Dirkx M L, Heijmen B J, Korevaar G A, van Os M J, Stroom J C, Koper P C, Levendag P C
Daniel den Hoed Cancer Center/University Hospital Rotterdam, The Netherlands.
Int J Radiat Oncol Biol Phys. 1997 Jul 15;38(5):1123-9. doi: 10.1016/s0360-3016(97)00287-3.
In axial, coplanar treatments with multiple fields, the superior and inferior ends of a planning target volume (PTV) are at risk to get underdosed due to the overlapping penumbras of all treatment fields. We have investigated a technique using intensity modulated x-ray beams that allows the use of small margins for definition of the superior and inferior field borders while still reaching a minimum PTV-dose of 95% of the isocenter dose.
The applied intensity modulated beams, generated with a multileaf collimator, include narrow (1.1-1.6 cm) boost fields to increase the dose in the superior and inferior ends of the PTV. The benefits of this technique have been assessed using 3D treatment plans for 10 prostate cancer patients. Treatment planning was performed with the Cadplan 3D planning system (Varian-Dosetek). Dose calculations for the narrow boost fields have been compared with measurements. The application of the boost fields has been tested on the MM50 Racetrack Microtron (Scanditronix Medical AB), which allows fully computer-controlled setup of all involved treatment fields.
Compared to our standard technique, the superior-inferior field length can be reduced by 1.6 cm, generally yielding smaller volumes of rectum and bladder in the high dose region. For the narrow boost fields, calculated relative dose distributions agree within 2% or 0.2 cm with measured dose distributions. For accurate monitor unit calculations, the phantom scatter table used in the Cadplan system had to be modified using measured data for square fields smaller than 4 x 4 cm2. The extra time needed at the MM50 for the setup and delivery of the boost fields is usually about 1 min.
The proposed use of intensity modulated beams yields improved conformal dose distributions for treatment of prostate cancer patients with a superior-inferior field size reduction of 1.6 cm. Treatments of other tumor sites can also benefit from the application of the boost fields.
在采用多个射野的轴向共面治疗中，计划靶区（PTV）的上端和下端有因所有治疗射野的半值层重叠而剂量不足的风险。我们研究了一种使用调强X射线束的技术，该技术允许在定义上下射野边界时使用较小的边界，同时仍能达到PTV最小剂量为等中心剂量的95%。
使用多叶准直器产生的调强射野包括窄（1.1 - 1.6厘米）的加量射野，以增加PTV上端和下端的剂量。已使用10例前列腺癌患者的三维治疗计划评估了该技术的益处。使用Cadplan三维治疗计划系统（瓦里安 - Dosetek）进行治疗计划。已将窄加量射野的剂量计算与测量结果进行了比较。已在MM50跑道式电子回旋加速器（斯堪的亚医疗公司）上测试了加量射野的应用，该加速器允许对所有相关治疗射野进行完全计算机控制的设置。
与我们的标准技术相比，上下射野长度可减少1.6厘米，通常在高剂量区域使直肠和膀胱的体积更小。对于窄加量射野，计算得到的相对剂量分布与测量剂量分布在2%或0.2厘米范围内相符。为了进行精确的监测单位计算，必须使用小于4×4平方厘米的方形射野的测量数据对Cadplan系统中使用的体模散射表进行修改。在MM50上设置和输送加量射野所需的额外时间通常约为1分钟。
所提议的调强射野的使用可为前列腺癌患者的治疗产生改善的适形剂量分布，上下射野尺寸减小1.6厘米。其他肿瘤部位的治疗也可从加量射野的应用中受益。
