Ade Nicholas, van Eeden D, du Plessis F C P
Medical Physics Department, University of the Free State, PO Box 339, Bloemfontein, 9300, South Africa.
Medical Physics Department, University of the Free State, PO Box 339, Bloemfontein, 9300, South Africa.
Appl Radiat Isot. 2020 Jan;155:108919. doi: 10.1016/j.apradiso.2019.108919. Epub 2019 Oct 7.
The tissue- or water-equivalence of dosimetry phantoms used as substitutes for water is essential for absorbed dose measurements in radiotherapy. At our institution, a heterogeneous pelvic phantom that consists of stacked Nylon-12 layers has recently been manufactured for Gafchromic film dosimetry. However, data on the use of Nylon as tissue-mimicking media for dosimetric applications are scarce. This study characterizes the water-equivalence of Nylon-12 for dosimetric measurements in therapeutic photon and electron beams. Employing an Elekta Synergy and SL25 linear accelerator (Linac), photon beam transmission measurements for 6 MV and 15 MV, acquired in narrow beam geometry with a 0.6 cm Farmer-type ion chamber showed that the mass attenuation coefficient μ of Nylon-12 agrees with the values of water, water-equivalent RW3 and Perspex phantom materials within 3%. For 6 MV, the μ values were 0.0477 ± 0.002 cm/g, 0.0490 ± 0.003 cm/g, 0.0482 ± 0.001 cm/g and 0.0479 ± 0.002cm/g for Nylon-12, water, RW3, and Perspex, respectively. Differences within 2% were attained between depth dose data measured in Nylon-12 slabs with Gafchromic EBT3 films and in water with a Roos ion chamber for 10 × 10 cm 6, 12 and 20 MeV electron beams produced by the Elekta Synergy and SL25 Linacs. Also, a good agreement within 2% was obtained between percent depth doses computed by DOSXYZnrc Monte Carlo simulations in water, Nylon-12 and RW3 materials for photon spectra between 250 kV and 15 MV. The discrepancies between the ratios of average, restricted stopping powers of Nylon to air and water to air for photon spectra ranging from 2 to 45 MV are typically within 1% signifying that Nylon and water have equivalent stopping power characteristics. This study highlights that Nylon-12 can be used as a tissue-mimicking phantom material for dosimetric measurements in clinical megavoltage photon and electron beams as it exhibits good water-equivalence.
用作水替代物的剂量学体模的组织等效性或水等效性对于放射治疗中的吸收剂量测量至关重要。在我们机构，最近制造了一种由堆叠的尼龙 - 12层组成的异质盆腔体模用于Gafchromic胶片剂量测定。然而，关于将尼龙用作剂量学应用中组织模拟介质的数据很少。本研究表征了尼龙 - 12在治疗性光子和电子束剂量测量中的水等效性。使用Elekta Synergy和SL25直线加速器（直线加速器），在窄束几何条件下，用0.6厘米Farmer型电离室进行的6兆伏和15兆伏光子束传输测量表明，尼龙 - 12的质量衰减系数μ与水、水等效物RW3和有机玻璃体模材料的值在3%以内相符。对于6兆伏，尼龙 - 12、水、RW3和有机玻璃的μ值分别为0.0477±0.002厘米/克、0.0490±0.003厘米/克、0.0482±0.001厘米/克和0.0479±0.002厘米/克。对于由Elekta Synergy和SL25直线加速器产生的10×10厘米6、12和20兆电子伏电子束，在尼龙 - 12平板中用Gafchromic EBT3胶片测量的深度剂量数据与在水中用Roos电离室测量的深度剂量数据之间的差异在2%以内。此外，对于250千伏至15兆伏的光子能谱，在水、尼龙 - 12和RW3材料中由DOSXYZnrc蒙特卡罗模拟计算得到的百分深度剂量之间在2%以内取得了良好的一致性。对于2至45兆伏的光子能谱，尼龙与空气以及水与空气的平均受限阻止本领之比之间的差异通常在1%以内，这表明尼龙和水具有等效的阻止本领特性。本研究强调，尼龙 - 12可作为临床兆伏级光子和电子束剂量测量的组织模拟体模材料，因为它表现出良好的水等效性。
