Johns Hopkins doctors said yesterday cigarette smokers over 40 should still get an annual chest X-ray "as their only chance" of surviving if they are among the many smokers who develop lung cancer.

The American Cancer Society said this week that Americans in general need not have annual physicals to detect cancer until after age 40, and that no sympton-free persons -- including smokers -- will benefit from an annual chest X-ray.

The society's lung cancer recommendation was based on experts' assessments of both the numbers of cancers found by X-rays and the low number of patients ultimately cured.

The dissenting Johns Hopkins doctors began screening 10,000 male smokers over 45 in 1974 in a long-term project to find the best way to detect early, hopefully curable lung cancer.

Some of the smokers get a sputum test, then a chest X-ray. Others get just a chest X-ray. Some early cancers have been found by the sputum test, said Dr. R. Robinson Baker. But, "surprisingly," even more have been found by X-ray alone, he said.

Twice as many of the study patients have had "operable cancers" -- cancers that could be attacked by surgery -- as did patients found in other John Hopkins clinics.

In men whose lung cancers were found at their first test, there are so far "30-some survivors," said Baker, "for a three-year survival rate of 75 percent, and we believe their five-year survival rate will be about 60 percent," compared with 30 percent in other men with operable cancers.

The Baltimore doctors' conclusion: "While at present there is not significant improvement in overall . . . mortality . . . the period of observation has been brief . . .We believe the ACS conclusion that screening for lung cancer lacks value was premature and lacked adequate scientific grounds."