Electronic Brachytherapy

We are billing 17999 (Electronic Brachytherapy on skin cancer) with correct destruction series code in box 19 on HCFA 1500 as requested by Medicare and modifier is being required for subsequent fractions billed on different dates of service, is it 58 or 76 or both?

Also, when 2 treatments are billed on same day with two different series code how should the two separate 17999 be appended with a modifier?

Electronic Brachytherapy