[4068944]
Problem with iba proton radiation therapy equipment. Range modulator slide draw in universal nozzle did not return to proper position for uniform scanning treatment after being retracted for pencil beam scanning mode. Slide drawer is inserted via pneumatic pressure until it reaches a mechanical stop. The mechanical stop allowed the drawer to be inserted 2mm more than i should have been due to wear or mechanical failure. An interlock micro-switch that detects the insertion of the slide drawer is mounted on the mechanical stop, so when the mechanical stop moved, the micro-switch moved with it and thereby failed to detect any error. The mis-positioning of the slide drawer resulted in improper modulation of the proton beam and radiation doses delivered to patients were different than intended. Because the iba system failed to produce any interlock, this situation was ongoing for an undetermined length of time affecting an unknown number of patients. The problem was eventually detected by making routine qa measurements of the beam properties by medical physics staff. Once the problem was detected, treatments were halted for 2 days (which also has a negative impact on patient outcomes) while repairs were made and tests were performed to verify the proper functioning of the system. No modifications have been made which will prevent such an error in the future. The staff physicists first discovered a problem when making depth-dose measurements with a multi-layer ion chamber on (b)(6) 2014. The measurements showed the spread-out bragg peak depth dose did not match the baseline data, differing by about 5% over the last several cm of range. The cause of this dose discrepancy was not understood at the time. To verify the measurement the same detection system was used in a second treatment room on (b)(6). Those measurements did not show the same discrepancy, which indicated there was no problem with the measuring device. Additionally, measurements were performed in the first treatment room with another system, a single parallel-plate ionization chamber in a water phantom. Those measurements confirmed the first measurements also indicating a dose discrepancy of about 5%. At this point, iba engineering staff determined the source of the problem was the positioning of the range modulator draw was in too far. This was determined by visual inspection, where it was seen that the mechanical stop had been displaced. Additional measurements were made of other spread-out bragg peaks to determine the extent of the dosimetric effect for the patients currently on treatment. After repairs were made, more measurements were made to confirm that the system again matched our baseline data. This is a radiation therapy producing machine that has been in continuous use for multiple patients since (b)(6) of 2013. It is made by iba and is their "universal nozzle" that failed. It did not produce any warning when it failed and so we do not know which patients were affected. Dates of use: (b)(6) 2013 - (b)(6) 2014. Diagnosis or reason for use: cancer.
Patient Sequence No: 1, Text Type: D, B5