I've heard a lot of people say yes, but I seriously doubt it... thymomas are very prone to recurrence though. If Thymus cells are left behind from a thymectomy, and you have cancer cells (thymoma) it is quite likely that they will grow back.

The thymus actually degrades to just fatty tissue (normally) shortly after puberty. We "lucky" few keep the thymus around longer than it is needed. I don't think it is actual Thymus tissue regrowing.

Of course, I could be very wrong... this is just my belief based on my understanding of the Thymus, thymomas, etc.