Department of Chemistry and Konstanz Research School Chemical Biology, University of Konstanz, Germany; Klinik und Praxis fr Strahlentherapie am Klinikum Konstanz, Konstanz, Germany.
Division of Medical Radiation Physics and Department of Radiation Oncology, Inselspital, Bern University Hospital and University of Bern, Switzerland.
Z Med Phys. 2021 Nov;31(4):365-377. doi: 10.1016/j.zemedi.2021.03.006. Epub 2021 Jun 29.
The present study investigates superficial in vivo dosimetry (IVD) by means of a previously proposed electron paramagnetic resonance (EPR) dosimetry system aiming at measuring and verifying total doses delivered by complex radiotherapy treatments. In view of novel regulatory requirements in Germany, differences between measured and planned total doses to the EPR dosimeters are analyzed and compared to reporting thresholds for significant occurrences.
EPR dosimeters, each consisting of one lithium formate monohydrate (LFM) and one polycrystalline l-alanine (ALA) pellet, were attached to the surface of an anthropomorphic head phantom. Three head and neck treatments with total target doses ranging from 30 to 64Gy were fully delivered to the phantom by helical tomotherapy. During each treatment, eight EPR dosimeters were placed at distinct spots: (i) within or next to the planning target volume (PTV), (ii) near to organs at risk including the parotids and the lenses, (iii) at the thyroid lying out-of-field. EPR read out was always performed after all fractions were delivered. EPR results were compared to thermoluminescence dosimeter (TLD) measurements and to the planned total doses derived from the treatment planning system (TPS). Planned total doses to the EPR dosimeters ranged from about 2 to 64Gy.
By taking uncertainties into account, the measured and planned doses were in good agreement. Exceptions occurred mainly at the thyroid (out-of-field) and lenses (extreme sparing). The maximum total dose difference between EPR results and corresponding planned doses was 1.3Gy occurring at the lenses. Remarkably, each LFM and ALA pellet placed within or next to the PTV provided dose values that were within ±4% of the planned dose. Dose deviations from planned dose values were comparable for EPR and TLD measurements.
The results of this proof of principle study suggests that superficial EPR-IVD is applicable in a wide dose range and in various irradiation conditions - being a valuable tool for monitoring cumulative total doses delivered by complex IMRT treatments. EPR-IVD in combination with helical tomotherapy is suitable to reliably detect local dose deviations at superficial dosimeter spots in the order of current national reporting thresholds for significant occurrences (i.e. 10%/4Gy).
本研究通过先前提出的电子顺磁共振（EPR）剂量测定系统进行表面体内剂量测定（IVD），旨在测量和验证复杂放射治疗的总剂量。鉴于德国的新监管要求，分析并比较了 EPR 剂量计的测量总剂量与计划总剂量之间的差异，以确定对重大事件的报告阈值。
将由一水合甲酸锂（LFM）和多晶 L-丙氨酸（ALA）颗粒各一个组成的 EPR 剂量计附着在体模的表面上。对体模进行了三种头颈部治疗，总靶剂量范围为 30 至 64Gy，采用螺旋断层放疗（Tomotherapy）完全输送。在每次治疗过程中，将 8 个 EPR 剂量计放置在不同的位置：（i）在计划靶区（PTV）内或附近，（ii）在危及器官附近，包括腮腺和晶状体，（iii）在甲状腺外野。每次治疗完成后都进行 EPR 读取。EPR 结果与热释光剂量计（TLD）测量值和治疗计划系统（TPS）得出的计划总剂量进行比较。EPR 剂量计的计划总剂量范围约为 2 至 64Gy。
考虑到不确定度的影响，测量剂量与计划剂量吻合良好。异常主要发生在甲状腺（场外）和晶状体（极端剂量 sparing）。EPR 结果与相应计划剂量之间的最大总剂量差异为 1.3Gy，发生在晶状体。值得注意的是，放置在 PTV 内或附近的每个 LFM 和 ALA 颗粒提供的剂量值在计划剂量的±4%以内。EPR 和 TLD 测量的剂量偏差相当。
这项原理验证研究的结果表明，表面 EPR-IVD 在广泛的剂量范围内和各种辐照条件下都是适用的，是监测复杂调强放疗总剂量的有用工具。EPR-IVD 与螺旋断层放疗相结合，适合可靠地检测体模表面深部剂量计点的局部剂量偏差，其量级符合当前国家对重大事件的报告阈值（即 10%/4Gy）。
