A former chief of pediatrics at Fort Belvoir was sentenced yesterday to four years in prison for sexually assaulting a 15-year-old male patient in 1983.

Former Lt. Col. Arthur C. Andreasen, a 40-year-old McLean resident, speaking publicly for the first time since his arrest in December, said in Alexandria Circuit Court yesterday that he regretted his actions. Sitting in the courtroom were members of his family and the youth he was convicted of assaulting.

"I knew there was something wrong since seventh grade," Andreasen said in a soft voice, asserting that he had tried to control his fantasies about young boys and is seeing a psychiatrist. He did not want to hurt anyone, Andreasen said, adding, "I'm really sorry."

Assistant Commonwealth's Attorney Mark R. Voss told the court that the 1983 incident was not isolated. "There were 24 or 25 victims that we know of," he said, most of them 13, 14 or 15 years old. All suffered psychological problems as a result, he said.

"We gave him access to our children and he used them," Voss said after the sentence was pronounced.

Andreasen's attorney, James M. Lowe, said prosecutors exaggerated allegations against his client.

"That's really not accurate," Lowe said of Voss' accusations. Andreasen "did make contact with a number of patients," the defense attorney said, but that "contact" was not as serious as the assault that led to Andreasen's arrest.

Andreasen, who is no longer licensed to practice medicine, pleaded guilty in March to sexually assaulting the 15-year-old during a physical examination at a private health clinic in Alexandria. At that time, he had just resigned from the Army and joined the Kaiser Permanente health organization.

Lowe said the prison term devastated Andreasen's wife and two daughters, who accompanied Andreasen to court. The attorney argued before Judge Alfred D. Swersky that his client should be placed on parole instead of in prison. "There is no question it happened. It is a question of what to do now . . . . The man was in the process of curing himself. That the 1983 assault was the final incident."

Federal officials are investigating allegations that Andreasen abused other boys while working at Fort Belvoir from 1980 to 1983. According to information obtained in a Defense Department investigation, the Army had enough evidence in 1983 to court-martial Andreasen but instead allowed him to resign.