Radiation Shielding Calculations 1.0.0 Download \/\/FREE\\\\
Millard Clenney
Purpose: To present shielding calculations for clinical digital breast tomosynthesis (DBT) rooms with updated workload data from a comprehensive survey and to provide reference shielding data for DBT rooms.
Methods: The workload survey was performed from eight clinical DBT (Hologic Selenia Dimensions) rooms at Massachusetts General Hospital (MGH) for the time period between 10/1/2014 and 10/1/2015. Radiation output related information tags from the DICOM header, including mAs, kVp, beam filter material and gantry angle, were extracted from a total of 310 421 clinical DBT acquisitions from the PACS database. DBT workload distributions were determined from the survey data. In combination with previously measured scatter fraction data, unshielded scatter air kerma for each room was calculated. Experiment measurements with a linear-array detector were also performed on representative locations for verification. Necessary shielding material and thickness were determined for all barriers. For the general purpose of DBT room shielding, a set of workload-distribution-specific transmission data and unshielded scatter air kerma values were calculated using the updated workload distribution.
Results: The workload distribution for Hologic DBT systems could be simplified by five different kVp/filter combinations for shielding purpose. The survey data showed the predominance of 45 gantry location for medial-lateral-oblique views at MGH. When taking into consideration the non-isotropic scatter fraction distribution together with the gantry angle distribution, accurate and conservative estimate of the unshielded scatter air kerma levels were determined for all eight DBT rooms. Additional shielding was shown to be necessary for two 4.5 cm wood doors.
Conclusions: This study provided a detailed workload survey and updated transmission data and unshielded scatter air kerma values for Hologic DBT rooms. Example shielding calculations were presented for clinical DBT rooms.
Further, the average number of patients that can be treated in 1 h by taking into account time required for patient set up, pretreatment imaging, and dose delivery, which came out to be 3.5, was rounded off to 4 patients/h to be conservatively safer. Assuming tomotherapy machine is operated for 8 h in a day and 5 days in a week, radiation leakage workload and primary workload can be estimated as follows:
As radiation dose is delivered slice-by-slice in helical tomotherapy, beam on time is very high for delivery of prescribed dose compared to conventional technique resulting high leakage workload. The IMRT factor accounts for the increase in monitor units (MUs) for IMRT dose delivery technique compared to conventional treatment technique. The manufacturer recommended IMRT factor for helical tomotherapy is based on the ratio of maximum and average leaf open time, ratio of maximum and average number of leaves open during treatment, and the ratio of maximum and average field width used in clinical treatment cases. On the basis of these, IMRT factor as recommended by manufacturer[9] for tomotherapy is 16. Hence, primary workload (WP) can be calculated by dividing leakage workload (WL) by the IMRT factor as shown below:
The slice-by-slice treatment approach of tomotherapy unit increases primary radiation incident on protective barrier by many folds. To reduce its impact on increase in thickness of barrier due to primary radiation, manufacturer has provided inbuilt lead beam stopper of thickness 12.7 cm opposite to source on the gantry[9] to reduce primary radiation incident on the primary barriers. Hence, the formula for calculating thickness for primary barrier (tP) as given in NCRP/IAEA reports[5,6] gets modified as:
As beam on time is significantly higher in tomotherapy as compared to conventional linear accelerator, shielding thickness due to head leakage radiation also needs to be considered for primary protective barrier. The thickness of wall required to achieve design goal (P) against head leakage radiation[5,6,7,10] can be expressed as follows:
To assess the adequacy of the calculated primary barrier thickness for patient scattered radiation component incident on primary barrier, the transmitted radiation dose (Dps) due to patient scattered radiation through calculated thickness of primary barrier wall can be calculated by following expression:
The width of primary barrier was calculated using formula ([SAD + d + t] F/SAD) + (2 30 cm), as described in NCRP/IAEA reports,[5,6] where SAD = source to axis distance; d = distance from isocenter to POI; t = primary barrier thickness; and F = maximum field width at isocenter, i.e. 5 cm for tomotherapy. The primary barrier width includes 30 cm margin added each side of the central beam axis to account scattered radiation.[5,6,7,10]
Use factor (U) is another important shielding optimizing parameter to consider for the radiation shielding calculations of a radiotherapy bunker, which primarily depends on treatment techniques and radiotherapy machine.[7] For example, the value of U for the primary barrier is assumed to be 0.25 for a conventional/standard medical linac vault mainly due to the four cardinal beam angles generally used in conventional radiotherapy treatments.[8,9,10] Similarly, the advanced treatment technique such as IMRT uses multiple beam angles generally ranging between 5 and 9; whereas RapidArc/VMAT treatment technique delivers dose over certain or entire range of treatment arc length ranging from a short arc length of 60 to full arc length of 360 to deliver the prescribed dose to tumor. Hence, VMAT treatment technique distributes the WP to certain/entire portion of primary barrier based on the chosen arc length in the treatment plan unlike a few fixed locations due to fixed static beam angles in case of conventional, 3DCRT, and IMRT treatments.[4,8,11] Therefore, the use factor needs to be determined for Halcyon linac.
