Summary: The licensee notified NRC that he/she identified two potential misadministration involving two patients that were treated with I-125 temporary brachytherapy eye implants. Both patients were prescribed a dose of 7000 cGy and their treatments started on September 15, 2000. On September 20, 2000, the eye plaques were removed at the conclusion of the treatment. The licensee reported that due to a calculation error, both patients received higher than the planned dose of 7000 cGy. The error resulted in an administered dose of 11,470 cGy to one patient and 10,866 cGy to the other patient.
Details: The error was made in converting the Air-Kerma strength of the I-125 sources to millicuries. The licensee’s treatment planning system is in Air-Kerma units, whereas the vendor requires the order to be placed in millicuries. The licensee stated that to convert I-125 source strength from Air-Kerma to millicuries, Air-Kerma value is divided by a conversion factor of 1.27. However, the dosimetrist erroneously multiplied the conversion factor to obtain the source strength and that resulted in the two misadministration. The error was identified on September 22, when the physicist recognized that the dosimetrist was ordering seeds for an upcoming case with an activity that was higher than expected.
Results: The conversion error was recognized, and all I-125 cases were reviewed. The review process identified the two misadministration and three recordable events. Specifically, the inspector noted that the licensee identified three recordable events involving the eye plaque treatments that were performed in April 2000. The recordable events occurred when the patients were planned to be treated for five days (120 hours), however, the implants were removed after four days of treatment. The error that Monday-Friday treatment would be a four-day treatment (implants started during the day on Monday, April 24, 2000 and explanted during the day on Friday, April 28, 2000) instead of the required 5-days (120 hours) treatment was not identified by the authorized physician or the support staff (physicist and dosimetrist) until September. A QMP audit in September 2000, identified the error and the consequent under-dosing of the patients by 12-15% of the prescribed dose.
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