SU‐F‐T‐07: Dosimetric Impact of the Dwell Position Inaccuracy in HDR Ring and Tandem Treatments with VarisourceIX Afterloader

Abstract

Purpose:The dwell positions in the VariSourceIX HDR ring applicator are known to be somewhat inaccurate with observed deviations from the planned position of up to 5mm. The details of how to correct this issue are suggested by the vendor and the dosimetric impact on volumetric HDR plans varies from patient to patient. In this study we present our experience with correcting this issue, and discuss the impact on plan quality if the planned dwell positions were left unchanged.Methods:A 60 degree ring was characterized using gafchromic film with the applicator in treatment orientation. The offsets as a function of dwell position in the ring were deduced. The variability in each observed dwell position was determined from three consecutive runs. A total of 10 ring and tandem treatments were analyzed in this work. The PTV and organs at risk were outlined and dose‐volume histograms were produced. The patient plans were modified for delivery according to the average shift determined in the characterization stage. D2cc for the organs at risk (rectum, bladder, and sigmoid) were used as dosimetric end‐points in this study. The dose at the locations of the ICRU points A was analyzed as well.Results:Each dwell position was corrected according to the measured offsets. For each fraction, three plans were produced: dosimetric plan with ideal dwell positions, treatment plan with corrected dwell positions, and a test plan with opposite offsets for evaluation of dose differences for organs at risk and PTV. Maximum and average dose deviations were deduced for each organ at risk. For rectum, sigmoid, and bladder the maximum D2cc dose deviations were 7.6%, 5.3%, and 1.5%, respectively.Conclusion:Correcting dwell position inaccuracy in an HDR ring is essential for maintaining quality of treatment. The dose differences are significant in the organs at risk.