Therac-25

A Therac-25 had been in operation for six months in Marietta, Georgia at the Kennestone Regional Oncology Center when, on June 3, 1985, applied radiation therapy treatment following a lumpectomy was being performed on a 61-year-old woman. The patient was set to receive a 10-MeV dose of electron therapy to her clavicle. When therapy began, she stated she experienced a "tremendous force of heat...this red-hot sensation." The technician entered the room, to whom the patient stated, "you burned me." The technician assured her this was not possible. The patient returned home where, in the following days, she experienced reddening of the treatment area. Shortly after, her shoulder became locked in place and she experienced spasms. Within two weeks, the aforementioned redness spread from her chest to her back, indicating that the source of the burn had passed through her, which is the case with radiation burns. The staff at the treatment center did not believe it was possible for the Therac-25 to cause such an injury, and it was treated as a symptom of her cancer. Later, the hospital physicist consulted the AECL about the incident. He calculated that the applied dose was between 15,000 and 20,000 rad (radiation absorbed dose) when she should have been dosed with 200 rad. A dose of 1000 rad can be fatal. In October 1985, the patient sued the hospital and the manufacturer of the machine. In November 1985, the AECL was notified of the lawsuit. It was not until March 1986, after another incident involving the Therac-25, that the AECL informed the FDA that it had received a complaint from the patient.

Due to the radiation overdose, her breast had to be surgically removed, an arm and shoulder were immobilized, and she was in constant pain. The treatment printout function was not activated at the time of treatment and there was no record of the applied radiation data. An out-of-court settlement was reached to resolve the lawsuit.[2]

Terminal DEC VT-100.

The Therac-25 had been in operation in the clinic for six months when, on July 26, 1985 a 40-year-old patient was receiving her 24th treatment for cervical cancer. The operator activated the treatment, but after five seconds the machine stopped with the error message "H-tilt", the treatment pause indication and the dosimeter indicating that no radiation had been applied. The operator pressed the P key (Proceed : continue). The machine stopped again. The operator repeated the process five times until the machine stopped the treatment. A technician was called and found no problem. The machine treated six other patients on the same day.

The patient complained of burning and swelling in the area and was hospitalized on July 30. She was suspected of a radiation overdose and the machine was taken out of service. On November 3, 1985 the patient herself died of cancer, although the autopsy mentioned that if she had not died then, she would have had to have a hip replacement due to damage from the radiation overdose. A technician estimated that she received between 13,000 and 17,000 rad.

The incident was reported to the FDA and the Canadian Radiation Protection Bureau.

The AECL suspected that there might be an error with three microswitches that reported the position of the turntable. The AECL was unable to replicate a failure of the microswitches and microswitch testing was inconclusive. They then changed the method to be tolerant of one failure and modified the software to check if the turntable was moving or in the treatment position.

Afterward, the AECL claimed that the modifications represented a five-order-of-magnitude increase in safety.[2]

In December 1985 a woman developed an erythema with a parallel band pattern after receiving treatment from a Therac-25 unit. Hospital staff sent a letter on January 31, 1986 to the AECL about the incident. The AECL responded in two pages detailing the reasons why radiation overdose was impossible on the Therac-25, stating both machine failure and operator error were not possible.

Six months later, the patient developed chronic ulcers under the skin due to tissue necrosis. She had surgery and skin grafts were placed. The patient continued to live with minor sequelae. [2]

Over two years, this hospital treated more than 500 patients with the Therac-25 with no incident. On March 21, 1986, a patient presented for his ninth treatment session for a tumor on his back. The treatment was set to be 22-MeV of electrons with a dose of 180 rad in an area of 10x17 cm, with an accumulated radiation in 6 weeks of 6000 rad.

The experienced operator entered the session data and realized that she had written an X instead of an E as the type of treatment. With the cursor she went up and changed the X to an E and since the rest of the parameters were correct she pressed ↵ Enter until she got down to the command box. All parameters were marked "Verified" and the message "Rays ready" was displayed. She hit the B key ("Beam on"). The machine stopped and displayed the message "Malfunction 54" (error 54). It also showed 'Treatment pause'. The manual said that the "Malfunction 54" message was a "dose input 2" error. A technician later testified that "dose input 2" meant that the radiation delivered was either too high or too low.

