Li K, Able A, Newton J, Cornell D
Associates In Medical Physics, Lanham, MD.
John R Marsh Cancer Center, Hagerstown, MD.
Med Phys. 2012 Jun;39(6Part12):3747. doi: 10.1118/1.4735253.
When the treatment couch-top contacts the patient, the skin dose to the patient also generates clinical significance as the radiation beam passes through the couch. In this study, the effect on entrance shallow depth dose was investigated for the carbon fiber exact couch-top from Varian Medical System.
An Accredited Dosimetry Calibration Laboratory (ADCL) calibrated PTW parallel-plate thin-window chamber was used to measure the doses in the build-up region. Firstly, the shallow depth doses at different field sizes, depths and incident angle beams were measured for 6MV and 18MV photon beams. To test the couch top contribution to the dose distribution, the depth of measurement went down to 10cm. The couch was simulated in the Eclipse treatment planning system and the related point dose was calculated with different depths. The CT number of the couch top was adjusted to generate agreement between measurement and simulation.
At a 2mm depth, the surface dose increased with the decrease of the field size. Comparing with and without the couch top, as field sizes vary from 20cm×20cm to 2cm×2cm, the dose increased from 35% to 53% of for 6MV, and from 50% to 113% for 18MV. At 0mm depth, the dose difference was most significant and is at the level of 300%. Angle incident beam dose increase due to couch top varied with complicated dose distribution. Extended Depth Dose measurement shows that couch top effect on dose in build-up region extends to depths of 1cm for 6MV and 2cm for 18MV. An appropriate CT number setting of the couch top is between -200HU to -300HU.
The couch simulation in Eclipse displays better dosimetric accuracy in couch contacted skin dose. However, calculation factors such as surface definition, heterogeneity, calculation resolution and algorithm also need extra consideration.
当治疗床面与患者接触时，由于辐射束穿过治疗床，患者的皮肤剂量也具有临床意义。在本研究中，研究了瓦里安医疗系统的碳纤维精确床面对入射浅表剂量的影响。
使用经认可的剂量测定校准实验室（ADCL）校准的PTW平行板薄窗电离室测量建成区的剂量。首先，测量了6MV和18MV光子束在不同射野大小、深度和入射角射束下的浅表剂量。为了测试床面对剂量分布的贡献，测量深度降至10cm。在Eclipse治疗计划系统中模拟治疗床，并计算不同深度的相关点剂量。调整床面的CT值以实现测量和模拟之间的一致性。
在2mm深度处，表面剂量随射野大小的减小而增加。与有无床面的情况相比，当射野大小从20cm×20cm变化到2cm×2cm时，6MV的剂量从35%增加到53%，18MV的剂量从50%增加到113%。在0mm深度处，剂量差异最为显著，达到300%的水平。由于床面导致的入射角射束剂量增加随复杂的剂量分布而变化。扩展深度剂量测量表明，床面对建成区剂量的影响在6MV时延伸至1cm深度，在18MV时延伸至2cm深度。床面的合适CT值设置在-200HU至-300HU之间。
Eclipse中的治疗床模拟在床面接触皮肤剂量方面显示出更好的剂量学准确性。然而，诸如表面定义、不均匀性、计算分辨率和算法等计算因素也需要额外考虑。
