A liquid ion chamber for clinical stereotactic output measurements
J. Mathews,1,2 P. Stevens,1,2 M. Zaini1,2
1Northwest Medical Physics Center, Lynnwood, WA; 2Skagit Valley Regional Health Center, Mount Vernon, WA
ASTRO Annual Scientific Meeting, San Francisco, CA (2014).
Purpose/Objectives: To measure output factors produced by the liquid-filled ion chamber at different field sizes and use the data to evaluate the liquid-filled ion chamber’s performance in a clinical setting compared to its gas-filled counterparts.
Materials/Methods: The setup of this experiment closely followed the method used to obtain the clinical data. The liquid-filled ion chamber’s cavity was positioned at isocenter (SSD = 90cm and d = 10cm) in a water tank on an Elekta Axesse medical linear accelerator. An HV power supply was set to –500V and connected to the liquid-filled ion chambervia an electrometer.The liquid-filled ion chamber had a nominal bias voltage of +/-800V (with +/-400V being the absolute minimum), but, due to equipment and budget constraints, only a -500 V bias could be applied. The ensuing collection inefficiency was negligible for the relative measurements of this study.
The liquid-filled ion chamber was pre-irradiated. Various rectangular fields were made by the MLC. This experiment was repeated at different timesin order to ensure accuracy and repeatability.The raw liquid ion chamber data were corrected by the published techniques for these types of measurements as well as the Daisy Chain method.
Results: Reproducibility of the measured results with this liquid ion chamber was better than 1%. There was a statistically significant difference between the clinical stereotactic output values and those of this liquid-filled ion chamber data. The clinical data were collected using four different chambers and diodes in multiple sessions by various physicists. These clinical output factors were also congruent with the published results for this linac type. The differences between the small stereotactic output factors for three photon beams of 6, 10, and 15 MV was determined using the liquid ion chamber versus those of the clinical values ranged between 0 and 7% for rectangular field sizes of 2 X 2 cm down to 0.8 X 0.8 cm. The largest discrepancies were observed for the smallest field sizes.
Conclusions: Although the liquid-filled ion chamber provided accurate output factors for large fields when compared to clinical data, it failed to do so for small fields (1.2×1.2cm and smaller). The inaccuracies of the stereotactic output factors measured with the liquid-filled ion chamberdid not seem to warrant its advantages of high signal and small collection volume.