The radiation monitor (dosimeter) marked 6 units supplied when it had demanded 202 units. The operator pressed P ( Proceed : continue). The machine stopped again with the message "Malfunction 54" (error 54) and the dosimeter indicated that it had delivered fewer units than required. The surveillance camera in the radiation room was offline and the intercom had been broken that day.

With the first dose the patient felt an electric shock and heard a crackle from the machine. Since it was his ninth session, he realized that it was not normal. He started to get up from the table to ask for help. At that moment the operator pressed P to continue the treatment. The patient felt a shock of electricity through his arm, as if his hand was torn off. He reached the door and began to bang on it until the operator opened it. A physician was immediately called to the scene, where they observed intense erythema in the area, suspecting that it had been a simple electric shock. He sent the patient home. The hospital physicist checked the machine and, because it was calibrated to the correct specification, it continued to treat patients throughout the day. The technicians were unaware that the patient had received a massive dose of radiation between 16,500 to 25,000 rads in less than a second over an area of one cm². The crackling of the machine had been produced by saturation of the ionization chambers, which had the consequence that they indicated that the applied radiation dose had been very low.

Over the following weeks the patient experienced paralysis of the left arm, nausea, vomiting, and ended up being hospitalized for radiation-induced myelitis of the spinal cord. His legs, mid-diaphragm and vocal cords ended up paralyzed. He also had recurrent herpes simplex skin infections. He died five months after the overdose.

From the day after the accident, AECL technicians checked the machine and were unable to replicate error 54. They checked the grounding of the machine to rule out electric shock as the cause. The machine was back in operation on April 7, 1986.[2]

On April 11, 1986, a patient was to receive electron treatment for skin cancer on the face. The prescription was 10 MeV for an area of 7x10 cm. The operator was the same as the one in the March incident, three weeks earlier. After filling in all the treatment data he realized that he had to change the mode from X to E. He did so and pressed ↵ Enter to go down to the command box. As "Beam ready" was displayed, he pressed P (Proceed : continue). The machine produced a loud noise, which was heard through the intercom. Error 54 was displayed. The operator entered the room and the patient described a burning sensation on his face. The patient died on May 1, 1986. The autopsy showed severe radiation damage to the right temporal lobe and brain stem.

The hospital physicist stopped the machine treatments and notified the AECL. After strenuous work, the physicist and operator were able to reproduce the error 54 message. They determined that speed in editing the data entry was a key factor in producing error 54. After much practice, he was able to reproduce the error 54 at will. The AECL stated they could not reproduce the error and they only got it after following the instructions of the physicist so that the data entry was very rapid.[2]

On January 17, 1987 a patient was to receive a treatment with two film-verification exposures of 4 and 3 rads, plus a 79-rad photon treatment for a total exposure of 86 rads. Film was placed under the patient and 4 rads were administered through a 22x18 cm opening. The machine was stopped, the aperture was opened to 35x35 cm and a dose of 3 rad was administered. The machine stopped. The operator entered the room to remove the film plates and adjust the patient's position. He used the hand control inside the room to adjust the turntable. He left the room, forgetting the film plates. In the control room, after seeing the "Beam ready" message, he pressed the B key to fire the beams. After 5 seconds the machine stopped and displayed a message that quickly disappeared. Since the machine was paused, the operator pressed P (Proceed : continue). The machine stopped, showing "Flatness" as the reason. The operator heard the patient on the intercom, but could not understand him, and entered the room. The patient had felt a severe burning sensation in his chest. The screen showed that he had only been given 7 rad. A few hours later, the patient showed burns on the skin in the area. Four days later the reddening of the area had a banded pattern similar to that produced in the incident the previous year, and for which they had not found the cause. The AECL began an investigation, but was unable to reproduce the event.

The hospital physicist conducted tests with film plates to see if he could recreate the incident. Two X-ray parameters with the turntable in field-light position. The film appeared to match the film that was left by mistake under the patient during the accident. It was found the patient was exposed to between 8,000 and 10,000 rad instead of the prescribed 86 rad. The patient died in April 1987 from complications due to radiation overdose. The relatives filed a lawsuit that ended with an out-of-court settlement.[2]