Iams is a good dry food, but your little Boston may have developed a need for extra omega-3 to help her skin be as healthy as possible. Your vet should actually have this on his shelf, but human omega-3s are fine too. She should probably get 1000mg daily.
She also may have an increased need for vitamin A and zinc. These should both be in the pet vitamins that you vet will have for her. The skin is an organ, and as we get older our organs need some extra help.
Before assuming that she has a nutritional disorder, I would have your vet check a blood profile to include a thyroid test. It is quite common for older dogs to have decreased thyroid activity and respond well to thyroid supplementation. If we don't determine this and it is ultimately the problem, then all of your other efforts will be for naught.
Also, at your vet appointment, I'd ask to have your Boston checked for ringworm. The scanning test is easily done with a black light. If this is negative, and your vet thinks this could still be her problem, then he could culture her for ringworm also. After this, your vet can do a skin scraping to look for demodectic mange
. This isn't an itchy disease, and it is also a skin disease that affects dogs with a decreased immune system (as does ringworm generally). Just by the fact that your little Boston is elderly, her immune system is less than when she was younger. Not saying that these are her problems, but it is very possible for only her to be affected by these two skin diseases.
There is an oral flea and tick medication that the veterinary dermatologists recommend and that is Comfortis. Your vet would be able to get this for you. Also, there are several different topicals available, so you may want to try a different topical. Since she isn't itchy though, her primary problem isn't flea allergy.
Has she had this dermatitis and alopecia for very long?