Development of a compact linear accelerator to generate ultrahigh dose rate high-energy X-rays for FLASH radiotherapy applications.

PURPOSE: In recent years, the FLASH effect, in which ultrahigh dose rate (UHDR) radiotherapy (RT) can significantly reduce toxicity to normal tissue while maintaining antitumor efficacy, has been verified in many studies and even applied in human clinical cases. This work evaluates whether a room-temperature radio-frequency (RF) linear accelerator (linac) system can produce UHDR high-energy X-rays exceeding a dose rate of 40 Gy/s at a clinical source-surface distance (SSD), exploring the possibility of a compact and economical clinical FLASH RT machine suitable for most hospital treatmentrooms. METHODS: A 1.65 m long S-band backward-traveling-wave (BTW) electron linac was developed to generate high-current electron beams, supplied by a commercial klystron-based power source. A tungsten-copper electron-to-photon conversion target for UHDR X-rays was designed and optimized with Monte Carlo (MC) simulations using Geant4 and thermal finite element analysis (FEA) simulations using ANSYS. EBT3 and EBT-XD radiochromic films, which were calibrated with a clinical machine Varian VitalBeam, were used for absolute dose measurements. A PTW ionization chamber detector was used to measure the relative total dose and a plane-parallel ionization chamber detector was used to measure the relative normalized dose of each pulse. RESULTS: The BTW linac generated 300-mA-pulse-current 11 MeV electron beams with 29 kW mean beam power, and the conversion target could sustain this high beam power within a maximum irradiation duration of 0.75 s. The mean energy of the produced X-rays was 1.66 MeV in the MC simulation. The measured flat-filter-free (FFF) maximum mean dose rate of the room-temperature linac exceeded 80 Gy/s at an SSD of 50 cm and 45 Gy/s at an SSD of 67.9 cm, both at a 2.1 cm depth of the water phantom. The FFF radiation fields at 50 cm and 67.9 cm SSD at a 2.1 cm depth of the water phantom showed Gaussian-like distributions with 14.3 and 20 cm full-width at half-maximum (FWHM) values, respectively. CONCLUSION: This work demonstrated the feasibility of UHDR X-rays produced by a room-temperature RF linac, and explored the further optimization of system stability. It shows that a simple and compact UHDR X-ray solution can be facilitated for both FLASH-RT scientific research and clinical applications.

OsPHS1 is required for both male and female gamete development in rice.

Meiosis plays an essential role in the production of male and female gametes. Extensive studies have elucidated that homologous chromosome association and pairing are essential for crossing-over and recombination of chromosomal segments. However, the molecular mechanism of chromosome recognition and pairing remains elusive. Here, we identified a rice male-female sterility mutant plant. Cytological observations showed that the development of both pollen and embryo sacs of the mutant were abnormal due to defects in homologous chromosome recognition and pairing during prophase I. Map-based cloning revealed that Os06g0473000 encoding a poor homologous synapsis 1 (PHS1) protein is the candidate target gene, which was confirmed by knockout using CRISPR/Cas9 technology. Sequence analysis revealed a single base mutation (G > A) involving the junction of the fourth exon and intron of OsPHS1, which is predicted to alter splicing, resulting in an Osphs1 mutant. Expression pattern analysis indicated that OsPHS1 expression levels were mainly expressed in panicles at the beginning of meiosis. Subcellular localization analysis demonstrated that the OsPHS1 protein is situated in the nucleus and cytoplasm. Taken together, our results suggest an important role for OsPHS1 in homologous chromosome pairing in both male and female gametogenesis in rice.

Generating Photoperiod-Sensitive Genic Male Sterile Rice Lines with CRISPR/Cas9.

Obtaining photoperiod-sensitive genic male sterility (PGMS) lines is one of the most important steps in two-line hybrid rice breeding. Traditionally, such lines were screened and developed with a classic rice breeding system under both long-day and short-day conditions. The isolation and backcross process used for this could easily last for more than 3 years with a very low success rate. Here, we describe a straightforward method for generating csa-based PGMS lines by using the CRISPR-Cas9 technology in rice.