It seems that you may be having vitiligo
And you had vitiligo patch on neck which was responded to placentax.
If it is not available, you may use tacrolimus oint. 0.1% twice a day. Rub it on the patch.
Along with it, you may take tab. Neopsoralen 25 mg in the morning daily. And expose the patch to sun at noon for 5 min.
50 mg at night.
I think, you may get good result. However, you may consult dermatologist
I hope you got my answer.
Dr. Ilyas Patel MD