Song Hong, He Bin, Prideaux Andrew, Du Yong, Frey Eric, Kasecamp Wayne, Ladenson Paul W, Wahl Richard L, Sgouros George
Division of Nuclear Medicine, Russell H Morgan Department of Radiology and Radiological Science, School of Medicine, Johns Hopkins University, Baltimore, MD 21231, USA.
J Nucl Med. 2006 Dec;47(12):1985-94.
The lungs are the most frequent sites of distant metastasis in differentiated thyroid carcinoma. Radioiodine treatment planning for these patients is usually performed following the Benua-Leeper method, which constrains the administered activity to 2.96 GBq (80 mCi) whole-body retention at 48 h after administration to prevent lung toxicity in the presence of iodine-avid lung metastases. This limit was derived from clinical experience, and a dosimetric analysis of lung and tumor absorbed dose would be useful to understand the implications of this limit on toxicity and tumor control. Because of highly nonuniform lung density and composition as well as the nonuniform activity distribution when the lungs contain tumor nodules, Monte Carlo dosimetry is required to estimate tumor and normal lung absorbed dose. Reassessment of this toxicity limit is also appropriate in light of the contemporary use of recombinant thyrotropin (thyroid-stimulating hormone) (rTSH) to prepare patients for radioiodine therapy. In this work we demonstrated the use of MCNP, a Monte Carlo electron and photon transport code, in a 3-dimensional (3D) imaging-based absorbed dose calculation for tumor and normal lungs.
A pediatric thyroid cancer patient with diffuse lung metastases was administered 37 MBq of (131)I after preparation with rTSH. SPECT/CT scans were performed over the chest at 27, 74, and 147 h after tracer administration. The time-activity curve for (131)I in the lungs was derived from the whole-body planar imaging and compared with that obtained from the quantitative SPECT methods. Reconstructed and coregistered SPECT/CT images were converted into 3D density and activity probability maps suitable for MCNP4b input. Absorbed dose maps were calculated using electron and photon transport in MCNP4b. Administered activity was estimated on the basis of the maximum tolerated dose (MTD) of 27.25 Gy to the normal lungs. Computational efficiency of the MCNP4b code was studied with a simple segmentation approach. In addition, the Benua-Leeper method was used to estimate the recommended administered activity. The standard dosing plan was modified to account for the weight of this pediatric patient, where the 2.96-GBq (80 mCi) whole-body retention was scaled to 2.44 GBq (66 mCi) to give the same dose rate of 43.6 rad/h in the lungs at 48 h.
Using the MCNP4b code, both the spatial dose distribution and a dose-volume histogram were obtained for the lungs. An administered activity of 1.72 GBq (46.4 mCi) delivered the putative MTD of 27.25 Gy to the lungs with a tumor absorbed dose of 63.7 Gy. Directly applying the Benua-Leeper method, an administered activity of 3.89 GBq (105.0 mCi) was obtained, resulting in tumor and lung absorbed doses of 144.2 and 61.6 Gy, respectively, when the MCNP-based dosimetry was applied. The voxel-by-voxel calculation time of 4,642.3 h for photon transport was reduced to 16.8 h when the activity maps were segmented into 20 regions.
MCNP4b-based, patient-specific 3D dosimetry is feasible and important in the dosimetry of thyroid cancer patients with avid lung metastases that exhibit prolonged retention in the lungs.
肺是分化型甲状腺癌远处转移最常见的部位。这些患者的放射性碘治疗计划通常按照贝努阿 - 利珀方法进行，该方法将给药活度限制在给药后48小时全身滞留量为2.96 GBq（80 mCi），以防止在存在吸碘性肺转移的情况下发生肺毒性。这个限制源于临床经验，对肺和肿瘤吸收剂量进行剂量学分析将有助于理解该限制对毒性和肿瘤控制的影响。由于肺密度和组成高度不均匀，以及当肺中含有肿瘤结节时活度分布不均匀，需要采用蒙特卡罗剂量学来估计肿瘤和正常肺的吸收剂量。鉴于当代使用重组促甲状腺素（甲状腺刺激激素）（rTSH）为患者准备放射性碘治疗，重新评估这个毒性限制也是合适的。在这项工作中，我们展示了使用蒙特卡罗电子和光子传输代码MCNP进行基于三维（3D）成像的肿瘤和正常肺吸收剂量计算。
一名患有弥漫性肺转移的儿童甲状腺癌患者在使用rTSH准备后给予37 MBq的（131）I。在给药后27、74和147小时对胸部进行SPECT/CT扫描。肺中（131）I的时间 - 活度曲线从全身平面成像得出，并与通过定量SPECT方法获得的曲线进行比较。重建并配准的SPECT/CT图像被转换为适合MCNP4b输入的3D密度和活度概率图。使用MCNP4b中的电子和光子传输计算吸收剂量图。根据正常肺的最大耐受剂量（MTD）27.25 Gy估计给药活度。使用简单分割方法研究了MCNP4b代码的计算效率。此外，使用贝努阿 - 利珀方法估计推荐的给药活度。根据该儿童患者的体重修改了标准给药方案，将2.96 GBq（80 mCi）的全身滞留量调整为2.44 GBq（66 mCi），以使48小时时肺中的剂量率相同，为43.6 rad/h。
使用MCNP4b代码，获得了肺的空间剂量分布和剂量 - 体积直方图。1.72 GBq（46.4 mCi）的给药活度使肺达到假定的MTD 27.25 Gy，肿瘤吸收剂量为63.7 Gy。直接应用贝努阿 - 利珀方法，得到的给药活度为3.89 GBq（105.0 mCi），当应用基于MCNP的剂量学时，肿瘤和肺吸收剂量分别为144.2和61.6 Gy。当将活度图分割为20个区域时，光子传输的逐体素计算时间从4642.3小时减少到16.8小时。
基于MCNP4b的、针对患者的3D剂量学在对肺中呈现长时间滞留的吸碘性肺转移甲状腺癌患者的剂量学中是可行且重要 的。
