Objective To measure radiation filed distribution in the treatment room of the Varian Halcyon medical linear accelerator, and to provide a basis for shielding design and potential exposure analysis of treatment rooms for this type of accelerator. Methods Under the 6 MV X-ray (FFF) mode at a maximum dose rate of 800 MU/min and a maximum irradiation field of 28.00 cm × 28.00 cm, a total of 540 MU was delivered during gantry rotation. Radiation field distribution was measured using thermoluminescence dosimeters located at multiple points in the room. The measured data were then applied to shielding calculations, and the results were compared with those obtained using empirical formulas. Results The overall radiation levels in the treatment room were in the range of 12.2 µGy/540 MU to 5.520 Gy/540 MU, with the highest dose (5.520 Gy/540 MU) observed at the isocenter, and the lowest dose (12.2 µGy/540 MU) recorded at approximately 6.5 m from the gantry head. The radiation levels at most points were within the range of 100-1000 µGy/540 MU. At heights of 0.5, 1.1, and 1.7 m, the radiation field exhibited a rapid attenuation from the beam center outward, with a faster decay along the treatment couch direction (Y-axis) than along the perpendicular direction (X-axis). Shielding calculations based on the measured radiation field yielded required wall thicknesses of 1219 mm (east wall), 949 mm (south wall), 1235 mm (west wall), and 1252 mm (north wall), all of which were smaller than those calculated using empirical formulas. Conclusion Thermoluminescence dosimeter is suitable for multi-point in situ measurement of radiation field distribution in Halcyon accelerator treatment rooms. The measurement results can be used to optimize the shielding design of Halcyon accelerator treatment rooms.

Objective:To evaluate the dosimetric effects of different aperture shape controller (ASC) parameters based on the Halcyon 3.0 accelerator in the hippocampal avoidance-whole brain radiotherapy (HA-WBRT) plans.Methods:This study enrolled 13 patients treated with WBRT using a Halcyon 3.0 accelerator at the Department of Radiotherapy of the Shuguang Hospital Affiliated to Shanghai University of Traditional Chinese Medicine. According to different aperture shape controller (ASC) parameters, the HA-WBRT plans were divided into six groups, namely non, very low, low, moderate, high, and very high ASC settings, marked by AO, AVL, AL, AM, AH, and AVH, respectively. This study presented a statistical evaluation of the effects of different ASC parameters on conformity index (CI), homogeneity index (HI), radiation doses to the hippocampus and other organs at risk, and the plans’ execution efficiency. Furthermore, using two dose verification tools, namely ArcCHECK and Portal Dosimetry, this study delivered a comprehensive analysis of the differences in beam delivery precision of the plans with different ASC parameters.Results:The six groups all met the requirements for clinical treatment. They had similar CI and HI values, with no statistically significant differences ( P > 0.05). The AVH group showed better protection effects on organs at risk. Compared to the control group, the AVL group showed lower Dmax of brainstems but higher Dmax of chiasma opticum ( F = 6.26, 8.04, P < 0.05). Compared to the control group, the AH group showed lower Dmax of eyeballs but higher Dmax of optic nerves ( F = 2.04, 1.37, P < 0.05). In contrast, the AVH group exhibited lower Dmax of brainstems, eyeballs, and lens than the control group ( F = 6.26, 2.04, 2.02, P < 0.05). No statistically significant differences were observed in dosimetric indices of other organs at risk ( P > 0.05). As verified using ArcCHECK and Portal Dosimetry, the γ passing rates of the six groups were over 98% at 2%/2 mm and 100% at 3%/3 mm. The overall γ passing rates verified using ArcCHECK were lower than those verified using Portal Dosimetry. The maximum difference in the monitor unit among the six groups was less than 15, and these groups did not show significant differences in terms of execution efficiency. Conclusions:The HA-WBRT plans based on the Halcyon 3.0 accelerator can meet the requirements for clinical treatment. Different ASC parameters can significantly optimize the dosimetric parameters. Among them, the AVH parameters can highly reduce the radiation dose to organs at risk. Furthermore, different ASC parameters show insignificant effects on beam delivery precision and plan execution efficiency, meeting the verification standards for clinical therapeutic doses.