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Thanks Scott for your concerning question.
The ultrasound very sensitive at finding testicular masses in the testicle especially if they are palpable.  Is it possible the ultrasound could miss a lesion the size of a