Leybovich Leonid B, Sethi Anil, Dogan Nesrin
Department of Radiation Oncology, Loyola University Medical Center, Maywood, Illinois 60153, USA.
Med Phys. 2003 Feb;30(2):119-23. doi: 10.1118/1.1536161.
IMRT plans are usually verified by phantom measurements: dose distributions are measured using film and the absolute dose using an ionization chamber. The measured and calculated doses are compared and planned MUs are modified if necessary. To achieve a conformal dose distribution, IMRT fields are composed of small subfields, or "beamlets." The size of beamlets is on the order of 1 x 1 cm2. Therefore, small chambers with sensitive volumes < or = 0.1 cm3 are generally used for absolute dose verification. A dosimetry system consisting of an electrometer, an ion chamber, and connecting cables may exhibit charge leakage. Since chamber sensitivity is proportional to volume, the effect of leakage on the measured charge is relatively greater for small chambers. Furthermore, the charge contribution from beamlets located at significant distances from the point of measurement may be below the small chambers threshold and hence not detected. On the other hand, large (0.6 cm3) chambers used for the dosimetry of conventional external fields are quite sensitive. Since these chambers are long, the electron fluence through them may not be uniform ("temporal" uniformity may not exist in the chamber volume). However, the cumulative, or "spatial" fluence distribution (as indicated by calculated IMRT dose distribution) may become uniform at the chamber location when the delivery of all IMRT fields is completed. Under the condition of "spatial" fluence uniformity, the charge collected by the large chamber may accurately represent the absolute dose delivered by IMRT to the point of measurement. In this work, 0.6, 0.125, and 0.009 cm3 chambers were used for the absolute dose verification for tomographic and step-and-shoot IMRT plans. With the largest, 0.6 cm3 chamber, the measured dose was equal to calculated within 0.5%, when no leakage corrections were made. Without leakage corrections, the error of measurement with a 0.125 cm3 chamber was 2.6% (tomographic IMRT) and 1.5% (step-and-shoot IMRT). When doses measured by a 0.125 cm3 chamber were corrected for leakage, the difference between the calculated and measured doses reduced to 0.5%. Leakage corrected doses obtained with the 0.009 cm3 chamber were within 1.5%-1.7% of calculated doses. Without leakage corrections, the measurement error was 16% (tomographic IMRT) and 7% (step-and-shoot IMRT).
调强放疗（IMRT）计划通常通过模体测量来验证：使用胶片测量剂量分布，使用电离室测量绝对剂量。将测量剂量与计算剂量进行比较，必要时修改计划的监测单位（MUs）。为了实现适形剂量分布，IMRT射野由小的子野或“子束”组成。子束的尺寸约为1×1平方厘米。因此，通常使用灵敏体积≤0.1立方厘米的小电离室进行绝对剂量验证。由静电计、电离室和连接电缆组成的剂量测定系统可能会出现电荷泄漏。由于电离室灵敏度与体积成正比，对于小电离室，泄漏对测量电荷的影响相对较大。此外，位于距测量点较远位置的子束的电荷贡献可能低于小电离室的阈值，因此无法检测到。另一方面，用于传统外照射野剂量测定的大（0.6立方厘米）电离室相当灵敏。由于这些电离室较长，穿过它们的电子注量可能不均匀（电离室内可能不存在“时间”均匀性）。然而，当所有IMRT射野的照射完成时，累积的或“空间”注量分布（如计算的IMRT剂量分布所示）在电离室位置可能会变得均匀。在“空间”注量均匀的条件下，大电离室收集的电荷可以准确代表IMRT在测量点处的绝对剂量。在这项工作中，使用0.6、0.125和0.009立方厘米的电离室对断层扫描和步进式IMRT计划进行绝对剂量验证。使用最大的0.6立方厘米电离室时，在未进行泄漏校正的情况下，测量剂量与计算剂量的误差在0.5%以内。未进行泄漏校正时，使用0.125立方厘米电离室测量的误差在断层扫描IMRT中为2.6%，在步进式IMRT中为1.5%。当对0.125立方厘米电离室测量的剂量进行泄漏校正后，计算剂量与测量剂量之间的差异降至0.5%。使用0.009立方厘米电离室获得的泄漏校正剂量在计算剂量的1.5% - 1.7%以内。未进行泄漏校正时，测量误差在断层扫描IMRT中为16%，在步进式IMRT中为7%。
