Nakanishi Kohei, Fujita Naotoshi, Iwanaga Haruna, Asano Yuki, Abe Shinji, Nishii Ryuichi, Kato Katsuhiko
Functional Medical Imaging, Biomedical Imaging Sciences, Division of Advanced Information Health Sciences, Department of Integrated Health Sciences, Nagoya University Graduate School of Medicine, 1-1-20 Daiko-Minami, Higashi-Ku, Nagoya, Aichi, Japan.
Department of Radiological Technology, Nagoya University Hospital, 65 Tsurumai-Cho, Showa-Ku, Nagoya, Aichi, Japan.
Ann Nucl Med. 2025 Feb;39(2):199-207. doi: 10.1007/s12149-024-01987-5. Epub 2024 Oct 1.
Dead-time loss is reported to be non-negligible for some patients with a high tumor burden in Lu-177 radionuclide therapy, even if the administered activity is 7.4 GBq. Hence, we proposed a simple method to shorten the apparent dead time and reduce dead-time loss using a thin lead sheet in previous work. The collimator surface of the gamma camera was covered with a lead sheet in our proposed method. While allowing the detection of 208-keV gamma photons of Lu-177 that penetrate the sheet, photons with energies lower than 208 keV, which cause dead-time loss, were shielded. In this study, we evaluated the usefulness of tungsten functional paper (TFP) for the proposed method using Monte Carlo simulation.
The count rates in imaging of Lu-177 administered to patients were simulated with the International Commission on Radiological Protection (ICRP) 110 phantom using the GATE Monte Carlo simulation toolkit. The simulated gamma cameras with a 0.5-mm lead sheet, 1.2-mm TFP, or no filter were positioned closely on the anterior and posterior sides of the phantom. The apparent dead times and dead-time losses at 24 h after administration were calculated for an energy window of 208 keV ± 10%. Moreover, the dead-time losses at 24-120 h were analytically assessed using activity excretion data of Lu-177-DOTATATE.
The dead-time loss without a filter was 5% even 120 h after administration in patients with a high tumor burden and slow excretion, while those with a lead sheet and TFP were 0.22 and 0.58 times less than those with no filter, respectively. The count rates with the TFP were 1.3 times higher than those with the lead sheet, and the TFP could maintain primary count rates at 91-94% of those without a filter.
Although the apparent dead time and dead-time loss with the lead sheet were shorter and less than those with TFP, those with TFP were superior to those without a filter. The advantage of TFP over the lead sheet is that the decrease in primary count rates was less.
据报道，在镥 - 177放射性核素治疗中，对于一些肿瘤负荷高的患者，即使给予的活度为7.4 GBq，死时间损失也不可忽略。因此，我们在先前的工作中提出了一种使用薄铅板缩短表观死时间并减少死时间损失的简单方法。在我们提出的方法中，γ相机的准直器表面覆盖有铅板。在允许检测穿透铅板的镥 - 177的208 keVγ光子的同时，屏蔽了导致死时间损失的能量低于208 keV的光子。在本研究中，我们使用蒙特卡罗模拟评估了钨功能纸（TFP）对于所提出方法的有效性。
使用GATE蒙特卡罗模拟工具包，以国际放射防护委员会（ICRP）110体模模拟给予患者镥 - 177后的成像计数率。将模拟的配有0.5 mm铅板、1.2 mm TFP或无滤过器的γ相机紧密放置在体模的前后两侧。计算给药后24小时在208 keV±10%能量窗口下的表观死时间和死时间损失。此外，使用镥 - 177 - DOTATATE的活度排泄数据对24 - 120小时的死时间损失进行分析评估。
对于肿瘤负荷高且排泄缓慢的患者，即使给药后120小时，无滤过器时的死时间损失为5%，而配有铅板和TFP时分别比无滤过器时少0.22倍和0.58倍。使用TFP时的计数率比使用铅板时高1.3倍，并且TFP可以将初始计数率维持在无滤过器时的91 - 94%。
虽然使用铅板时的表观死时间和死时间损失比使用TFP时更短且更小，但使用TFP时优于无滤过器的情况。TFP相对于铅板的优势在于初始计数率的降低较少。
