We developed and validated a dedicated small field back–projection portal dosimetry model for pretreatment and in vivo verification of stereotactic plans entailing small unflattened photon beams. For this purpose an aSi–EPID was commissioned as a small field dosimeter. Small field output factors for 6 MV FFF beams were measured using the PTW microDiamond detector and the Agility 160–leaf MLC from Elekta. The back-projection algorithm developed in our department was modified to better model the small field physics. The feasibility of small field portal dosimetry was validated via absolute point dose differences w.r.t. small static beams, and 5 hypofractionated stereotactic VMAT clinical plans measured with the OCTAVIUS 1000 SRS array dosimeter and computed with the treatment planning system Pinnacle v16.2. Dose reconstructions using the currently clinically applied back–projection model were also computed for comparison. We found that the latter yields underdosage of about −8% for square beams with cross section near 10 mm x 10 mm and about −6% for VMAT treatments with PTV volumes smaller than about 2cm³. With the methods described in this work such errors can be reduced to less than the 3.0% recommendations for clinical use. Our results indicate that aSi–EPIDs can be used as accurate small field radiation dosimeters, offering advantages over point dose detectors, the correct positioning and orientation of which is challenging for routine clinical QA.

我们开发并验证了用于预处理和体内的专用小场反投影门户剂量学模型验证需要小的未展平光子束的立体定向计划。为此目的，aSi-EPID 被用作小型场剂量计。使用 PTW microDiamond 检测器和 Elekta 的 Agility 160-leaf MLC 测量 6 MV FFF 光束的小场输出因子。我们部门开发的反投影算法经过修改，以更好地模拟小场物理。通过小静态光束的绝对点剂量差异和 5 个大分割立体定向 VMAT 临床计划，使用 OCTAVIUS 1000 SRS 阵列剂量计测量并使用治疗计划系统 Pinnacle v16.2 计算，验证了小野门剂量测定的可行性。还计算了使用当前临床应用的反投影模型的剂量重建以进行比较。³ . 使用本工作中描述的方法，此类错误可以减少到低于临床使用建议的 3.0%。我们的结果表明 aSi-EPID 可用作精确的小场辐射剂量计，与点剂量探测器相比具有优势，点剂量探测器的正确定位和方向对常规临床 QA 具有挑战性。