[Multi-institutional Analysis of MLC Parameters for Commissioning of IMRT].

Multi-leaf collimator (MLC) parameters, which are registered with radiation treatment planning systems, are very important for intensity modulated radiation therapy (IMRT). In this study, we investigated MLC parameters of respective institutions for efficient commissioning of IMRT. Data of linac models, MLC types, nominal energy, irradiation technique, calculation algorithm, dosimetric leaf gap (DLG) values, and MLC transmission values were collected from each institution in which Varian linac and Eclipse were owned, and analyzed. The numbers of responses from institutions to questionnaire were 15, and the total number of linac was 22. In most institutions, volumetric modulated arc therapy (VMAT) technique was used, and the most used nominal energy was 10 MV. The higher nominal energy was, the higher values of MLC parameters (DLG and MLC transmission) were. In addition, values of MLC parameters of flattening filter free (FFF) beams were smaller than those of flattening filter (FF) beams, even when nominal energy was same. Values of DLG of VMAT tended to be greater than those of multi-field IMRT. These results are expected to be useful for institutions, in which IMRT is implemented.

Comparison of the effects of high-energy photon beam irradiation (10 and 18 MV) on 2 types of implantable cardioverter-defibrillators.

PURPOSE: Radiation therapy for cancer may be required for patients with implantable cardiac devices. However, the influence of secondary neutrons or scattered irradiation from high-energy photons (≥10 MV) on implantable cardioverter-defibrillators (ICDs) is unclear. This study was performed to examine this issue in 2 ICD models. METHODS AND MATERIALS: ICDs were positioned around a water phantom under conditions simulating clinical radiation therapy. The ICDs were not irradiated directly. A control ICD was positioned 140 cm from the irradiation isocenter. Fractional irradiation was performed with 18-MV and 10-MV photon beams to give cumulative in-field doses of 600 Gy and 1600 Gy, respectively. Errors were checked after each fraction. Soft errors were defined as severe (change to safety back-up mode), moderate (memory interference, no changes in device parameters), and minor (slight memory change, undetectable by computer). RESULTS: Hard errors were not observed. For the older ICD model, the incidences of severe, moderate, and minor soft errors at 18 MV were 0.75, 0.5, and 0.83/50 Gy at the isocenter. The corresponding data for 10 MV were 0.094, 0.063, and 0 /50 Gy. For the newer ICD model at 18 MV, these data were 0.083, 2.3, and 5.8 /50 Gy. Moderate and minor errors occurred at 18 MV in control ICDs placed 140 cm from the isocenter. The error incidences were 0, 1, and 0 /600 Gy at the isocenter for the newer model, and 0, 1, and 6 /600Gy for the older model. At 10 MV, no errors occurred in control ICDs. CONCLUSIONS: ICD errors occurred more frequently at 18 MV irradiation, which suggests that the errors were mainly caused by secondary neutrons. Soft errors of ICDs were observed with high energy photon beams, but most were not critical in the newer model. These errors may occur even when the device is far from the irradiation field.

[Evaluating photonuclear activation for clearance of decommissioned medical linear accelerators].

In a linear accelerator (linac) that operates at greater than an accelerating energy of 10 MV, neutrons are generated by a photonuclear reaction and the head section of the linac becomes radioactive. The purpose of this research is to obtain data for ensuring the safety of linac decommissioning and upgrading. The decommissioned linac investigated in this study was a Clinac 2100 C/D (Varian) installed in April 1999. Its total time of use was 2757.7 h (equivalent to 496,386 Gy). The dosage for its last three months of use was 7213.67 Gy. After being allowed to sit for a 7-day cooling period, the apparatus was disassembled and the parts of the gantry head portion were removed. The ambient dose equivalent rates, H*(10), (microSv/h) from the removed parts were measured in air, at a location with low background, by using a gamma ray scintillation survey meter. The target was also analyzed with an HP-Ge semiconductor detector, in order to identify the nuclides responsible for the observed radiation. On day 7 after the last use of the linac, the ambient dose equivalent rates, H*(10), (microSv/h) in air at the surface of all parts, except the target and the beryllium window, were within the limit of normal background radiation. The measured value (microSv/h) for the beryllium window decreased to within the background limit on day 10. The measured value (microSv/h) of the target decreased to about 1.5 times the background on day 19. At a distance of 10 cm, all the parts were within the background limit after the initial 7-day cooling period. In the analysis of the target with the HP-Ge semiconductor detector, peaks at 125, 333, 352, 356, 426, 511, 583, 609, 689, 811, 835, 911, 969, 1091, 1099, 1120, 1173, 1238, 1292, 1333, 1461 and 1764keV were detected on day 23. Seven months after the linac was last used, peaks were detected at 352, 511, 583, 609, 835, 911, 969, 1120, 1173, 1238, 1333, 1461 and 1764 keV. From these results, the natural radioactive nuclides can be assigned as 40K, 208Tl, 214Pb, 214Bi and 228Ac; the short half-life nuclides can be assigned as 59Fe, 58Co, 185W and 196Au; and the long half-life nuclides can be assigned as 54Mn and 60Co. These results show that photonuclear activation of parts is important in regard to clearance. Currently, there are no regulations that specify criteria for evaluating radioactivation. Such criteria are needed to establish suitable protocols for the clearance of radioactivated materials.