The thickness of radiation shielding material around the treatment unit head determines amount of head leakage radiation which influences the barrier shielding requirements. The maximum head leakage is generally specified by the manufacturer; however, the values of maximum head leakage may be different in different directions around the machine, which may be due to variation in head shielding surrounding the target/source. Therefore, it may be useful to find out the maximum head leakage in different directions around the machine for optimizing the shielding requirements of vault. Caravani et al. reported maximum head leakage for Halcyon medical linac determined using ionization chamber based survey meter with accuracy 20%.[12] Cai et al. reported the maximum head leakage measured with EBT3 GafChromic films.[13]
The patient scatter fractions (denoted as, αs) are the radiation doses scattered from a human size phantom in a particular direction with target to phantom distance of 100 cm and field size of 20 cm 20 cm.[8,9] These values are useful to find out barrier thicknesses due to patient scattered radiation component. The patient scatter fractions as a function of scattering angles with respect to central beam axis for 6 MV-FF X-ray beam energy are already published in NCRP/IAEA reports.[8,9] However, similar data for 6 MV-FFF X-ray beam are not available in these reports. Therefore, in the present study, the patient scatter fractions are determined experimentally in various angular directions from isocenter around the Halcyon machine similar to the method described by Balog et al.[14] for the Helical Tomotherapy machine. A study by Caravani et al. reported dose rates (mSv/h) of phantom scattered radiation as a function of room angles (deg.) around Halcyon machine measured using an ionization chamber-based survey meter (Fluke Biomedical, USA).[12]
As the size of Halcyon machine is reduced in comparison with the size of a standard linac; therefore, the space requirements for installation and smooth operation of this machine are reduced as compared to a standard linac. The manufacturer provided an integrated primary beam block (beam-stopper) of size 75.4 cm (length) 66 cm (width) 17.2 cm (thickness) in the Halcyon unit which is made up of lead with 3% antimony encased in 10 mm thick steel placed diametrically opposite to source/target perpendicular to the central beam axis. This beam block reduces the primary radiation component reaching primary protective barrier/wall significantly resulting in reduced primary radiation shielding requirements. The transmission factor of the beam block needs to be validated against the transmission factor value reported by the manufacturer before using the same in the shielding calculations of primary barrier thicknesses for Halcyon vault. Cai et al. reported primary beam block transmission for the Halcyon medical linac determined using GafChromic EBT3 films[13] and Caravani et al. measured the same with Farmer type ionization chamber (PTW 30013).[12]
In view of the above, a comprehensive study is carried out to determine radiation shielding parameters required for calculations of wall/barrier thicknesses of a Halcyon vault. The primary and leakage workloads are estimated from patient treatment planning and treatment delivery data of the Halcyon facilities. A newer approach to determine the effective use factor for a facility treating patients with various types of treatment techniques is proposed in this paper. The primary use factor for Halcyon facility is determined. The parameters useful in shielding calculations such as primary beam-block transmission, maximum head leakage directed towards the walls/barriers, patient scatter fractions at various angular directions (or room angles) around the Halcyon machine in a horizontal plane passing through isocenter and primary TVLs of 6 MV-FFF X-ray beam energy for ordinary concrete (2.35 g/cc) are determined experimentally using a 30 cm3 ionization chamber (PTW 23361) having better accuracy in comparison with ion chamber based survey meter.[17,18]
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