Vos P H
Department of Clinical Physics, Dr. Bernard Verbeeten Institute, Tilburg, The Netherlands.
Radiother Oncol. 1994 May;31(2):169-75. doi: 10.1016/0167-8140(94)90397-2.
The accuracy of field shaping using standard shielding blocks has been investigated in 157 fields in the thoracic region, the majority for treatment of lung cancer. Accuracy was defined as the width of the tolerance margin around the prescribed field outline that would be needed to accept the full megavoltage field boundary. The use of several positioning aids was evaluated, including a cardboard template that was created with the data already specified for field shape verification of megavoltage fields. Block positioning via tattoos on the skin gives large random field shape errors: assuming a tolerance margin of +/- 5 mm, approximately 50% of these fields would have to be rejected. When a transparent sheet with a drawing of the required field outline was used as positioning aid on the tray without an explicit quality control of the sheet itself approximately 24% of the fields would not be accepted because of an error in the sheet (systematic error) and approximately 20% because of a discrepancy in block position relative to the actual prescription of the sheet (random error). If block positioning is performed via a cardboard template of which the prescription was checked to be better than 3 mm measured in the plane of the isocentre, approximately 96% of the fields will be accepted assuming a tolerance margin of +/- 5 mm: the risk of misplacement of blocks appeared to be very low. Integration of prescription, preparation of the cardboard template, quality control of the template and verification of the megavoltage field shape results in an easy, accurate and reliable way of field shape definition using standard blocks.
在157个胸部区域的射野中研究了使用标准屏蔽挡块进行射野塑形的准确性，其中大多数射野用于肺癌治疗。准确性定义为在规定射野轮廓周围所需的容差范围宽度，以接受整个兆伏级射野边界。评估了几种定位辅助工具的使用情况，包括一个用已指定用于兆伏级射野形状验证的数据制作的硬纸板模板。通过皮肤上的纹身进行挡块定位会产生较大的随机射野形状误差：假设容差范围为±5毫米，大约50%的这些射野将不得不被拒收。当在托盘上使用带有所需射野轮廓图的透明薄片作为定位辅助工具，而未对薄片本身进行明确的质量控制时，由于薄片中的误差（系统误差），大约24%的射野将不被接受，由于挡块位置相对于薄片实际处方的差异（随机误差），大约20%的射野将不被接受。如果通过在等中心平面测量处方优于3毫米的硬纸板模板进行挡块定位，假设容差范围为±5毫米，大约96%的射野将被接受：挡块误放置的风险似乎非常低。将处方、硬纸板模板的制备、模板的质量控制和兆伏级射野形状的验证整合在一起，就形成了一种使用标准挡块进行射野形状定义的简单、准确且可靠的方法。
