I'm 67 yrs. old. recently had transvaginal us, showed fibroid, and thickening of endometrium 10mm. endometrial biopsy to be scheduled .i have been on tamoxifen for 4 months . what are my chances that this is benign, from a polyp or tamoxifen?

Tamoxifen is known to cause endometrial thickening though you should repeat the US in 3 months or so since this seemed to have developed so quickly. You may also want to discuss alternatives with your prescribing doctor. It is not wrong to do, but in the absence of vaginal bleeding the endometrial biopsy is unlikely to be helpful and it will certainly be painful.