Spatial dose distributions in solid tumors from ¹⁸⁶Re transported by liposomes using HS radiochromic media

Purpose: A procedure for the measurement of spatial dose rate distribution of beta particles emitted by ¹⁸⁶Re-liposomes in tumoral tissue, using HS GafChromic films, is presented. Methods: HNSCC xenografts were intratumorally injected with 3.7 or 11.1 MBq of ¹⁸⁶Re-liposomes, and planar gamma camera images were acquired to determine the liposome retention in the tumor. After imaging, rats were sacrificed and tumors were excised and processed in slices; HS film sections were placed between slices and the tumor lobe was reassembled. Tumors and films were kept in the dark at 4°C for 18 h. After irradiation, films were removed and response was read using a transmission scanner. Films were analyzed to determine two-dimensional spatial dose rate distributions and cumulative dose volume histograms. Dose rate distributions were quantified using a ⁶⁰Co calibration curve, the ¹⁸⁶Re physical half-life, and a perturbation factor that takes into account the effect of the film protective layer. Results: Dose rate distributions are highly heterogeneous with maximal dose rates about 0.4 Gy h^-1 in tumors injected with 3.7 MBq and 1.3 Gy h^-1 in tumors injected with 11.1 MBq. Dose volume histograms showed dose distributed in more than 95% and 80% of the tumor when injected with the lower and the higher activity, respectively. Conclusion: The described procedures and techniques have shown the potential and utility of HS GafChromic film for determination of dose rate distributions in solid tumors injected intratumorally with ¹⁸⁶Re-liposomes. The film's structure and the liposomes' biodistribution must be taken into account to obtain quantitative dose measurements.