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Pre-Treatment Simulation Tolerance and Dosimetry Effects On OARs for the Cylinder High Dose Rate (HDR) Brachytherapy Treatment of Vaginal Cuff Cancer

C Shi

C Shi*, D Fang, C Iannuzzi, Saint Vincent Medical Center, BRIDGEPORT, CT

SU-E-T-351 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: To statistically find out the achievable and practical simulation tolerance for HDR cylinder treatment using scout AP/LAT images and the corresponding organ at risks dosimetry effects.

Methods:Five previously treated vaginal cuff cancer patients were randomly selected. All of the patients had prescription of 5 mm above the cylinder surface with 500 cGy per fraction for 5 fractions. The treatment plans were developed using the Oncentra treatment planning system with graphical local optimization method. Two scouts (AP/LAT) were taken during CT scan day and each fraction treatment day. The dummy source was placed in the cylinder and the tip of the dummy source to the top of the pubic bone vertical distance on the AP scout was recorded. The cylinder diameter was also measured. The LAT dummy source angle relative to the horizontal line was also recorded. The treatment for each fraction was then reconstructed using AP/LAT scout images. The source position was registered back to the planning CT. A dummy cumulative plan was then created with a prescription of 2500 cGy for 1 fraction.

Results:The AP shifts were ranging from -5 mm to 10 mm and the LAT shifts were ranging from -13 deg to 9 deg. The percentage dose difference can be ranging up 0.3%-14% for the rectum and -17.8%-8.0% for the bladder. Based on the statistics results, we propose the pre-treatment simulation tolerance: trying to maintain the cylinder in AP direction shift under 5 mm, a warning zone is 5 mm-10 mm. For the lateral angle, the tolerance is trying to maintain the angle difference under 5 degree with 5-10 degree as warning zone.

Conclusion:We have done pre-treatment simulation quality assurance study using AP/LAT scout images. The tolerance for this pre-treatment simulation was proposed. The relative dosimetry effects for the rectum and bladder were reported.

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