Technical note: Evaluation of the dose enhancement effect for a novel transmission-type x-ray tube using the Monte Carlo method.

PURPOSE: Transmission-target x-ray tubes generate more x-rays than reflection thick-target x-ray tubes. A transmission x-ray tube combined with radiosensitizers has a better radiation enhancement effect. This study investigated the feasibility of using a transmission x-ray tube with radiosensitizers in clinical radiotherapy and its effect on radiation dose enhancement. METHODS: This study used MCNP6.2 to simulate the model of a transmission x-ray tube and Co-60 beam.   The radiation enhancement effect of radiosensitizers was examined with iodine-127 (I-127), radioiodinated iododeoxyuridine (IUdR), and gold nanoparticles (GNPs). RESULTS: The study results showed that the dose enhancement factor (DEF) of the transmission x-ray tube with GNPs was 10.27, which was higher than that of I-127 (6.46) and IUdR (3.08). The DEF of the Co-60 beam with GNPs, I-127, and IUdR was 1.23, 1.19, and 1.2, respectively. The Auger electron flux of the transmission x-ray tube with GNPs was 1.19E+05 particles/cm2 . CONCLUSIONS: This study found that a transmission x-ray tube with appropriate radiosensitizers could produce a high rate of Auger electrons to fulfill the radiation enhancement effect, and this procedure has the potential to become a radiotherapy modality.

Incidence of low-level viremia and its impact on virologic failure among people living with HIV-1 who switched to elvitegravir-based antiretroviral therapy.

OBJECTIVES: We aimed to investigate the incidence of low-level viremia (LLV) and its impact on virologic failure (VF) in people living with HIV (PLWH) on stable antiretroviral therapy (ART) who switched to co-formulated elvitegravir, cobicistat, emtricitabine and tenofovir alafenamide (EVG/c/FTC/TAF). METHODS: PLWH aged 18 years or older who had received ART with plasma HIV RNA load (PVL) <50 copies/mL for 6 months or longer and switched to EVG/c/FTC/TAF between March and October 2018 were retrospectively included. The incidence of LLV (defined as PVL of 50-999 copies/mL) and VF (PVL ≥1000 copies/mL) was calculated and represented by Kaplan-Meier plots. The generalised estimating equation model was constructed to identify factors associated with LLV and VF. Resistance-associated mutations were determined using population sequencing. RESULTS: A total of 1078 PLWH were included. The incidence rates of LLV and VF after the switch to EVG/c/FTC/TAF were 3.5 and 0.8 events per 100 person-years of follow-up, respectively, whereas the respective cumulative incidence of LLV and VF reached 11.7% and 2.9% within 3 years of follow-up. LLV was associated with any LLV episode before or after the switch and prior exposure to integrase strand transfer inhibitor-based ART. VF was associated with any LLV before or after the switch and prior exposure to raltegravir, but not the level or frequency of LLV. CONCLUSION: The risks of LLV and VF were low in PLWH who had achieved virologic suppression and switched to EVG/c/FTC/TAF, and the presence of LLV and prior exposure to raltegravir increased the risk of VF.