Department of Health Sciences, Faculty of Life Sciences, Kumamoto University, 4-24-1 Kuhonji, Chuo-ku, Kumamoto, 862-0976, Japan. Author to whom any correspondence should be addressed.
Phys Med Biol. 2018 Sep 17;63(18):185018. doi: 10.1088/1361-6560/aad9c0.
The purpose of this study was to develop a new ionization chamber dosimetry formalism for diagnostic kilovoltage x-ray beams based on a Co absorbed dose-to-water calibration coefficient [Formula: see text]. To validate the new chamber dosimetry, 39 fluence spectra of kilovoltage x-rays were calculated, using a SpekCalc program, for Al half-value-layer (Al-HVL) values of 1.4-8.5 mm and tube voltages of 50-137.6 kVp. We used these spectra to calculate the beam quality conversion factor [Formula: see text] for kilovoltage x-ray beams Q to a Co beam for a PTW 30013 Farmer ionization chamber in a reference geometrical setup (depth = 1 cm in water, field size = 20 × 20 cm). The absorbed dose to water for kilovoltage x-rays was obtained using [Formula: see text] and [Formula: see text]. Meanwhile, the water surface dose based on the air kerma calibration coefficient N was obtained using a Monte Carlo-calculated correction factor CF (corresponding to [Formula: see text] in AAPM TG-61) for the 39 x-ray fluence spectra. The absorbed dose based on [Formula: see text] was validated by comparing the water surface dose for the 39 x-ray fluence spectra with the water surface dose based on N . The water surface dose based on [Formula: see text] was obtained using Monte Carlo-calculated percentage depth doses. For Al-HVL ranging from 1.4 to 8.5 mm, [Formula: see text] ranged from 0.884 to 0.956 and CF ranged from 1.264 to 1.663. The water surface dose based on [Formula: see text] for the x-ray fluence spectra was 1.0% higher on average than that based on N , except for Al-HVLs of 1.4 and 1.5 mm and 1% was within their uncertainty ranges. Therefore, it is possible to use the absorbed dose-to-water value based on [Formula: see text] instead of on N . With the new chamber dosimetry, the chamber reading can be easily converted to the absorbed dose using [Formula: see text] and the calculated [Formula: see text].
本研究旨在基于 Co 吸收剂量-水校准系数 [Formula: see text] ，为诊断千伏 X 射线束开发新的电离室剂量学公式。为了验证新的腔室剂量学，使用 SpekCalc 程序计算了 39 种千伏 X 射线的通量谱，Al 半价层（Al-HVL）值为 1.4-8.5 毫米，管电压为 50-137.6 kVp。我们使用这些光谱，根据参考几何设置（水深为 1 厘米，射野尺寸为 20 × 20 厘米），计算了 PTW 30013 Farmer 电离室的千伏 X 射线束 Q 与 Co 束的束质转换系数 [Formula: see text]。使用 [Formula: see text] 和 [Formula: see text] 获得了千伏 X 射线的水吸收剂量。同时，基于空气比释动能校准系数 N 获得了水表面剂量，方法是使用 Monte Carlo 计算的修正因子 CF（对应于 AAPM TG-61 中的 [Formula: see text] ），用于 39 种 X 射线通量光谱。通过将 39 种 X 射线通量光谱的水表面剂量与基于 N 的水表面剂量进行比较，验证了基于 [Formula: see text] 的吸收剂量。使用 Monte Carlo 计算的百分深度剂量获得了基于 [Formula: see text] 的水表面剂量。对于 Al-HVL 范围为 1.4 至 8.5 毫米，[Formula: see text] 范围为 0.884 至 0.956，CF 范围为 1.264 至 1.663。除了 Al-HVL 为 1.4 和 1.5 毫米之外，基于 X 射线通量光谱的基于 [Formula: see text] 的水表面剂量平均比基于 N 的水表面剂量高 1%，并且 1% 在其不确定度范围内。因此，有可能使用基于 [Formula: see text] 的水吸收剂量值代替基于 N 的水吸收剂量值。使用新的腔室剂量学，腔室读数可以通过使用 [Formula: see text] 和计算的 [Formula: see text] 轻松转换为吸收剂量。
