Rana Vijay, Gill Kamaljit, Rudin Stephen, Bednarek Daniel R
Department of Physiology and Biophysics, Toshiba Stroke Research Center, University at Buffalo (State University of New York), 3435 Main St., Buffalo, NY USA 14214.
Proc SPIE Int Soc Opt Eng. 2012 Feb 23;8313:83131N-. doi: 10.1117/12.911528.
The current version of the real-time skin-dose-tracking system (DTS) we have developed assumes the exposure is contained within the collimated beam and is uniform except for inverse-square variation. This study investigates the significance of factors that contribute to beam non-uniformity such as the heel effect and backscatter from the patient to areas of the skin inside and outside the collimated beam. Dose-calibrated Gafchromic film (XR-RV3, ISP) was placed in the beam in the plane of the patient table at a position 15 cm tube-side of isocenter on a Toshiba Infinix C-Arm system. Separate exposures were made with the film in contact with a block of 20-cm solid water providing backscatter and with the film suspended in air without backscatter, both with and without the table in the beam. The film was scanned to obtain dose profiles and comparison of the profiles for the various conditions allowed a determination of field non-uniformity and backscatter contribution. With the solid-water phantom and with the collimator opened completely for the 20-cm mode, the dose profile decreased by about 40% on the anode side of the field. Backscatter falloff at the beam edge was about 10% from the center and extra-beam backscatter decreased slowly with distance from the field, being about 3% of the beam maximum at 6 cm from the edge. Determination of the magnitude of these factors will allow them to be included in the skin-dose-distribution calculation and should provide a more accurate determination of peak-skin dose for the DTS.
我们研发的实时皮肤剂量跟踪系统（DTS）的当前版本假定照射剂量包含在准直束内，除了平方反比变化外是均匀的。本研究调查了导致束不均匀性的因素的重要性，如足跟效应以及从患者到准直束内外皮肤区域的反向散射。将剂量校准的Gafchromic胶片（XR - RV3，ISP）放置在东芝Infinix C型臂系统等中心管侧15 cm处患者检查床平面的束中。分别在胶片与一块20 cm厚的固体水块接触以提供反向散射以及胶片悬浮在空气中无反向散射的情况下进行曝光，两种情况均在束中有或没有检查床时进行。扫描胶片以获得剂量分布，通过比较各种条件下的分布可以确定射野不均匀性和反向散射贡献。对于20 cm模式且准直器完全打开的固体水模体，射野阳极侧的剂量分布下降了约40%。束边缘处的反向散射从中心处下降约10%，束外反向散射随离射野距离缓慢下降，在离边缘6 cm处约为束最大值的3%。确定这些因素的大小将使它们能够被纳入皮肤剂量分布计算中，并应为DTS提供更准确的皮肤峰值剂量测定。
