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Using Matrixx to Determine Transit Dose Contribution Over Clinically Useful Limits of HDR Source Activity

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M Bhagwat

M Bhagwat*, D O'Farrell , M Wagar , I Buzurovic , S Friesen , A Damato , P Devlin , R Cormack , Dana-Farber Brigham and Women's Cancer Center, Harvard Medical School, Boston, MA


SU-E-T-783 (Sunday, July 12, 2015) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: Most HDR brachytherapy treatment planning systems (TPS) use TG-43 formalism to calculate dose without including transit dose corrections. Historically, measurement of this contribution has required sophisticated apparatus unavailable in most hospitals. We use Matrixx to investigate several scenarios where transit dose contribution may effect a clinical treatment.

Methods: Treatment plans were generated using Oncentra Brachy TPS (Version, Nucletron ) on a CT scan of a 24-catheter Freiburg applicator (Nucletron ) laid flat on the MatriXX (IBA) detector. This detector is an array of 1020 parallel plate ion chambers. All 24 catheters were digitized and dwells within a central square region of 5x5cm of the applicator were activated. Each of the active catheters had 6 dwells in increments of 1.0cm. The plans were normalized to 10mm. This places the 100% isodose line at the correct effective point of measurement, which lies half-way between the parallel plates of the ion chambers. It is also within the clinically relevant treatment depth for superficial applications. A total of 6 plans were delivered for 3 prescription doses, 1Gy, 2Gy and 4Gy using source activities of 2.9Ci and 11.2Ci. The MatriXX array was operated to capture dosimetric snaps every 500ms and yielded an integral dose at the end of treatment.

Results: A comparison of integral dose from 2 different source activities shows that the transit dose contribution is larger when the source activity is higher. It is also observed that the relative transit dose contribution decreases as prescription dose increases. This is quantified by the Gamma analysis.

Conclusion: We have demonstrated that the Matrixx detector can be used to evaluate the contribution for a HDR source during transit from the HDR afterloader to a dwell location, and between adjacent dwell locations.

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