Recall of C-Series High Energy Linear Accelerator Models: Novalis Tx, Trilogy, Trilogy Tx, Clinac iX, Clinac CX, Clinac 2100 c/D, Clinac 2300C/D, Clinac DX, Clinac 21 EX, Clinac 23 EX

Recall

RC-2014-RN-00700-1

Class I

2014-07-02

Australia

DHTGA

https://apps.tga.gov.au/Prod/sara/arn-detail.aspx?k=RC-2014-RN-00700-1

Varian has seen a trend in reports of unexpected decrease in beam output in c-series high energy linacs for 6mv photon treatment mode. varian has determined the cause of the unexpected variations in beam output to be degradation of the 6mv target. specifically, the effects of modern, highly modulated treatment modes can create high levels and frequency of stress cycles in the targets particularly if the beam spot size is small. this can lead to the targets' deterioration and failure at an accelerated rate resulting in a rapid change in the beam output and symmetry. specifically:1. the photon generation, or bremsstrahlung yield, decreases as fewer electrons are converted to photons in the target, and;2. due to a resulting escape of primary electrons the output of photons, as measured by the ion chamber, might appear to be constant, but the actual photon output is decreasing.This failure mode in the target only affects the 6 mv photon treatment modes (6srs, 6fff and 6x).

Varian strongly recommends that all sites implement daily output constancy checks of photon beams as recommended by the AAPM. Specifically those provided by: 1. AAPM, Task Group 142 Report: Quality Assurance of Medical Accelerators, Medical Physics publication 36 (9), September 2009, and 2. AAPM Report No. 46, Comprehensive QA for Radiation. It is particularly important that these daily output constancy checks include all 6MV beams [6SRS, 6FFF and 6X]. Sites should particularly check for any sudden decrease in dose output = 3% per day, or =6% per week. If any sudden decrease in dose output is observed, cease use of all 6MV beams and contact Varian immediately. A Varian service representative will visit the site and investigate whether the target is degrading, or has failed. A technical fix is planned for implementation in a future Clinac system control software release.