The originality of our study lies in the evaluation of use factors according to the treatment technique and treatment site based on the MU weight according to the gantry angle with various intervals. The method of acquiring use factors through DICOM RT plan files was first attempted by Choi et al. [21], and the use factors were recalculated at 90° intervals. However, in this study, more detailed analysis was performed at intervals of 10°, 30°, 45°, and 90° for each treatment technique and treatment site.
Our study clearly shows that the use factor varies depending on the treatment technique and treatment site. This suggests that facilities with a high treatment rate for specific diseases, sites, treatment techniques, and machines may need to evaluate their use factors for radiation safety purposes [2, 12]. It's worth noting that certain machines, like Tomotherapy, Halcyon, and Cyberknife, use a beam delivery method that differs from the general C-arm type LINAC. Therefore, it's crucial to conduct multicenter research to evaluate the use factor value for these machines [1, 2, 12]. Our findings underscore the importance of ensuring radiation safety in medical facilities, especially those that use radiation for cancer treatment. Further research and evaluation are needed to promote the safe and effective use of radiation therapy in the clinical setting.
The use factor recommended by NCRP 151, currently the most referenced, and that calculated from actual patient data in our study show a large difference of 30% or more at most in case of 3D CRT. In case of VMAT, the difference is not as significant as that in case of 3D CRT. However, differences are observed depending on the angle. According to other reported studies, the use factor of a stereotactic LINAC that is 100% used for VMAT or dynamic conformal arc therapy should be 0.08, which is a reasonable value, instead of 0.25 recommended by NCRP 151 [22]. Further, comparing the use factors for IMRT and non-IMRT-based treatment of prostate cancer, it is found that at 0°, 75°, 135°, 225°, and 325°, the contribution of IMRT to the output exceeds that of the non-IMRT treatment [23].
According to a different study, use factors are highly dependent on treatment sites [1, 11]. However, even in the same treatment site, the MU delivery weight at a specific angle is different depending on the treatment technique used for each organ; thus, the use factor values may be different. Representatively, the results of the use factor used for breast are listed in Table 5. In the treatment room of institution A and the treatment room of institution C, the use factors for both the VMAT treatment techniques were more than 75%. However, the trend of the use factors used in the two treatment rooms of Institution A was similar; nevertheless, in case of Institution C, the use factor at 0° was approximately twice that of Institution A. Institution B, which mainly uses IMRT treatment, features a high value at 0° and the value at approximately 6° is higher than that at 90°, 180°, and 270°. Although the use factor approaches 0.25 when multiple modalities and techniques are combined, paraspinal stereotactic body radiation therapy (SBRT) features a larger use factor in the ceiling direction [12]. However, as a result of the use factor according to the SBRT treatment technique in Institution B, the value of the use factor facing the floor was the highest at 34.9% and 18% higher than the use factor facing the ceiling (Table 6). Table 7 shows the results of the recalculated use factor in the previous study and our study, and the percentage difference between the recalculated use factor and the NCRP 151 value. The treatment room with the most difference from the NCRP 151 value is Vault 4 of Choi 2022, showing a difference of 33.8% at 0°. In Choi 2022, Vault 4 explained that the rate of 3D-CRT was higher than other treatment rooms, these results show that even if the treatment area is the same and a similar treatment technique is used, the use factors should be recalculated for each institution.
Table 5
Proportion of treatment techniques and use factor in breast cancer treatment by institution
| | Treatment technique (%) | | Use factor |
| | 3D CRT | IMRT | VMAT | | 0 ° | 90 ° | 180 ° | 270 ° |
A-1 | | 5.2 | 0 | 94.8 | | 24.1 | 30.0 | 15.2 | 30.7 |
A-2 | | 17.7 | 0 | 82.3 | | 23.0 | 21.9 | 15.0 | 40.1 |
B | | 26.1 | 69.6 | 4.3 | | 43.1 | 15.6 | 20.0 | 21.4 |
C | | 5.7 | 16.7 | 77.6 | | 46.7 | 28.8 | 22.1 | 22.3 |
Table 6
Percent difference between the results of use factors and the NCRP 151 values according to 90° and 45° intervals for the SBRT treatment technique in Institution B.
| Interval | 90º | | 45º | |
| Angle | 0º | 90º | 180º | 270º | | 0º | 45º | 90º | 135º | 180º | 225º | 270º | 315º |
Institution | NCRP 151 | 31.0 | 21.3 | 26.3 | 21.3 | | 25.6 | 5.8 | 15.9 | 4.0 | 23.0 | 4.0 | 15.9 | 5.8 |
B | Use factor | 34.9 | 14.3 | 16.9 | 33.9 | | 13.4 | 8.3 | 5.9 | 6.8 | 10.1 | 15.8 | 18.1 | 21.5 |
% difference | -3.9 | 7.0 | 9.4 | -12.6 | | 12.2 | -2.5 | 10.0 | -2.8 | 12.9 | -11.8 | -2.2 | -15.7 |
Table 7
In the previous study and our study, the result of the recalculated use factor for each treatment room and the result of the percentage difference with the NCRP 151 value.
