Radiation Systems Inc. has reported fiscal first-quarter profits of $470,000 (25 cents a share) compared with $180,000 (15 cents) for the same quarter last year. Sales for the three months ended Sept. 30 increased to $2.6 million from $1.8 million.

In other reports, Federal Realty Investment Trust reported third-quarter profits of $551,000 (29 cents a share) on revenues of $3.53 million, up sharply from profits of $212,000 (12 cents) and sales of $3.1 million for the same quarter last year.

For the first nine months, the Chevy Chase real-estate trust--which owns 19 shopping centers and four apartment developments--recorded a profit of $2.06 million ($1.04) on revenues of $10.59 million, up from 1980 nine-month profits of $1.01 million (65 cents) and sales of $8.07 million.

Washington Corp., a Chevy Chase-based real estate development compnay, reported third-quarter profits of $125,787 (5 cents a share) compared with $222,442 (10 cents) for the 1980 third quarter. For the first nine months, the company reported profits of $1.17 milllion (50 cents), up from $.104 million (46 cents) for the same period last year.