Department of Oncology, Helsinki University Central Hospital, Helsinki, Finland.
Radiother Oncol. 2009 Oct;93(1):80-6. doi: 10.1016/j.radonc.2009.04.016. Epub 2009 May 9.
Isocentric treatment technique is a standard method in photon radiotherapy with the primary advantage of requiring only a single patient set-up procedure for multiple fields. However, in electron treatments the size of the standard applicators does not generally allow to use an isocentric treatment technique. In this work we have modified and dosimetrically tested electron applicators for isocentric treatments in combination with photons. An isocentric treatment technique with photons and electrons for postmastectomy radiation therapy (PMRT) has been developed with special emphasis on improving the dose uniformity in the field gap area.
Standard electron applicators of two Varian Clinac 2100CD linear accelerators were shortened by 10cm allowing isocentric treatments of 90cm<SSD<100cm in electron fields. Shortened applicators were commissioned and configured for the electron calculation algorithm of the treatment planning system. The field arrangement of PMRT was modified by combining three photon field segments with different gaps and overlaps with the electron field to improve dose uniformity. The developed technique and two other methods for PMRT were compared with each other in the group of 20 patients.
Depth dose characteristics of the shortened applicators remained unchanged from those of the standard applicators. Penumbrae were broadened by 0-3mm depending on electron energy and depth as the air gap was increased from 5cm (standard applicator at SSD=100cm) to 10cm (shortened applicator at SSD=95cm). The dose calculation performance of the modified applicators at 95cm<SSD<100cm was considered similar as with standard applicators at SSD=100cm using the Gaussian pencil beam electron dose calculation algorithm of the treatment planning system (Varian Eclipse). The modified isocentric treatment technique for PMRT was superior than the traditional two-dimensional technique. However, with the tangential photon fields without electrons the even better dose uniformity within PTV could be achieved but with increased irradiation of healthy tissues (lung, heart, and contralateral breast). The modified isocentric technique was also found faster than the traditional technique with SSD=100cm fields.
It is possible to apply an isocentric treatment technique in PMRT with electrons and photons. The homogeneity of the dose distribution can be improved by adding more photon field segments. With the isocentric technique it is possible to achieve even some time sparing in treatment delivery compared with the traditional SSD=100cm technique.
等中心治疗技术是光子放射治疗的标准方法，其主要优点是仅需进行一次患者摆位，即可用于多个射野。然而，在电子治疗中，标准施源器的尺寸通常不允许采用等中心治疗技术。在这项工作中，我们对电子施源器进行了修改，并对其进行了等中心治疗的剂量学测试，同时结合了光子治疗。我们开发了一种适用于乳腺癌根治术后放射治疗（PMRT）的等中心光子-电子治疗技术，特别强调改善射野间隙区域的剂量均匀性。
我们缩短了两台瓦里安 Clinac 2100CD 直线加速器的标准电子施源器 10cm，使电子野在 90cm<SSD<100cm 时能够进行等中心治疗。缩短后的施源器经过认证并配置到治疗计划系统的电子计算算法中。通过结合三个具有不同间隙和重叠的光子野段来修改 PMRT 的射野排列，以改善剂量均匀性。在 20 例患者中，我们比较了开发的技术与另外两种 PMRT 方法。
缩短后的施源器的深度剂量特性与标准施源器相同。随着空气间隙从 5cm（标准施源器在 SSD=100cm 时）增加到 10cm（缩短施源器在 SSD=95cm 时），电子能量和深度的半影分别增宽 0-3mm。在治疗计划系统（瓦里安 Eclipse）的高斯铅笔束电子剂量计算算法中，95cm<SSD<100cm 时修改后的施源器的剂量计算性能被认为与 SSD=100cm 时的标准施源器相似。PMRT 的改良等中心治疗技术优于传统二维技术。然而，对于没有电子的切线光子野，可以实现更好的靶区均匀性，但会增加健康组织（肺、心脏和对侧乳房）的照射。改良的等中心技术也比传统的 SSD=100cm 野技术更快。
可以在 PMRT 中应用电子和光子的等中心治疗技术。通过增加更多的光子野段，可以改善剂量分布的均匀性。与传统的 SSD=100cm 技术相比，等中心技术甚至可以在治疗时间上获得一些优势。
