直线加速器小照野物理数据的测量和比较

背景与目的：对于精确的肿瘤放射治疗特别是立体定向和调强放射治疗，为了建立可靠的治疗计划系统剂量计算模型，提供准确的小照射物理数据尤其重要。本研究通过测量不同能量下小照野的物理数据，分析和比较不同方法和不同电离室之间相应的测量误差。方法：在直线加速器4、6、8MV光子线下，采用0．65、0．13、0．01cm^3的三种指形电离室，在30cm×30cm×30cm的固体水体模中测量了1cm×1cm～10cm×10cm照射野的总散射因子（total scatter factor，Scp）、准直器散射因子（collimator scatter factor，Sc）和组织最大剂量比（tissue-maximum ratio，TMR）等物理数据。对相应的测量结果进行了分析和比较。结果：照射野〉3cm×3cm时，不同电离室的Scp和Sc测量结果偏差在0．8％以内：3cm×3cm以下的照射野的测量结果差别较大（最大64％）；在4、6、8MV光子线1cm×1cm和2cm×2cm照射野的Sc测量中。0．13cm^3电离室拉长源皮距（〉150cm）比标准源皮距处（100cm）的测量结果分别大25．4％、6．9％、24．6％和1．4％、1．4％、2．2％；两种电离室0．01cm^3和0．13cm^3拉长距离测量的Sc对≥2cm×2cm照射野没有明显的偏差．对1cm×1cm照射野0．13cm^3比0．01cm^3测量值小0．2％、8．5％、3．4％。在1cm×1cm照射野的TMR测量中，0．01cm，和0．13cm^3电离室在15cm以下区域的测量偏差较大，约为4％左右。对于2cm×2cm及以上照射野TMR的测量结果偏差较小（〈1％）。〉3cm×3cm的照射野中，TMR测量的结果与百分深度剂量（percentage depth dose，PDD）转换得到的TMR数据在深度15cm之前一致性较好。15cm深度之后有明显的偏差（〉2％）。结论：测量小照射野物理数据时。由于侧向电子散射不够，需要谨慎选择测量探头。不同的测量探头对小照野物理数据的准确性可能存在较大的影响。当侧向电子平衡不能建立时，测量?

Measurements and comparisons for data of small beams of linear accelerators

Background and Objective： Accurate data acquisition is very important to establish a reliable dose calculation model of the treatment planning system for small radiation fields in intensity modulated radiation therapy （IMRT） and stereotactic radiotherapy （SRT）. This study was to analyze and compare small-field measurements using different methods and ionization chambers. Methods： Three types of farmer chambers were used, with active volumes of 0.65 co, and 0.13 cc, 0.01 cc, respectiveley. The beam data, including the total scatter factor （Scp）, collimator scatter factor （Sc）, tissue-maximum ratio （TMR）, were acquired in a 30 cm×30 cm×30 cm water phantom under two linear accelerators. Measurements were performed at accelerating potentials of 4, 6, and 8 MV with the beam size ranging from 1 cm ×1 cm to 10 cm ×10 cm. The measurements were analyzed and compared. Results： For the beam size of ≥3 cm×3 cm, the differences in Sep and Sc measurements of the 0.65 cc, 0.13 cc and 0.01 cc ion chambers were within 0.8%, while the differences were much greater for the beam size of less than 3 cm×3 cm （the maximum difference reached 64%）. Using 4, 6 and 8 MV X-rays, Sc measured by the 0.13 cc chamber with an elongated source-to-surface distance （SSD） （〉150 cm） were 25.4%, 6.9%, 24.6%, and 1.4%, 1,4%, 2.2% greater than those measured by a standard SSD （100 cm） for 1 cm×1 cm and 2 cm×2 cm beams respectively; although there was no significant difference in Sc measurements for the beams of ≥2 cm×2 cm using the elongated SSD of the 0.13 cc and the 0.01 cc ion chambers, Sc measured by the 0.13 cc ion chamber were 0.2%, 8.5%, 3.4% less than those measured by the 0.01 cc ion chamber for the 1 cm×1 cm beam. For the 1 cm×1 cm beam, the TMR of the depth deeper than 15 cm measured with the 0.01 cc ion chamber was about 4% different compared with that measured with the 0.13cc ion chamber; for radiation fields of ≥2 cm×2 cm, the differences of TMR between the 0.01 cc and 0.13 cc c