Chiu-Tsao Sou-Tung, Medich David, Munro John
Quality MediPhys LLC, 17 Jade Lane, Denville, New Jersey 07834, USA.
Med Phys. 2008 Aug;35(8):3787-99. doi: 10.1118/1.2955746.
Radiochromic film dosimetry has been extensively used for intravascular brachytherapy applications for near field within 1 cm from the sources. With the recent introduction of new model of radiochromic films, GAFCHROMIC EBT, with higher sensitivity than earlier models, it is promising to extend the distances out to 5 cm for low dose rate (LDR) source dosimetry. In this study, the use of new model GAFCHROMIC EBT film for 125I seed dosimetry in Solid Water was evaluated for radial distances from 0.06 cm out to 5 cm. A multiple film technique was employed for four 125I seeds (Implant Sciences model 3500) with NIST traceable air kerma strengths. Each experimental film was positioned in contact with a 125I seed in a Solid Water phantom. The products of the air kerma strength and exposure time ranged from 8 to 3158 U-h, with the initial air kerma strength of 6 U in a series of 25 experiments. A set of 25 calibration films each was sequentially exposed to one 125I seed at about 0.58 cm distance for doses from 0.1 to 33 Gy. A CCD camera based microdensitometer, with interchangeable green (520 nm) and red (665 nm) light boxes, was used to scan all the films with 0.2 mm pixel resolution. The dose to each 125I calibration film center was calculated using the air kerma strength of the seed (incorporating decay), exposure time, distance from seed center to film center, and TG43U1S1 recommended dosimetric parameters. Based on the established calibration curve, dose conversion from net optical density was achieved for each light source. The dose rate constant was determined as 0.991 cGy U(-1)h(-1) (+/-6.9%) and 1.014 cGy U(-1)h(-1) (+/-6.8%) from films scanned using green and red light sources, respectively. The difference between these two values was within the uncertainty of the measurement. Radial dose function and 2D anisotropy function were also determined. The results obtained using the two light sources corroborated each other. We found good agreement with the TG43U1S1 recommended values of radial dose function and 2D anisotropy function, to within the uncertainty of the measurement. We also verified the dosimetric parameters in the near field calculated by Rivard using Monte Carlo method. The radial dose function values in Solid Water were lower than those in water recommended by TG43U1S1, by about 2%, 3%, 7%, and 14% at 2, 3, 4, and 5 cm, respectively, partially due to the difference in the phantom material composition. Radiochromic film dosimetry using GAFCHROMIC EBT model is feasible in determining 2D dose distributions around low dose rate 125I seed. It is a viable alternative to TLD dosimetry for 125I seed dose characterization.
放射性变色薄膜剂量测定法已广泛应用于血管内近距离放射治疗，用于距源1厘米以内的近场。随着新型放射性变色薄膜GAFCHROMIC EBT的推出，其灵敏度高于早期型号，有望将低剂量率（LDR）源剂量测定的距离扩展至5厘米。在本研究中，评估了新型GAFCHROMIC EBT薄膜在固体水介质中对125I种子进行剂量测定时，从0.06厘米到5厘米的径向距离的情况。采用多薄膜技术对四个具有NIST可溯源空气比释动能强度的125I种子（植入科学公司3500型）进行研究。每个实验薄膜都与固体水模体中的一个125I种子接触放置。空气比释动能强度与照射时间的乘积范围为8至3158 U-h，在一系列25次实验中，初始空气比释动能强度为6 U。一组25张校准薄膜依次在距一个125I种子约0.58厘米的距离处接受0.1至33 Gy的剂量照射。使用基于电荷耦合器件（CCD）相机的微密度计，配备可互换的绿色（520纳米）和红色（665纳米）灯箱，以0.2毫米像素分辨率扫描所有薄膜。使用种子的空气比释动能强度（考虑衰变）、照射时间、从种子中心到薄膜中心的距离以及TG43U1S1推荐的剂量学参数计算每个125I校准薄膜中心的剂量。基于建立的校准曲线，实现了每个光源下从净光密度到剂量的转换。分别从使用绿色和红色光源扫描的薄膜中确定剂量率常数为0.991 cGy U(-1)h(-1)（±6.9%）和1.014 cGy U(-1)h(-1)（±6.8%）。这两个值之间的差异在测量不确定度范围内。还确定了径向剂量函数和二维各向异性函数。使用两种光源获得的结果相互印证。我们发现，在测量不确定度范围内，径向剂量函数和二维各向异性函数的值与TG43U1S1推荐值吻合良好。我们还验证了里瓦德使用蒙特卡罗方法计算的近场剂量学参数。固体水中的径向剂量函数值低于TG43U1S1推荐的水中值，在2、3、4和5厘米处分别低约2%、3%、7%和14%，部分原因是模体材料成分的差异。使用GAFCHROMIC EBT型号的放射性变色薄膜剂量测定法在确定低剂量率125I种子周围的二维剂量分布方面是可行的。它是用于125I种子剂量表征的热释光剂量测定法的可行替代方法。
