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On Wednesday May 18th, the licensee's radiation safety officer contacted the agency (Illinois Emergency Management Agency) to advise that a medical event had likely occurred the prior Friday. Based on their initial evaluation, 1 of the 10 fractions prescribed for a patient was not delivered as intended such that less than 50% of the expected dose was delivered during a fraction. The written directive indicates that the patient was scheduled to undergo 10 treatments to her breast twice a day over a period of 5 days. Each treatment involved 340 centiGy for a total written directive prescription of 3,400 centiGy. During the 6th fraction after completion of treatments in channels 1 and 2, the unit experienced a power supply error. Sensing the power variation, the device automatically withdrew the source and did not complete the treatment for channel 3. Although additional attempts were made to clear the error and continue the treatment, none of the remaining 4 channels could be finished. The licensee had experienced similar problems with the device the previous week which had resulted in a service call on May 10. At that time the power supply was adjusted to ensure operation within specifications. The unit operated normally following the repair until the event on the 13th. The power supply as well as main controller board was subsequently replaced by the manufacturer Saturday morning and the patient returned that afternoon to complete the remaining portion of the 6th fraction. All the remaining fractions were delivered without incident. The fraction which finished the overall treatment was completed Tuesday. There have been no errors or incidents noted since the replacement of the above components. The licensee is still investigating this incident with the manufacturer to conduct an engineering evaluation of the power supply failure, however the 'fail safe' design of the system operated as expected. The licensee's initial estimate was that the patient received 133 centiGy of the 340 centiGy intended fractional dose. The patient and the referring physician were advised of the event but since the remaining fractions of the treatment were completed under the revised treatment plan, no adverse affect on the patient is expected and no other additional action is anticipated. The intended dose for this fraction was 3.4 Gy. The dose received was 1.3 Gy. Illinois Report Number: 11058 A Medical Event may indicate potential problems in a medical facility's use of radioactive materials. It does not necessarily result in harm to the patient.

• * * RETRACTION FROM DAREN PERRERO TO VINCE KLCO ON 5/24/11 @ 0902 EDT * * *

The following information was received via email: On May 23, 2011 the licensee's medical physicist provided information to the agency (Illinois Emergency Management Agency) which demonstrated that the target volume involved in the interrupted fraction received a dose of 217 cGy of the prescribed 340 cGy rather than the conservatively estimated 130 cGy. This dose to the patient treatment volume represents 63% of the prescribed dose and therefore does NOT constitute a medical event as identified in regulations. Notified R3DO (Cameron) and FSME EO (McIntosh)