Part V. Conclusions, recommendations and lessons to be learned
3. THE ACCIDENTAL OVEREXPOSURE
3.1. DESCRIPTION OF THE EVENT
On 27 February 2001, a malfunction of the NEPTUN 10P® linear accelerator occurred following a sudden power cut. The machine was used for
radiation therapy at the BOC. Five patients, all of whom had been undergoing radiation treatment with an electron beam to the surgical wound following surgery for breast cancer, were overexposed. The prescribed doses were 2–2.5 Gy per fraction with an 8 MeV electron beam. The machine shut down during a mains power cut while a patient (subsequently designated as Patient 1) was on the treatment couch and had received only 5 MU of the prescribed 155 MU, which corresponded to 0.08 Gy of the prescribed 2.5 Gy.
The radiation technologist operating the machine contacted the chief physicist. The physicist restarted the machine, allowing the required 5 minute minimum warm-up time. He checked the machine controls, including the voltage levels of the power supply panel, and found that they were functioning as expected. The user’s manual for the NEPTUN 10P® requires that detailed machine checks, including output measurements, be performed after emergency shut-offs of an accelerator caused by malfunction of any of the machine systems, but shut-off due to a power cut was not included in the list of emergency shut-offs in this manual. Machine shut-offs due to power cuts had happened many times in the past. The AC mains voltage in the hospital area was quite unstable and as many as two power cuts a day had occurred occasionally. The experience of the radiation technologist with previous power cuts indicated that, after resuming operation, the machine performed normally, i.e. without any change in its beam parameters. After the warm-up had been completed, the patient’s treatment continued with the remaining 150 MU. The analogue dose rate indicator on the machine console fluctuated around 100 MU/min instead of showing the usual 290–300 MU/min. The MU counters worked slowly, which corresponded to the low dose rate indication at the machine console. The physicist adjusted the secondary timer on the console from 1.3 min to 1.5 min to allow completion of the treatment (150 MU/
(100 MU/min) = 1.5 min).
Towards the end of the treatment of Patient 1, the radiation technologist noted a minor asymmetry of the radiation field indicated on the NEPTUN 10P® console and adjusted it. When the next patient (Patient 2) was being treated, the physicist was summoned to the mould room because of an emergency.
The next three patients treated (Patients 3, 4 and 5) reported an abnormal skin reaction. Patient 4 mentioned an unusual feeling during treatment and Patient 3 returned after treatment to complain of an itching and a burning sensation in the area of the irradiated field. After Patient 5 was treated, the radiation oncologist on duty examined all three patients. She noted a post-irradiation reaction for Patient 5 which could not be attributed to a dose of just a few grays that the patient ought to have received in the course of treatment.
The physicist discussed the problem with the chief radiation oncologist and further treatments with this machine were stopped.
3.2. THE DISCOVERY OF THE PROBLEM
As the machine seemed to be functioning abnormally, the physics team made dose output measurements of those radiation beams of the accelerator that were clinically used3, i.e. the 8 and 10 MeV electrons and 9 MV photons.
Initially, an old Siemens dosimeter used for regular quality control measure-ments was employed for output measuremeasure-ments. Its readings were off the scale for 300 MU, which is the usual number of MUs preset for output measurements in the water phantom under reference conditions. The local dosimetry standard, comprising an NE2581 ionization chamber and a 2500/3A IONEX® electrometer, was subsequently used. Again, the first readings for 300 MU were off the scale. On the low MU settings (100 MU for photons, 25 MU for 8 MeV, 50 MU for 10 MeV electron beams), the dose rate, without corrections applied for lower ion collection efficiency and non-linearity of monitor unit counter, was 37 times higher than the normal dose rate for an 8 MeV electron beam, 17 times higher for 10 MeV electrons and 3.5 times higher for 9 MV X rays. The individual measurements fluctuated within approximately 10% for each beam.
The physicists noticed an increase in the current measured in the electron gun filament on the analogue display of the accelerator cabinet: 1.46 A instead of 1.20 A for 8 MeV electrons, 1.20 A instead of 1.00 A for 10 MeV electrons and 1.60 A instead of 1.50 A for the 9 MV photon beam. The chief physicist informed the manufacturer’s service unit of the measured doses and requested assistance.
On the next day, 28 February, the local engineer examined the machine and discovered a broken fuse connecting the AC voltage supply to the DC power supply panel of the dose monitoring system of the accelerator and also a break in the D29 diode in a machine safety interlock system. After replacement of the faulty parts, the machine appeared to be ‘back to normal’. The physicists performed the basic set of measurements of the characteristics of the photon and electron beams, such as percentage depth doses, beam profiles and dose outputs. The results for all beams were close to normal, although the energy of the 8 MeV electron beam was slightly lower.
3A 6 MeV electron beam generally available with NEPTUN machines was not used clinically at the BOC.
The chief physicist reported the abnormal machine output measured on the day of the accident (i.e. 27 February) to the director of the hospital and initiated the procedure for informing the emergency response unit of the regulatory body’s Office for the Control of Radiation Sources (Applications Department).
According to the information provided to the IAEA team, the reliability of the dose measurements performed on 27 February in Białystok was questioned by the manufacturer which stated that the construction of the NEPTUN 10P® accelerator prevented the machine from generating a dose rate as high as that measured on the day of the accident.