Cellular Dosimetry of 197 Hg, 197m Hg and 111 In - Comparison of Dose Deposition and Identification of the Cell and Nuclear Membrane as Important Targets.

PURPOSE: Our aims were to examine the reliability of using MCNP6 to model cellular S-values for the Auger electron (AE) emitters 111 In, 197 Hg and 197m Hg compared to MIRDcell and examine their relative dose deposition in subcellular targets and the AE contribution.

METHODS: A model cell was defined as four concentric spheres consisting of the nucleus (N, radius =2-11μm), cytoplasm (Cy, cell radius =5-12μm), cell membrane (CM) and nuclear membrane (NM) (10 and 20 nm thickness, respectively) in which 111 In, 197 Hg and 197m Hg were assumed to distribute homogeneously. The transport of AE, internal conversion electrons (CE) and photons emitted by 111 In, 197 Hg and 197m Hg were simulated by MCNP6 with the energy cut-off at 20 eV for AE and 1 keV for CE and photons. The electron transport by the condensed-history algorithms was switched to a single-event electron transport method at 10 keV. Cellular S values (SN←CM , SN←Cy , SN←NM , SN←N , SCM←CM , SNM←NM ) were calculated and compared. For comparison, SN←CM , SN←Cy and SN←N were calculated with MIRDcell assuming a cell and its cell nucleus as two concentric spheres.

RESULTS: SN←N calculated by MIRDcell and MCNP6 were in excellent agreement for all three radionuclides, but a slight discrepancy was observed between MIRDcell and MCNP6 calculated SN←Cy and SN←CM . The ratios of SN←Cy and SN←CM by MIRDcell to MCNP6 were 0.751-0.999 and 0.602-1.900 (111 In), 1.036-1.248 and 0.903-1.386 (197 Hg), 1.060-1.234 and 0.968-1.330 (197m Hg), respectively. Using the same cell and nucleus size, the ratios of SCM←CM or SNM←NM to SN←N were highest for 111 In (9.7-51.0 or 10.5-37.4), followed by 197 Hg (7.9-41.8 or 8.4-31.8), then 197m Hg (7.2-36.9 or 8.0-28.1). The mean ratios of S values of 197 Hg or 197m Hg to those of 111 In were 2.5 ± 0.5 and 2.5 ± 0.6, respectively. The highest ratio of SN←CM. was 5.14 (197 Hg) and 6.25 (197m Hg) at RC =12 μm and RN =6 μm. AE contributed >99.6, 98.8 and 98.1% to the total SNM←NM or SCM←CM for 111 In, 197 Hg and 197m Hg, respectively. The contribution of AE to SN←N was 93.5-98.3%, 78.1-94.7% and 70.4-92.2% for 111 In, 197 Hg and 197m Hg, respectively.

CONCLUSIONS: MCNP6 is a reliable method to model cellular S-values for 111 In, 197 Hg and 197m Hg. Targeting a single cell, 197 Hg and 197m Hg deposit two fold more dose than 111 In. All AE emitters deposit a higher self dose in the CM and NM than in the N, which warrants studies on the radiobiological effects of targeting the CM and NM with AE emitters.