McNairn Connor, Pasricha Prarthana, Milligan Kirsty, Mansour Iymad R, Cassol Edana, Chauhan Vinita, Andrews Jeffrey L, Subedi Sanjeena, Jirasek Andrew, Muir Bryan R, Thomson Rowan M, Murugkar Sangeeta
Department of Physics, Carleton University, Ottawa, Ontario, Canada.
Department of Physics, University of British Columbia - Okanagan Campus, Kelowna, Canada.
Med Phys. 2025 Jul;52(7):e17900. doi: 10.1002/mp.17900.
Micrometer-scale dosimetry is crucial when estimating the energy deposited within micrometer-scale biological targets exposed to low doses or high dose gradients. Raman micro-spectroscopy read-out of radiochromic films (RCFs) permits micrometer-scale resolution; this presents a novel opportunity to explore its feasibility for experimental microdosimetry.
The purpose of this work was to develop a novel approach towards generating data for experimental microdosimetry. The objective was to develop a method based on high (1-2 µm) spatial resolution Raman micro-spectroscopy of RCFs, ensuring reproducibility of data while producing two-dimensional intensity maps of the Raman response.
EBT3 RCFs were irradiated to doses between 0.2 Gy and 2 Gy using a clinical linear accelerator. Raman spectra were collected using a custom Raman microscope fitted with 40× and 60× water immersion (WI) objectives, and a commercial Raman microscope utilizing a 100× dry objective. The excitation source of the custom setup was circularly polarized to minimize the influence of polarization on the film read-out. The Raman response of the RCFs was measured over a 100 × 100 µm region of interest (ROI) with a 10 × 10 grid. The Raman response of the active layer of the film was normalized to the radiation-insensitive monomer peak at 2260 cm. The Raman intensities of the 1445 cm and 2060 cm peaks were used to generate dose response curves for each microscope setup. Maps of the Raman intensity over the 100 × 100 µm ROI for the 60× WI setup were used to quantify the heterogeneity in the film response. Higher resolution point-scans were performed over a 20 × 20 µm ROI for 0 Gy and 2 Gy samples.
The dose response of each Raman microscope setup over the 0-2 Gy dose range was linear (r of 0.98) after normalization to the 2260 cm Raman peak in the active layer. The slope of the dose response curve of the commercial microscope exhibited dependence on the film orientation; this was minimized with the custom Raman setup by using the circularly polarized excitation source. The relative standard deviation (RSD) of the 1445 cm peak Raman intensity over the 100 × 100 µm ROIs was significant (∼11%) for each microscope setup, and independent of dose. The Raman intensity distribution maps revealed that the heterogeneity in Raman response across the ROI was on the same size-scale (1.62 µm × 9.4 µm) of the lithium salt of pentacosa-10,12-diynoic acid (LiPCDA) crystals comprising the active layer of the film.
This work explored the feasibility of a Raman micro-spectroscopy-based read-out technique of RCFs for experimental microdosimetry. Utilizing the 2260 cm peak in the Raman spectrum as an internal standard for normalization produced a linear (r of 0.98) dose-response curve in the 0-2 Gy dose range. Utilizing circularly polarized laser excitation minimized the polarization dependence of the film and increased the reproducibility of the Raman measurements. Spatial heterogeneity in the concentration of PCDA crystals in the active layer was visualized based on two-dimensional maps of the Raman intensity response to explore the implications on microdosimetry.
在估计暴露于低剂量或高剂量梯度下的微米级生物靶标内沉积的能量时，微米级剂量测定至关重要。拉曼显微光谱法对放射变色薄膜（RCFs）进行读出可实现微米级分辨率；这为探索其在实验微剂量测定中的可行性提供了一个新机会。
本研究的目的是开发一种用于生成实验微剂量测定数据的新方法。目标是开发一种基于对RCFs进行高（1 - 2微米）空间分辨率拉曼显微光谱分析的方法，确保数据的可重复性，同时生成拉曼响应的二维强度图。
使用临床直线加速器将EBT3 RCFs辐照至0.2 Gy至2 Gy的剂量。使用配备40×和60×水浸（WI）物镜的定制拉曼显微镜以及使用100×干物镜的商用拉曼显微镜收集拉曼光谱。定制装置的激发源采用圆偏振，以最小化偏振对薄膜读出的影响。在100×100微米的感兴趣区域（ROI）上，以10×10的网格测量RCFs的拉曼响应。将薄膜活性层的拉曼响应归一化为2260厘米处对辐射不敏感的单体峰。使用1445厘米和2060厘米处峰的拉曼强度为每个显微镜装置生成剂量响应曲线。对于60×WI装置，在100×100微米ROI上的拉曼强度图用于量化薄膜响应中的不均匀性。对0 Gy和2 Gy样品在20×20微米ROI上进行了更高分辨率的点扫描。
在将活性层中2260厘米处的拉曼峰归一化后，每个拉曼显微镜装置在0 - 2 Gy剂量范围内的剂量响应呈线性（r = 0.98）。商用显微镜剂量响应曲线的斜率表现出对薄膜取向的依赖性；通过使用圆偏振激发源的定制拉曼装置，这种依赖性得以最小化。对于每个显微镜装置，在100×100微米ROI上1445厘米处峰的拉曼强度的相对标准偏差（RSD）显著（约11%），且与剂量无关。拉曼强度分布图显示，整个ROI上拉曼响应的不均匀性与构成薄膜活性层的二十五碳-10,12-二炔酸锂盐（LiPCDA）晶体的尺寸尺度相同（1.62微米×9.4微米）。
本研究探索了基于拉曼显微光谱法读出技术的RCFs用于实验微剂量测定的可行性。利用拉曼光谱中2260厘米处的峰作为归一化的内标，在0 - 2 Gy剂量范围内产生了线性（r = 0.98）剂量响应曲线。利用圆偏振激光激发最小化了薄膜的偏振依赖性，并提高了拉曼测量的可重复性。基于拉曼强度响应的二维图可视化了活性层中PCDA晶体浓度的空间不均匀性，以探索其对微剂量测定的影响。
