Zhang Rongxiao, Baer Esther, Jee Kyung-Wook, Sharp Gregory C, Flanz Jay, Lu Hsiao-Ming
Massachusetts General Hospital, Boston, MA 02114, United States of America.
Phys Med Biol. 2017 Jun 23;62(14):5640-5651. doi: 10.1088/1361-6560/aa782c.
For proton therapy, an accurate conversion of CT HU to relative stopping power (RSP) is essential. Validation of the conversion based on real tissue samples is more direct than the current practice solely based on tissue substitutes and can potentially address variations over the population. Based on a novel dose extinction method, we measured water equivalent path lengths (WEPL) on animal tissue samples to evaluate the accuracy of CT HU to RSP conversion and potential variations over a population. A broad proton beam delivered a spread out Bragg peak to the samples sandwiched between a water tank and a 2D ion-chamber detector. WEPLs of the samples were determined from the transmission dose profiles measured as a function of the water level in the tank. Tissue substitute inserts and Lucite blocks with known WEPLs were used to validate the accuracy. A large number of real tissue samples were measured. Variations of WEPL over different batches of tissue samples were also investigated. The measured WEPLs were compared with those computed from CT scans with the Stoichiometric calibration method. WEPLs were determined within ±0.5% percentage deviation (% std/mean) and ±0.5% error for most of the tissue surrogate inserts and the calibration blocks. For biological tissue samples, percentage deviations were within ±0.3%. No considerable difference (<1%) in WEPL was observed for the same type of tissue from different sources. The differences between measured WEPLs and those calculated from CT were within 1%, except for some bony tissues. Depending on the sample size, each dose extinction measurement took around 5 min to produce ~1000 WEPL values to be compared with calculations. This dose extinction system measures WEPL efficiently and accurately, which allows the validation of CT HU to RSP conversions based on the WEPL measured for a large number of samples and real tissues.
对于质子治疗而言，将CT的Hounsfield单位（HU）准确转换为相对阻止本领（RSP）至关重要。基于真实组织样本对这种转换进行验证，比当前仅基于组织替代物的做法更为直接，并且有可能解决人群中的差异问题。基于一种新颖的剂量消光方法，我们在动物组织样本上测量了水等效路径长度（WEPL），以评估CT的HU到RSP转换的准确性以及人群中的潜在差异。一束宽质子束向夹在水箱和二维电离室探测器之间的样本传递扩展布拉格峰。样本的WEPL通过作为水箱水位函数测量的透射剂量分布来确定。使用具有已知WEPL的组织替代物插入物和有机玻璃块来验证准确性。测量了大量真实组织样本。还研究了不同批次组织样本中WEPL的变化。将测量的WEPL与使用化学计量校准方法从CT扫描计算得到的结果进行比较。对于大多数组织替代物插入物和校准块，确定的WEPL的百分比偏差在±0.5%以内，误差在±0.5%以内。对于生物组织样本，百分比偏差在±0.3%以内。对于来自不同来源的相同类型组织，未观察到WEPL有显著差异（<1%）。除了一些骨组织外，测量的WEPL与CT计算值之间的差异在1%以内。根据样本大小，每次剂量消光测量大约需要5分钟，以产生约1000个可与计算值进行比较的WEPL值。这种剂量消光系统能够高效且准确地测量WEPL，这使得基于大量样本和真实组织测量的WEPL对CT的HU到RSP转换进行验证成为可能。
