Eeva Boman
Blood & Cancer Centre, Wellington Hospital
Senior Medical Physicist

Dean Paterson
radiation therapist
Blood & Cancer Centre, Wellington Hospital

Lynne Greig
Head physicist
Blood & Cancer Centre, Wellington Hospital

Background and Purpose The timer accuracy of the VariSource iX HDR brachytherapy afterloader (Varian Medical Systems, CA) is stated to be better than 1% or 0.1 second (s) whichever is greater. However, a conservative approach is applied in our centre so that dwell times <1s are manually edited to either 0 or 1s. With small doses per fraction, such as in interstitial breast treatments, and especially with a new source, this minimum dwell time restriction can be a limiting factor in achieving a good dose distribution. The aim of this study is to investigate possible errors in delivered dose when dwell times <1s are used. Methods Timer linearity was measured by positioning the source at a fixed location in a well chamber (HDR 1000 Plus with source holder 70010, Standard Imaging, WI), altering the dwell times from 60s to 0.1s and measuring charge. The differences in dose to the plan reference point for plan geometries created with and without dwell times < 1s, for the same calculated dose, were measured with a microDiamond detector (PTW, Germany). Plans without small dwell times were considered as reference plans. A single channel plan geometry used a Freiburg flab (Elekta, Netherlands) and a multichannel geometry used five needles (ProGuide 6F, Elekta). Figure 1 shows the phantom geometry and dwell times used. The plan doses were calculated with the AcurosTM BV model (Brachyvision 11.0, Varian Medical Systems). Results Results are shown in Figure 2. Timer linearity measurements resulted in a maximum difference to linear fit of 2.5% at dwell time 0.1s and -1.2% at dwell time 1s. With a 370GBq source, these timer errors would result in dose errors of 0.4mGy and 2 mGy respectively at 1 cm distance from the source. In microDiamond measurements, the maximum difference between measured values for plans containing small dwell times and reference plans was -1.3% with the plan that had ≥1s dwell times and ≤1% in the plans that had dwell times <1s. Conclusions The errors in timer linearity and dose measurements with small dwell times were all considered to be negligible in the context of total treatment accuracy. The errors were smaller with the plans containing dwell times <1s than in the plans containing 1s dwell times. It is therefore not necessary to apply a restriction to the use of small dwell times with the VariSource iX afterloader.

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