MarkOasis

To answer my own question. It seems that radiotherapy will chiefly impact ordinary white blood cells, whereas the T-cells and such-like, which are more important for keeping cancer in check, are not so affected. So, there is not an obvious scientific argument for radiotherapy being detrimental in this case. Although for my staging of resected testicular seminoma, we came to close monitoring as the preferred option. So considering the mm too, I am now monitoring two-for-the-price-of-one. 

Radiotherapy was offered as on option, but because the 80-85% of patients who would in-any-case not recur would receive unnecessary radiation, with some evidence that the treatment increases incidence of cardiovascular disease later in life, and with recurrent seminoma responding very well to subsequent treatment if necessary, the balance of the consensus is for watch-and-wait.

I was in two minds if to go for the radiotherapy for the seminoma. But the melanoma is still the most dangerous recurrence risk for me. So, conscious of the law of unintended consequences, and my current NED, I went with the "If it works don't fix it" approach.