Cazzaniga L F, Frigerio M
Divisione di Radioterapia Oncologica, Azienda Ospedaliera S. Anna, Como.
Radiol Med. 1997 Dec;94(6):664-70.
Quality controls in radiotherapy allow to check the correct running of treatment units and to test our procedures. Portal films taken during the first treatment session are used in quality assurance programs to compare scheduled to administered doses.
To analyze the accuracy of patient positioning in pelvic cancer irradiation, a retrospective study was carried out on 50 treatment schedules carried out at the Radiotherapy Department of S. Anna Hospital (Como, Italy) from June to December, 1996. We checked field mispositioning for correlations with patient or treatment variables, such as patient age, sex, weight, thickness, or unit type. We took one portal film at the first treatment session and checked field positioning by measuring the distance of the isocenter from fixed anatomical structures on the simulation film and on the portal film taken in anteroposterior and lateral projections.
Four vectors were defined to evaluate field mispositioning along right-left (ADS), craniocaudal (ACC, LCC) and anteroposterior (AP) directions. The average values of these four vectors were respectively 2.94, 5.23, 5.54 and 3.20 mm. We found a major shift in field centering leftward and toward the patient's feet. To obtain more information about the total isocenter displacement, a vector T was calculated by summing the vectors ADS, ACC and LAP; a further evidence of field mispositioning is given by the vector T mean value (8.66 +/- 4.95 mm). No correlation was found between vector T values and any patient or treatment variable.
The acquaintance with uncertainties requires adequate statistical tools. A single check at treatment beginning could show a systematic error, but not the random fluctuations which can be recognized only with periodic portal films. To correct a possible systematic error without likely worsening set-up conditions, an adequate threshold value must be chosen for field mispositioning, according to each center's historical data.
One portal film at the beginning of treatment is the minimum requirement in a quality assurance program. We feel the need to change our protocol and acquire more than one portal film, because the higher the number of portal films the easier the distinction of systematic from random errors. Using serial portal films, all at the first session, we will be able to introduce quantitative criteria for various action levels.
放射治疗中的质量控制有助于检查治疗设备的正常运行情况并测试我们的程序。首次治疗期间拍摄的射野片用于质量保证计划，以比较计划剂量与实际给予的剂量。
为分析盆腔癌放疗中患者定位的准确性，对1996年6月至12月在意大利科莫市圣安娜医院放疗科进行的50个治疗计划进行了回顾性研究。我们检查了射野错位情况，以确定其与患者或治疗变量的相关性，如患者年龄、性别、体重、体厚或设备类型。在首次治疗时拍摄一张射野片，并通过测量模拟片以及前后位和侧位射野片上固定解剖结构与等中心的距离来检查射野定位。
定义了四个向量来评估沿左右（ADS）、头脚（ACC、LCC）和前后（AP）方向的射野错位。这四个向量的平均值分别为2.94、5.23、5.54和3.20毫米。我们发现射野中心向左和向患者脚部有较大偏移。为获取更多关于等中心总位移的信息，通过将向量ADS、ACC和LAP相加计算出向量T；向量T的平均值（8.66±4.95毫米）进一步证明了射野错位。未发现向量T值与任何患者或治疗变量之间存在相关性。
了解不确定性需要适当的统计工具。在治疗开始时进行一次检查可能会显示出系统误差，但无法发现只有通过定期射野片才能识别的随机波动。为了在不太可能恶化摆位条件的情况下纠正可能的系统误差，必须根据每个中心的历史数据为射野错位选择一个合适的阈值。
治疗开始时拍摄一张射野片是质量保证计划的最低要求。我们认为有必要改变我们的方案并获取不止一张射野片，因为射野片数量越多，区分系统误差和随机误差就越容易。在首次治疗时使用系列射野片，我们将能够为不同的行动水平引入定量标准。
