Should additional radiation be added to the total course in a HN patient who has an extended break during treatment?

Extending standard RT overall time, without chemotherapy, results in a "loss" of about 0.7 Gy/day for each day treatment is extended beyond standard (on the other hand, acceleration results in a "gain" of 0.7 Gy per each day). These data are based both on pre-clinical experiments and clinical experience such as the extent of benefit from acceleration demonstrated in the RTOG randomized study of accelerated vs standard RT (Fu et al). It is likely that concurrent chemo mitigates partly the effect of extending treatment time by blunting accelerated repopulation of tumor cells. Evidence for this are recent randomized studies showing no benefit for accelerated RT in HN ca when concurrent chemotherapy is delivered. These considerations are relevant for extending overall treatment time by 1-2 weeks (for example, assuming 0.7 Gy/day, you need to increase the total tumor dose by 7 Gy if there is a 10-day delay, and likely less if concurrent chemo is delivered). However, 2 month break is very long and does not fit into the experimental or clinical studies that established a potential loss of 0.7 Gy/day. I dont know how effective was the cisplatin alone over 2 months in blunting accelerated repopulation. The limiting factor in raising the total dose is obviously late effects, which theoretically are not affected by extending overall time (apart for consequential effects that relate to severe acute effects such as severe mucositis). This risk of late complications should determine how much more you can deliver. 76-80 Gy total dose seems to me reasonable. Assuming you continue with concurrent chemo, I don't think BID fractionation will be beneficial. PET scan now, 2 months after the partial RT, may show you the extent of the metabolically active tumor and lead you to decide how intensive further treatment should be .