| | User factor by treatment room | Percentage difference from NCRP 151 |
| | 0° | 90° | 180° | 270° | 0° | 90° | 180° | 270° |
Saleh, 2017 | Vault #1 | 28.4% | 23.0% | 26.0% | 22.6% | -2.6% | 1.7% | -0.3% | 1.3% |
Vault #2 | 26.1% | 24.7% | 25.1% | 24.1% | -4.9% | 3.4% | -1.2% | 2.8% |
Vault #3 | 25.8% | 27.7% | 21.9% | 24.6% | -5.2% | 6.4% | -4.4% | 3.3% |
Vault #4 | 24.1% | 12.4% | 18.6% | 45.0% | -6.9% | -8.9% | -7.7% | 23.7% |
Vault #5 | 15.0% | 26.8% | 32.7% | 25.4% | -16.0% | 5.5% | 6.4% | 4.1% |
Vault #6 | 11.6% | 39.0% | 10.6% | 38.8% | -19.4% | 17.7% | -15.7% | 17.5% |
Vault #7 | 33.2% | 20.9% | 24.4% | 21.5% | 2.2% | -0.4% | -1.9% | 0.2% |
Vault #8 | 37.4% | 20.2% | 22.7% | 19.7% | 6.4% | -1.1% | -3.6% | -1.6% |
Vault #9 | 30.4% | 29.1% | 23.8% | 16.7% | -0.6% | 7.8% | -2.5% | -4.6% |
Vault #10 | 32.7% | 21.4% | 23.6% | 22.3% | 1.7% | 0.1% | -2.7% | 1.0% |
Choi, 2022 | Vault #1 | 26.5% | 22.8% | 24.0% | 26.8% | -4.5% | 1.5% | -2.3% | 5.5% |
Vault #2 | 30.8% | 22.9% | 22.2% | 24.1% | -0.2% | 1.6% | -4.1% | 2.8% |
Vault #3 | 64.8% | 10.6% | 10.3% | 14.3% | 33.8% | -10.7% | -16.0% | -7.0% |
Vault #4 | 26.5% | 21.8% | 17.5% | 34.2% | -4.5% | 0.5% | -8.8% | 12.9% |
Vault #5 | 22.8% | 19.9% | 23.4% | 34.0% | -8.2% | -1.4% | -2.9% | 12.7% |
Vault #6 | 34.8% | 21.0% | 22.7% | 21.5% | 3.8% | -0.3% | -3.6% | 0.2% |
Vault #7 | 41.0% | 22.7% | 16.0% | 20.3% | 10.0% | 1.4% | -10.3% | -1.0% |
Our study | Vault #1 | 25.1% | 24.7% | 23.6% | 26.6% | -5.9% | 3.4% | -2.7% | 5.3% |
Vault #2 | 26.3% | 24.8% | 20.4% | 28.5% | -4.7% | 3.5% | -5.9% | 7.2% |
Vault #3 | 28.1% | 24.3% | 23.5% | 24.1% | -2.9% | 3.0% | -2.8% | 2.8% |
Vault #4 | 29.1% | 35.9% | 16.5% | 18.5% | -1.9% | 14.6% | -9.8% | -2.8% |
When shielding facilities are developed, they are generally used without additional re-evaluation until the machine is discarded at the end of its lifetime. Therefore, as mentioned in NCRP 151, considering the purpose of the treatment room at the time of design is recommended. However, it may be difficult to predict the use factor in advance because the shielding facility is designed prior to the onset of treatments. Thus, it is necessary to reevaluate the use factor periodically, when the treatment techniques undergo many changes in each institution, or the number of specific treatment sites is increased. The difference between the result obtained at 90° for Institution C and that reported in NCRP was 14.6, which is the largest difference. If the shielding evaluation is performed using the NCRP report value without recalculating the use factor, the agency can underestimate the transmitted dose at a specific point by up to 14.6%. In a treatment room with many specific treatment techniques and treatment sites, dividing the treated patients into different treatment rooms can be an effective alternative.
The results obtained in this study do not reflect data from many institutions, diverse techniques for total body irradiation, total skin electron beam therapy, SRS, and the quality assurance output. However, the findings of this study are crucial because it shows the necessity to reevaluate the use factor when the treatment techniques or the number of specific treatment sites change and can be used to analyze the trend of use factor to the most widely used treatment techniques in recent times.