Muñoz Luis, Kron Tomas, Petasecca Marco, Bucci Joseph, Jackson Michael, Metcalfe Peter, Rosenfeld Anatoly B, Biasi Giordano
Genesiscare Flinders Private Hospital, Bedford Park, SA, Australia.
Centre for Medical Radiation Physics, University of Wollongong, NSW, Australia.
J Appl Clin Med Phys. 2021 Feb;22(2):185-193. doi: 10.1002/acm2.13160. Epub 2021 Jan 13.
Stereotactic radiosurgery (SRS) can be delivered with a standard linear accelerator (linac). At institutions having more than one linac, beam matching is common practice. In the literature, there are indications that machine central axis (CAX) matching for broad fields does not guarantee matching of small fields with side ≤2 cm. There is no indication on how matching for broad fields on axis translates to matching small fields off axis. These are of interest to multitarget single-isocenter (MTSI) SRS planning and the present work addresses that gap in the literature.
We used 6 MV flattening filter free (FFF) beams from four Elekta VersaHD® linacs equipped with an Agility™ multileaf collimator (MLC). The linacs were strictly matched for broad fields on CAX. We compared output factors (OPFs) and effective field size, measured concurrently using a novel 2D solid-state dosimeter "Duo" with a spatial resolution of 0.2 mm, in square and rectangular static fields with sides from 0.5 to 2 cm, either on axis or away from it by 5 to 15 cm.
Among the four linacs, OPF for fields ≥1 × 1 cm ranged 1.3% on CAX, whereas off axis a maximum range of 1.9% was observed at 15 cm. A larger variability in OPF was noted for the 0.5 × 0.5 cm field, with a range of 5.9% on CAX, which improved to a maximum of 2.3% moving off axis. Two linacs showed greater consistency with a range of 1.4% on CAX and 2.2% at 15 cm off axis. Between linacs, the effective field size varied by <0.04 cm in most cases, both on and off axis. Tighter matching was observed for linacs with a similar focal spot position.
Verification of small-field consistency for matched linacs used for SRS is an important task for dosimetric validation. A significant benefit of concurrent measurement of field size and OPF allowed for a comprehensive assessment using a novel diode array. Our study showed the four linacs, strictly matched for broad fields on CAX, were still matched down to a field size of 1 x 1 cm on and off axis.
立体定向放射外科手术（SRS）可通过标准直线加速器（直线加速器）进行。在拥有多台直线加速器的机构中，射束匹配是常见的做法。在文献中，有迹象表明，对于大射野的机器中心轴（CAX）匹配并不能保证边长≤2 cm的小射野的匹配。目前尚无关于轴向上大射野匹配如何转化为轴外小射野匹配的相关说明。这些对于多靶点单等中心（MTSI）SRS计划很重要，而本研究旨在填补文献中的这一空白。
我们使用了四台配备Agility™多叶准直器（MLC）的Elekta VersaHD®直线加速器的6 MV无均整器（FFF）射束。这些直线加速器在CAX上针对大射野进行了严格匹配。我们使用一种空间分辨率为0.2 mm的新型二维固态剂量仪“Duo”，同时测量了边长从0.5到2 cm的方形和矩形静态射野在轴上以及离轴5至15 cm处的输出因子（OPF）和有效射野尺寸。
在这四台直线加速器中，≥1×1 cm射野的OPF在CAX上的范围为1.3%，而在离轴15 cm处观察到的最大范围为1.9%。对于0.5×0.5 cm的射野，OPF的变化更大，在CAX上的范围为5.9%，离轴后最大改善至2.3%。两台直线加速器在CAX上的范围为1.4%，离轴15 cm处为2.2%，显示出更高的一致性。在直线加速器之间，有效射野尺寸在轴上和离轴情况下大多数情况下变化<0.04 cm。对于焦点位置相似的直线加速器，观察到更紧密的匹配。
验证用于SRS的匹配直线加速器的小射野一致性是剂量验证的一项重要任务。同时测量射野尺寸和OPF的一个显著优点是可以使用新型二极管阵列进行全面评估。我们的研究表明，在CAX上针对大射野进行严格匹配的这四台直线加速器，在轴上和离轴情况下，直至1×1 cm的射野尺寸仍能保持匹配。
