SU-F-T-332: Dose Impact of Rectal Gas On Prostate VMAT

Abstract

Purpose: The aim of this study to evaluate influence of rectal gas on dose distribution during prostate VMAT. Methods: Our subjects were 10 patients who have received VMAT for prostate cancer at our hospital. In this study, we made four types of VMAT plan. The angles of rotation were as follows: clockwise from 181–179° and 179–181° (Full arc), clockwise from 200–160° and counter‐clockwise from 160–200° (Partial arc1), clockwise from 220‐ 140° and counter‐clockwise from 140–220° (Partial arc2), clockwise from 240–120° and counter‐clockwise from 120–240° (Partial arc3). The rectal contour used for the reference treatment plan each had 5% or less rectal gas. In order to evaluate the effects of the rectal gas on the dose distribution, we created a rectal contour for assessment separate from the contour used for the reference treatment plans. In the contour for evaluation, the Hounsfield unit (HU) value of the gas was assigned for the total volume of the rectal contour. A HU value of −950 was adopted for simulating the rectal gas. The 3DVH version 2.2 was used for evaluation, and evaluation was performed based on the concordance rate between the contour being evaluated and that of the reference treatment plans. The dose difference (DD), distance to agreement (DTA), and gamma analysis (GA) were used to obtain the concordance rate. The contours being evaluated were CTV, PTV, rectum and bladder. Results: The results of DD, DTA, and GA showed that the rectum, the CTV and rectum had the lowest concordance rates. Irrespective of DD, DTA, or GA, the treatment plan based on full arc had a higher concordance rate. Conclusion: With respect to the effect of rectal gas on the dose distribution in prostate VMAT, it was shown that full arc might be the least susceptible.