Nyflot M, Holdsworth C, Kalet A, Chvetsov A, Sandison G
University of Washington, Seattle, WA.
Med Phys. 2012 Jun;39(6Part13):3758. doi: 10.1118/1.4735303.
In vivo dosimetry (IVD) assessment of treatment dose is important when delivering total body irradiation (TBI). One method is to average AP and PA surface diode measurements and compare them to prescribed midline doses. We designed phantom studies to examine the impact of patient thickness on surface IVD measurements under TBI conditions.
Phantom studies were designed to assess the effects of patient thickness on diode IVD. Sun Nuclear QED diodes with inherent buildup were placed on anterior and posterior surfaces of a solid water phantom. Phantom thickness was varied between 20 and 40 cm. A PTW farmer chamber was inserted in the center of the phantom at 425 SSD to reflect prescribed midline dose, and 50 cGy was delivered to midline with 18 MV photons. Averaged entrance and exit diode doses were then compared to farmer chamber measurements of phantom midline dose.
A trend of increased deviation with increasing umbilicus thickness was observed between averaged surface diodes and midline farmer chamber measurements. Averaged surface diode dose ranged from 49.6 cGy (20 cm thickness) to 52.1 cGy (40 cm thickness). Interpolation of diode measurements to midline resulted in linear overestimation of delivered dose relative to farmer chamber measurements at midline, up to 6.8% at 40 cm umbilicus thickness.
Accurate in vivo dosimetry at time of patient TBI is important to allow individual correction of MU exposure and tissue compensation. Without patient thickness correction, overresponse of surface diodes may lead to unnecessary clinical intervention to treatment MU or compensation and insufficient midline dose. Additionally, SAD setup is preferable to SSD setup to minimize thickness non-linearity. In conclusion, thickness correction factors should be used to generate expected diode readings for patients with thickness greater than 30 cm.
在进行全身照射（TBI）时，对治疗剂量进行体内剂量测定（IVD）评估很重要。一种方法是对前后表面二极管测量值进行平均，并将其与规定的中线剂量进行比较。我们设计了体模研究，以检查患者厚度对TBI条件下表面IVD测量的影响。
设计体模研究以评估患者厚度对二极管IVD的影响。将具有固有建成效应的Sun Nuclear QED二极管放置在固体水体模的前后表面。体模厚度在20至40厘米之间变化。将一个PTW电离室插入体模中心，源皮距（SSD）为425，以反映规定的中线剂量，并用18兆伏光子向中线输送50厘戈瑞的剂量。然后将平均的入射和出射二极管剂量与电离室对体模中线剂量的测量值进行比较。
在平均表面二极管和中线电离室测量值之间，观察到随着脐部厚度增加偏差增大的趋势。平均表面二极管剂量范围从49.6厘戈瑞（厚度20厘米）到52.1厘戈瑞（厚度40厘米）。将二极管测量值内插到中线会导致相对于中线电离室测量值，输送剂量出现线性高估，在脐部厚度为40厘米时高达6.8%。
在患者接受TBI时进行准确的体内剂量测定对于允许对监测单位（MU）照射进行个体化校正和组织补偿很重要。如果不进行患者厚度校正，表面二极管的过度响应可能导致对治疗MU或补偿进行不必要的临床干预，以及中线剂量不足。此外，源轴距（SAD）设置比SSD设置更可取，以尽量减少厚度非线性。总之，对于厚度大于30厘米的患者，应使用厚度校正因子来生成预期的二极管读数。
