Mansouri Zahra, Salimi Yazdan, Koutsouvelis Nikolaos, Zaidi Habib
Division of Nuclear Medicine and Molecular Imaging, Diagnostic Department, Geneva University Hospital, Geneva, Switzerland.
Division of Radiation Oncology, Geneva University Hospital, Geneva, Switzerland.
Eur J Nucl Med Mol Imaging. 2025 Aug 8. doi: 10.1007/s00259-025-07461-2.
Selective Internal Radiation Therapy (SIRT) with Y-microspheres, and Stereotactic Body Radiation Therapy (SBRT), are well-established treatment strategies for liver malignancies. Combining these two modalities has proven to be an effective and safe approach for addressing undertreated tumor regions from initial SIRT through a boost dose from SBRT. The complex dosimetry process, which includes image-based dosimetry of Y-SIRT and SBRT dose painting, requires precise dosimetry verification. In this study, gel dosimetry is proposed as a valuable novel tool to capture the 3D absorbed dose contributions from both treatments with high spatial resolution.
The MAGIC-f polymer gel dosimeter was used in three experiments: external beam radiation therapy (EBRT), Y-SIRT, and combination therapy. For EBRT, eight calibration vials filled with gel were irradiated with absorbed doses ranging from 0 to 21 Gy. In the Y-SIRT experiments, eleven vials were filled with gel containing varying concentrations of Y-citrate, delivering doses from 0 to 44 Gy after 68 h. For combination therapy, eight vials received 0-7 Gy from Y-citrate after 68 h, followed by an additional 0-7 Gy from SBRT (final doses 0-14 Gy). A phantom, designed to simulate a tumor within a normal liver, was created with the sphere receiving 10 Gy after 68 h from SIRT and another 10 Gy from SBRT using 6MV photons (in total 20 Gy). PET/CT imaging was conducted before SBRT, and the Medical Internal Radiation Dose (MIRD) schema was used for dose calculations. For each experiment, MR T2-weighted imaging was performed using a 3 Tesla scanner, and R2 maps (1/s) were analyzed to establish a dose-response relationship between absorbed dose (Gy) and R2. The gel's sensitivity to each irradiation was measured, and phantom's dose maps were evaluated using mean absorbed dose, dose volume histograms (DVHs), line profiles, and isodose maps.
The dose response of the gel was linear within the irradiated ranges for EBRT and combination therapy. The linear range for Y-SIRT was between 0 and 16.75 Gy. The sensitivity of the gel was 0.380, 0.758, and 0.713 s⁻¹/Gy for EBRT, Y-SIRT, and combination experiments, respectively. In the phantom irradiated with combination therapy, a saturated area was observed in the central core of the sphere, surrounded by an underestimation area with a diameter of 4.5 mm. The mean absorbed dose values within the sphere were 9.83 Gy, 9.71 Gy, and 18.58 Gy from SBRT, SIRT, and combination therapy, respectively. For the cylinder, these values were 1.29 Gy, 0.61 Gy, and 2.68 Gy, respectively. The DVHs, line profiles, and isodose lines for the combination therapy demonstrated the cumulative effects of the absorbed dose from both treatments.
This is the first study demonstrating the feasibility of using MAGIC-f gel dosimetry to directly measure 3D dose distributions from combined Y-SIRT and SBRT. Compared to PET/CT-based dosimetry, the gel method offers superior spatial resolution and enables objective physical verification of complex dose distributions. This technique has strong potential for quality assurance in theranostic protocols, particularly for beta emitters and, potentially, alpha-emitting radiotracers in future applications.
使用钇微球的选择性体内放射治疗（SIRT）和立体定向体部放射治疗（SBRT）是治疗肝脏恶性肿瘤的成熟治疗策略。事实证明，将这两种方式结合起来是一种有效且安全的方法，可通过SBRT的增敏剂量来处理初始SIRT治疗后未充分治疗的肿瘤区域。复杂的剂量测定过程，包括基于图像的Y-SIRT剂量测定和SBRT剂量描绘，需要精确的剂量测定验证。在本研究中，凝胶剂量测定法被提议作为一种有价值的新型工具，以高空间分辨率获取两种治疗的三维吸收剂量贡献。
MAGIC-f聚合物凝胶剂量计用于三个实验：外照射放疗（EBRT）、Y-SIRT和联合治疗。对于EBRT，八个装有凝胶的校准瓶接受0至21 Gy的吸收剂量照射。在Y-SIRT实验中，十一个瓶中装有含不同浓度柠檬酸钇的凝胶，68小时后剂量范围为0至44 Gy。对于联合治疗，八个瓶在68小时后接受来自柠檬酸钇的0至7 Gy剂量，随后再接受来自SBRT的0至7 Gy剂量（最终剂量0至14 Gy）。设计了一个模拟正常肝脏内肿瘤的体模，球体在68小时后接受来自SIRT的10 Gy剂量，以及使用6MV光子来自SBRT的另外10 Gy剂量（总共20 Gy）。在SBRT之前进行PET/CT成像，并使用医学内部辐射剂量（MIRD）模式进行剂量计算。对于每个实验，使用3特斯拉扫描仪进行MR T2加权成像，并分析R2图（1/秒）以建立吸收剂量（Gy）与R2之间的剂量响应关系。测量凝胶对每次照射的敏感性，并使用平均吸收剂量、剂量体积直方图（DVH）、线轮廓和等剂量图评估体模的剂量图。
在EBRT和联合治疗的照射范围内，凝胶的剂量响应呈线性。Y-SIRT的线性范围在0至16.75 Gy之间。凝胶在EBRT、Y-SIRT和联合实验中的敏感性分别为0.380、0.758和0.713 s⁻¹/Gy。在接受联合治疗照射 的体模中，在球体的中心核心观察到一个饱和区域，周围是一个直径为4.5毫米的低估区域。球体内部来自SBRT、SIRT和联合治疗的平均吸收剂量值分别为9.83 Gy、9.71 Gy和18.58 Gy。对于圆柱体，这些值分别为1.29 Gy、0.61 Gy和2.68 Gy。联合治疗的DVH、线轮廓和等剂量线显示了两种治疗吸收剂量的累积效应。
这是第一项证明使用MAGIC-f凝胶剂量测定法直接测量联合Y-SIRT和SBRT的三维剂量分布可行性的研究。与基于PET/CT的剂量测定相比，凝胶方法具有更高的空间分辨率，并能够对复杂的剂量分布进行客观的物理验证。该技术在治疗诊断方案的质量保证方面具有强大潜力，特别是对于β发射体，并且在未来应用中可能对α发射放射性示踪剂也有潜力。
