PO89

Abstract

The objective was to evaluate radioactive angles and dose distribution of partially shielded GYN cylinders during HDR Brachytherapy in a commercial treatment planning system (TPS). Patient treatment plans with partial shielded cylinders were generated in Brachytherapy planning using Eclipse TPS. The AAPM TG-43 dose calculation formalism was commonly used to provide rapid patient treatment. The Acuros algorithm in AAPM TG-186 MBCDAs with the same source position and planned dwell time was used to compare shielding effects and unblocked dose distributions. Acuros calculations employed solid applicators with shielded angles of 90°, 180°, and 270° from the TPS library, with tungsten alloy shielded segments. All calculations were performed in the water phantom. The differences between the AAPM TG-43 and TG-186 algorithms in these applicators were evaluated in terms of shielded angle, radioactive range, low dose bath background, and dose distribution. Patient planning using various shielded angle cylinders anteriorly or posteriorly to minimize local bladder or rectal dose on patient CT images for vaginal cuff treatment. The dwell time at the source position was calculated according to the 3D TG-43 algorithm, ignoring the shielding effect of the applicator to result in a cylindrically symmetrical dose distribution along the cylinder axis. Acuros dose calculation implemented Monte-Carlo (MC) algorithm approach. The 3D asymmetric dose distribution was shown in the sagittal view as expected. Low dose bath in the shielded areas was observed at approximately 10-15% of the prescribed dose. The low dose bath level changed slowly with shielding angle. The physical shielding angle was effectively shielded within the dose distribution range. The radioactive angle can be defined as the angle of the radioactive coverage portion from the source, or the angle between the designed isodose points to the center of cylindrical section. As showed in the figure below, a 180° shielded cylinder can shield a maximum range of about 210° in the posterior portion. When the sum of the shieling angle and radioactive angle was 360°, the radioactive angle was equal to 150°. This behavior was confirmed for shielding angles of 90°, 180°, and 270°, respectively in Acuros calculations. The radioactive angle was found to be smaller than the physical shielding angle. The presence of the metal block appeared to prevent deposition of scattered doses to unshielded tissue. This reduced scatter made the blocking angle projection wider than its physical angle. A slightly longer Acuros dwell time at a few percent was required to achieve the same dose level calculated for TG-43. Acuros calculations for shielded cylinder suggest that the shielded region in dose distribution is wider than the physically shielded part of segment. The radioactive angle is smaller than the physical shielded angle. Shielded areas are kept in low dose baths of prescribed doses. A slightly longer dwell time is required to achieve the same dose level calculated for TG-43.