Somigliana A, Cattaneo G M, Fiorino C, Borelli S, del Vecchio A, Zonca G, Pignoli E, Loi G, Calandrino R, Marchesini R
Divisione di Fisica Sanitaria, Istituto Nazionale per lo Studio e la Cura dei Tumori, Milan, Italy.
Phys Med Biol. 1999 Apr;44(4):887-97. doi: 10.1088/0031-9155/44/4/006.
In stereotactic radiosurgery the choice of appropriate detectors, whether for absolute or relative dosimetry, is very important due to the steep dose gradient and the incomplete lateral electronic equilibrium. For both linac-based and Leksell Gamma Knife radiosurgery units, we tested the use of calibrated radiochromic film to measure absolute doses and relative dose distributions. In addition a small diode was used to estimate the relative output factors. The data obtained using radiochromic and diode detectors were compared with measurements performed with other conventional methods of dosimetry, with calculated values by treatment planning systems and with data prestored in the treatment planning system supplied by the Leksell Gamma Knife (LGK) vendor. Two stereotactic radiosurgery techniques were considered: Leksell Gamma Knife (using gamma-rays from 60Co) and linac-based radiosurgery (LR) (6 MV x-rays). Different detectors were used for both relative and absolute dosimetry: relative output factors (OFs) were estimated by using radiochromic and radiographic films and a small diode; relative dose distributions in the axial and coronal planes of a spherical polystyrene phantom were measured using radiochromic film and calculated by two different treatment planning systems (TPSs). The absolute dose at the sphere centre was measured by radiochromic film and a small ionization chamber. An accurate selection of radiochromic film was made: samples of unexposed film showing a percentage standard deviation of less than 3% were used for relative dose profiles, and for absolute dose and OF evaluations this value was reduced to 1.5%. Moreover a proper calibration curve was made for each set of measurements. With regard to absolute doses, the results obtained with the ionization chamber are in good correlation with radiochromic film-generated data, for both LGK and LR, showing a dose difference of less than 1%. The output factor evaluations, performed using different methods, are in good agreement with a maximum difference of 1.5% for all field sizes considered (LGK and LR) except the 4 mm helmet used in the LGK unit. In this case, differences exist between diode and radiochromic film measurements and both detectors show data values larger than the prestored OF value of 0.80. Dose profiles measured by radiochromic film and calculated are in excellent agreement for both LGK and LR with a maximum deviation of less than 1.0 mm, when full widths of the dose profiles at 20%, 50%, 80% levels are considered. When external photon beams are used in stereotactic radiosurgery, the 'well selected' radiochromic films are very accurate detectors both for relative and absolute dosimetry. The experimental results, obtained using both radiochromic and diode detectors, show that the 4 mm helmet relative output factor could be underestimated.
在立体定向放射外科中，由于剂量梯度陡峭和横向电子平衡不完全，选择合适的探测器（无论是用于绝对剂量测定还是相对剂量测定）非常重要。对于基于直线加速器的和Leksell伽玛刀放射外科设备，我们测试了使用校准的放射变色胶片来测量绝对剂量和相对剂量分布。此外，还使用了一个小二极管来估计相对输出因子。将使用放射变色和二极管探测器获得的数据与使用其他传统剂量测定方法进行的测量、治疗计划系统的计算值以及Leksell伽玛刀（LGK）供应商提供的治疗计划系统中预先存储的数据进行了比较。考虑了两种立体定向放射外科技术：Leksell伽玛刀（使用来自60Co的伽马射线）和基于直线加速器的放射外科（LR）（6MV X射线）。相对和绝对剂量测定使用了不同的探测器：相对输出因子（OFs）通过使用放射变色和射线照相胶片以及一个小二极管来估计；使用放射变色胶片测量球形聚苯乙烯体模轴向和冠状面的相对剂量分布，并由两个不同的治疗计划系统（TPSs）进行计算。球体中心的绝对剂量通过放射变色胶片和一个小电离室进行测量。对放射变色胶片进行了精确选择：对于相对剂量分布，使用未曝光胶片样品，其百分比标准偏差小于3%，对于绝对剂量和OF评估，该值降至1.5%。此外，为每组测量制作了适当的校准曲线。关于绝对剂量，对于LGK和LR，电离室获得的结果与放射变色胶片生成的数据具有良好的相关性，剂量差异小于1%。使用不同方法进行的输出因子评估具有良好的一致性，对于所有考虑的射野尺寸（LGK和LR），最大差异为1.5%，但LGK设备中使用的4mm头盔除外。在这种情况下，二极管和放射变色胶片测量之间存在差异，并且两个探测器显示的数据值都大于预先存储的OF值0.80。当考虑剂量分布在20%、50%、80%水平的全宽时，放射变色胶片测量和计算的剂量分布对于LGK和LR都具有极好的一致性，最大偏差小于1.0mm。当在立体定向放射外科中使用外部光子束时，“精心挑选”的放射变色胶片对于相对和绝对剂量测定都是非常准确的探测器。使用放射变色和二极管探测器获得的实验结果表明，4mm头盔的相对输出因子可能被低估。
