Almond P R, Biggs P J, Coursey B M, Hanson W F, Huq M S, Nath R, Rogers D W
Brown Cancer Center, Louisville, Kentucky 40202, USA.
Med Phys. 1999 Sep;26(9):1847-70. doi: 10.1118/1.598691.
A protocol is prescribed for clinical reference dosimetry of external beam radiation therapy using photon beams with nominal energies between 60Co and 50 MV and electron beams with nominal energies between 4 and 50 MeV. The protocol was written by Task Group 51 (TG-51) of the Radiation Therapy Committee of the American Association of Physicists in Medicine (AAPM) and has been formally approved by the AAPM for clinical use. The protocol uses ion chambers with absorbed-dose-to-water calibration factors, N(60Co)D,w which are traceable to national primary standards, and the equation D(Q)w = MkQN(60Co)D,w where Q is the beam quality of the clinical beam, D(Q)w is the absorbed dose to water at the point of measurement of the ion chamber placed under reference conditions, M is the fully corrected ion chamber reading, and kQ is the quality conversion factor which converts the calibration factor for a 60Co beam to that for a beam of quality Q. Values of kQ are presented as a function of Q for many ion chambers. The value of M is given by M = PionP(TP)PelecPpolMraw, where Mraw is the raw, uncorrected ion chamber reading and Pion corrects for ion recombination, P(TP) for temperature and pressure variations, Pelec for inaccuracy of the electrometer if calibrated separately, and Ppol for chamber polarity effects. Beam quality, Q, is specified (i) for photon beams, by %dd(10)x, the photon component of the percentage depth dose at 10 cm depth for a field size of 10x10 cm2 on the surface of a phantom at an SSD of 100 cm and (ii) for electron beams, by R50, the depth at which the absorbed-dose falls to 50% of the maximum dose in a beam with field size > or =10x10 cm2 on the surface of the phantom (> or =20x20 cm2 for R50>8.5 cm) at an SSD of 100 cm. R50 is determined directly from the measured value of I50, the depth at which the ionization falls to 50% of its maximum value. All clinical reference dosimetry is performed in a water phantom. The reference depth for calibration purposes is 10 cm for photon beams and 0.6R50-0.1 cm for electron beams. For photon beams clinical reference dosimetry is performed in either an SSD or SAD setup with a 10x10 cm2 field size defined on the phantom surface for an SSD setup or at the depth of the detector for an SAD setup. For electron beams clinical reference dosimetry is performed with a field size of > or =10x10 cm2 (> or =20x20 cm2 for R50>8.5 cm) at an SSD between 90 and 110 cm. This protocol represents a major simplification compared to the AAPM's TG-21 protocol in the sense that large tables of stopping-power ratios and mass-energy absorption coefficients are not needed and the user does not need to calculate any theoretical dosimetry factors. Worksheets for various situations are presented along with a list of equipment required.
已制定一项协议，用于外照射放射治疗的临床参考剂量测定，该治疗使用标称能量在60Co至50 MV之间的光子束以及标称能量在4至50 MeV之间的电子束。该协议由美国医学物理学家协会（AAPM）放射治疗委员会的第51任务组（TG - 51）编写，并已得到AAPM的正式批准可用于临床。该协议使用具有吸收剂量 - 水校准因子N(60Co)D,w的电离室，这些校准因子可追溯到国家一级标准，以及方程D(Q)w = MkQN(60Co)D,w，其中Q是临床射束的射束质，D(Q)w是在参考条件下放置电离室的测量点处水的吸收剂量，M是完全校正后的电离室读数，kQ是质量转换因子，它将60Co射束的校准因子转换为质量为Q的射束的校准因子。对于许多电离室，kQ的值表示为Q的函数。M的值由M = PionP(TP)PelecPpolMraw给出，其中Mraw是原始的、未校正的电离室读数，Pion校正离子复合，P(TP)校正温度和压力变化，Pelec在校准单独进行时校正静电计的不准确性，Ppol校正电离室极性效应。射束质Q的规定如下：（i）对于光子束，通过%dd(10)x规定，即对于在100 cm源皮距（SSD）下模体表面10x10 cm2射野尺寸，10 cm深度处的百分深度剂量的光子分量；（ii）对于电子束，通过R50规定，即在100 cm SSD下模体表面射野尺寸≥10x10 cm2（对于R50>8.5 cm为≥20x20 cm2）的射束中，吸收剂量降至最大剂量50%处的深度。R50直接由I50的测量值确定，I50是电离降至其最大值50%处的深度。所有临床参考剂量测定均在水模体中进行。用于校准目的的参考深度对于光子束为10 cm，对于电子束为0.6R50 - 0.1 cm。对于光子束，临床参考剂量测定在SSD或SAD设置下进行，对于SSD设置，在模体表面定义10x10 cm2的射野尺寸，对于SAD设置，在探测器深度处进行。对于电子束，临床参考剂量测定在90至110 cm的SSD下，射野尺寸≥10x10 cm2（对于R50>8.5 cm为≥20x20 cm2）的条件下进行。与AAPM的TG - 21协议相比，该协议有重大简化，即不需要大量的阻止本领比和质能吸收系数表，用户也无需计算任何理论剂量学因子。文中给出了各种情况的工作表以及所需设备清单。
