Swalec J J, Leavitt D D, Moeller J H
Department of Radiology, University of Utah Health Sciences Center, Salt Lake City 84132.
Int J Radiat Oncol Biol Phys. 1994 Aug 30;30(1):205-10. doi: 10.1016/0360-3016(94)90536-3.
The diffuse shape of the electron beam used for arc therapy requires collimation on the patient's surface to sharply define treatment field edges. The electron beam arcs 10-15 degrees past field edges defined by a custom fitted and manufactured cast which acts as a tertiary collimator. This allows the entire beam profile to be integrated at the field edge. The tertiary collimator is heavy and bulky, requiring two therapists to lift and position the cast. An alternative technique for field edge definition would require the electron arc collimators to dynamically close to zero, while maintaining the projection of the leading edge of the field coincident with the geometric edge of the treatment field. This would allow integration of the entire electron arc profile and maintain a sharply defined treatment edge at the medial and lateral margins of the arc. The present customized cast could be replaced by generic lead strips at only the superior and inferior treatment field borders. This study investigates the dosimetry of dynamically collimated electron arc treatment volumes at field margins and its potential for eliminating the need for tertiary collimation at the arc field margins.
Electron arc isodose distributions were calculated using a pencil beam algorithm for treatment volumes defined by tertiary collimation at the surface of a cylindrical phantom and compared to distributions generated by simulating dynamic collimation to define the same field edges. Phantom measurements were performed using film densitometry to verify computer predictions.
Penumbra width is one measure of the sharpness of dose fall off at a treatment field edge. We define it as the distance between the 90% and 20% isodose lines at the field edge measured orthogonal to the incident electron beam. Calculations and phantom film densitometry measurements were performed for electron energies from 6-20 MeV. Dynamic and tertiary collimation both reduce penumbra width by approximately 50% compared to no collimation. There is a small advantage in minimizing penumbra width at low electron energy with tertiary collimation. This shifts to a small advantage with dynamic collimation at high electron energy.
Dynamic collimation produces a field edge isodose distribution equivalent to tertiary collimation for clinical purposes. These results suggest that tertiary collimation at medial and lateral electron are treatment field margins can be eliminated with dynamic collimation. This should result in greater clinical acceptance of breast electron arc therapy as the capacity for dynamic collimation is added to the next generation of linear accelerators.
用于弧形治疗的电子束呈弥散状，需要在患者体表进行准直，以清晰界定治疗野边缘。电子束弧形会超出由定制铸造且用作三级准直器的铸件所界定的野边缘10 - 15度。这使得整个射野轮廓能在场边缘处整合。三级准直器沉重且体积庞大，需要两名治疗师抬起并放置铸件。一种界定野边缘的替代技术要求电子弧形准直器动态关闭至零，同时保持野前缘的投影与治疗野的几何边缘重合。这将允许整合整个电子弧形轮廓，并在弧形的内侧和外侧边缘保持清晰界定的治疗边缘。当前的定制铸件仅在上部和下部治疗野边界处可由通用铅条替代。本研究调查了在野边缘处动态准直的电子弧形治疗体积的剂量学，以及其在弧形野边缘消除三级准直需求的潜力。
使用笔形束算法计算圆柱形模体表面由三级准直界定的治疗体积的电子弧形等剂量分布，并与通过模拟动态准直以界定相同野边缘生成的分布进行比较。使用胶片密度测定法进行模体测量以验证计算机预测结果。
半值层宽度是治疗野边缘处剂量下降锐度的一种度量。我们将其定义为在与入射电子束正交方向上测量的野边缘处90%和20%等剂量线之间的距离。对6 - 20 MeV的电子能量进行了计算和模体胶片密度测定测量。与不进行准直相比，动态准直和三级准直均使半值层宽度减小约50%。在低电子能量下，三级准直在最小化半值层宽度方面有小优势。在高电子能量下，这种优势转向动态准直。
就临床目的而言，动态准直产生与三级准直等效的野边缘等剂量分布。这些结果表明，通过动态准直可消除电子弧形治疗野内侧和外侧边缘的三级准直。随着下一代直线加速器增加动态准直能力，这应会使乳腺电子弧形治疗在临床上更易被接受。
