Objective To compare the single-shot fields irradiated by three focusing modes of γ-knife and explore the approaches for improving the quality of stereotactic radiosurgery.Methods GAFCHROMIC(R) EBT3 mode flushing-free film was used to measure the single-shot fields irradiated by multi-source static focusing modes,multi-source single-axis rotating focusing mode and single-source double-axis rotating focusing mode of γ-knife.Also the uniformity and penumbra of the single-shot fields were compared.Results The 2D dose distribution of the single-shot fields irradiated by three focusing modes of γ-knife was different.In the axis (x,y,z),the rang of penumbra axial length ratios of multisource static focusing modes,multi-source single-axis rotating focusing mode and single-source double-axis rotating focusing mode were 0.13-0.48,0.17-0.33 and 0.28-0.54,in the diagonal direction of the wings plane (NSD,PSD),were 0.31-0.39,0.38-0.43 and 0.54-0.72,respectively;the penumbra axial length ratio of single-source double-axis rotating focusing mode was bigger than in multi-source static focusing modes and multi-source single-axis rotating focusing mode.On the no-wings plane,the area ratios of 80％ dose curve enveloped and 50％ dose curve enveloped(A80％/A50％)were 0.40,0.47 and 0.19,on the wings plane,were 0.61,0.53 and 0.35,respectively.The field uniformity of multi-source static focusing modes and multi-source single-axis rotating focusing mode were superior to single-source doubleaxis rotating focusing mode.Conclusions Considering dose distribution of the single-shot fields,the multi-source static focusing modes devices and the multi-source single-axis rotating focusing mode devices should be preferred,when important tissues and organs are adjacent to the target areas.Compared with single-source double-axis rotating focusing mode,both multi-source static focusing modes and multi-source single-axis rotating focusing mode could make more target areas to be surrounded by high dose region.

Objective:To understand the activity concentration of 131I aerosol in the air of the iodine treatment workplace and estimate the internal dose of 131I inhaled by medical staff in nuclear medicine. Methods:Using CF-1001BRL portable large capacity air sampler, the 131I aerosol in the iodine treatment workplace of nuclear medicine department of 6 hospitals in Shandong province was collected by iodine box, and the HPGe-γ energy spectrometer was used to measure the samples. The 131I activity concentration in iodine treatment workplace at 6 hospitals was obtained, and the internal dose to medical staff was estimated. Results:The 131I activity concentration in the air in iodine treatment workplaces at 6 hospitals ranged from 3.64 to 2.94×10 3 Bq/m 3. The 131I activity concentration in the controlled area (ward, patient passageway, subpacking room, operation administration room) was significantly higher than that in the supervised area. The highest 131I activity concentration, 2.62×10 2 Bq/m 3, in the supervised area was found in the medical care passageway. The estimated effective dose to nuclear medicine workers was 0.07-5.68 mSv, not exceeding the national limit. Conclusions:The phenomenon of 131I aerosol contamination still exists in the iodine treatment workplaces of nuclear medicine departments in hospitals, so it is necessary to carry out internal radiation monitoring for nuclear medicine departments all around the country, and explore more reasonable protection standards and methods.

Objective:To explore the causes of the crosstalk in the gross α and gross β measurement using an MPC 9604 low background α/β counter.Methods:With the A4 copy paper (70 g/m 2), polyethylene (PE) films (8.7 g/m 2), and 304 stainless steel seperately as shielding materials, the gross α and gross β experiments, gamma spectrometry experiments and solid state nuclear track detection (SSNTD) experiments were conducted by using 241Am and 40K standard materials. A comprehensive analysis encompassing statistical analysis and nuclear physics analysis was performed to reveal the impact of contributing factors on the crosstalk in the gross α and gross β measurement with an MPC 9604 low background α/β counter. Results:241Am powder source experimental result: when two sheets of copy paper were used in the experiment, α-rays did not generate one count in the β channel of the low background α/β counter. The same test with the shielding material of two layers of PE films showed that the α count rate further decreased by about 36.5%, while the β count rate hardly changed. The gross α and gross β experiments and γ spectrometry with the shielding material of stainless steel demonstrated that the characteristic γ ray peaking at 59.5 keV of the 241Am powder source did not generate one count in the β channel. 40K powder source experimental result: when the source was covered with steel of total thickness of 0.965 mm in the gross α and gross β experiments, the γ rays of 40K did not generate one count in the β channel. Compared with naked 40K powder source, when source was covered with one and two sheets of copy paper, the gross α count rate decreased approximately from 3.30 × 10 -3 to 1.50 × 10 -3 and 1.75 × 10 -3, respectively. The SSNTD indicated the presence of other α nuclides in 40K powder source. Conclusions:The β counting in the β channel with the 241Am powder source using MPC 9604 low background α/β counter was, instead of α-rays, caused by the internal conversion electrons and the characteristic X rays of 11.870-22.402 keV emitted from the 241Am powder source, thus this is not a true α/β crosstalk. The α counting in α channel with the 40K powder source, except the contribution of impurity α nuclides, was mainly attributed to the α signals arising from β particles when the amplitude of the piled-up β pules exceeded the discrimination threshold of the detector, therefore it is a true crosstalk.

Objective To discuss ⁷Be and a 77.2 keV full-energy peak with short half-life found in the water sample from the 3D water phantom of a proton therapy system. Methods We measured the water sample from the 3D water phantom of a proton therapy system according to Determination of Radionuclides in Water by Gamma Spectrometry (GB/T 16140—2018). Results The activity concentration of ⁷Be in the water sample was 1.30 × 10¹ Bq·L⁻¹ on December 24, 2018; 4.3 × 10¹ Bq·L⁻¹ on March 22, 2019; and 1.41 × 10¹ Bq·L⁻¹ at the time of sampling on December 19, 2018. On December 24, 2018, the net peak area of the 77.2 keV full-energy peak in the sample was 683 ± 45, and the measurement time was 26123.02 s; on March 22, 2019, the net peak area decreased to the background level of 194 ± 49, and the measurement time was 86400.00 s. Conclusion In the 3D water phantom of the proton therapy system, ⁷Be can be generated from the spallation reaction between high-energy neutrons and oxygen in water. In addition, we find a full-energy peak at 77.2 keV with short half-life. The activity concentration of ⁷Be in the water sample is lower than the exemption level, but the activity concentration at sampling may not be the maximum activity concentration in the process of quality control. The inductive radionuclide ⁷Be produced in the 3D water phantom should be identified and properly evaluated in the assessment of occupational radiation hazards of proton therapy system.