Primary barrier determinations for the shielding of medical radiation therapy facilities are generally made assuming normal beam incidence on the barrier, since this is geometrically the most unfavorable condition for that shielding barrier whenever the occupation line is allowed to run along the barrier. However, when the occupation line (for example, the wall of an adjacent building) runs perpendicular to the barrier (especially roof barrier), then two opposing factors come in to play: increasing obliquity angle with respect to the barrier increases the attenuation, while the distance to the calculation point decreases, hence, increasing the dose. As a result, there exists an angle (alpha(max)) for which the equivalent dose results in a maximum, constituting the most unfavorable geometric condition for that shielding barrier. Based on the usual NCRP Report No. 151 model, this article presents a simple formula for obtaining alpha(max), which is a function of the thickness of the barrier (t(E)) and the equilibrium tenth-value layer (TVL(e)) of the shielding material for the nominal energy of the beam. It can be seen that alpha(max) increases for increasing TVL(e) (hence, beam energy) and decreases for increasing t(E), with a range of variation that goes from 13 to 40 deg for concrete barriers thicknesses in the range of 50-300 cm and most commercially available teletherapy machines. This parameter has not been calculated in the existing literature for radiotherapy facilities design and has practical applications, as in calculating the required unoccupied roof shielding for the protection of a nearby building located in the plane of the primary beam rotation.

Purpose: To validate a recently installed portal dosimetry system that uses an in‐house developed analysis application for IMRT QAMethods: 6 MV X‐ray Intensity modulated radiation therapy fields were delivered by a Varian 6EX linear accelerator (Varian Associates, Palo Alto, CA, USA) equipped with Millennium 120 multileaf collimator. An aS1000 Portal Vision electronic portal imaging device (EPID) was installed on the accelerator. In‐house gamma analysis software was designed to compare the dosimetryc distributions predicted by the XiO 4.50 treatment planning system (Elekta, Stockholm, Sweden) and those measured by the EPID system. The EPID was calibrated in absolute dose following the vendor recommended procedure. IMRT fields were acquired at a source to detector distance of 105 cm. To validate the results of the in‐house software the same IMRT fields were also acquired using a commercial 2D array of 445 n‐type diodes system arranged in a 22 cm side cm2 area subdivided in two density regions. A central 10 cm side region containing 221 diodes spaced 7.07 mm and an outer region containing 221 diodes spaced 14.14 mm. Gamma distribution map and percentage of points with gamma <1 using a 3% 3mm criterion were used in both cases as QA pass/fail criteria. Results: For one given field, gamma distribution maps of both detectors were similar in the inner region of the field but EPID based images showed more discrepancies in the borders of the fields due to higher resolution of the EPID panel. Percentage of points with gamma < 1 was between 97.2 and 99.7 for EPID images and between 96.0 and 100% for 2D diode array. Conclusion: The software developed ad hoc for EPID based IMRT QA proved to be a valuable and affordable solution. Similar results confirm soundness of the analysis performed by the software and validates its clinical use.

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Purpose: To use end‐to‐end testing to validate a 6 MV high dose rate photon beam, configured for Eclipse AAA algorithm using Golden Beam Data (GBD), for SBRT treatments using RapidArc. Methods: Beam data was configured for Varian Eclipse AAA algorithm using the GBD provided by the vendor. Transverse and diagonals dose profiles, PDDs and output factors down to a field size of 2×2 cm2 were measured on a Varian Trilogy Linac and compared with GBD library using 2% 2mm 1D gamma analysis. The MLC transmission factor and dosimetric leaf gap were determined to characterize the MLC in Eclipse. Mechanical and dosimetric tests were performed combining different gantry rotation speeds, dose rates and leaf speeds to evaluate the delivery system performance according to VMAT accuracy requirements. An end‐to‐end test was implemented planning several SBRT RapidArc treatments on a CIRS 002LFC IMRT Thorax Phantom. The CT scanner calibration curve was acquired and loaded in Eclipse. PTW 31013 ionization chamber was used with Keithley 35617EBS electrometer for absolute point dose measurements in water and lung equivalent inserts. TPS calculated planar dose distributions were compared to those measured using EPID and MapCheck, as an independent verification method. Results were evaluated with gamma criteria of 2% dose difference and 2mm DTA for 95% of points. Results: GBD set vs. measured data passed 2% 2mm 1D gamma analysis even for small fields. Machine performance tests show results are independent of machine delivery configuration, as expected. Absolute point dosimetry comparison resulted within 4% for the worst case scenario in lung. Over 97% of the points evaluated in dose distributions passed gamma index analysis. Conclusion: Eclipse AAA algorithm configuration of the 6 MV high dose rate photon beam using GBD proved efficient. End‐to‐end test dose calculation results indicate it can be used clinically for SBRT using RapidArc.

Purpose: To implement an end‐to‐end QA test of the radiation therapy imaging, planning and delivery process, aimed to assess the dosimetric agreement accuracy between planned and delivered treatment, in order to identify and correct possible sources of deviation. To establish an internal standard for machine commissioning acceptance. Methods: A test involving all steps of the radiation therapy: imaging, planning and delivery process was designed. The test includes analysis of point dose and planar dose distributions agreement between TPS calculated and measured dose. An ad hoc 16 cm diameter PMMA phantom was constructed with one central and four peripheral bores that can accommodate calibrated electron density inserts. Using Varian Eclipse 10.0 and Elekta XiO 4.50 planning systems, IMRT, RapidArc and 3DCRT with hard and dynamic wedges plans were planned on the phantom and tested. An Exradin A1SL chamber is used with a Keithley 35617EBS electrometer for point dose measurements in the phantom. 2D dose distributions were acquired using MapCheck and Varian aS1000 EPID.Gamma analysis was performed for evaluation of 2D dose distribution agreement using MapCheck software and Varian Portal Dosimetry Application.Varian high energy Clinacs Trilogy, 2100C/CD, 2000CR and low energy 6X/EX where tested.TPS‐CT# vs. electron density table were checked for CT‐scanners used. Results: Calculated point doses were accurate to 0.127% SD: 0.93%, 0.507% SD: 0.82%, 0.246% SD: 1.39% and 0.012% SD: 0.01% for LoX‐3DCRT, HiX‐3DCRT, IMRT and RapidArc plans respectively. Planar doses pass gamma 3% 3mm in all cases and 2% 2mm for VMAT plans. Conclusion: Implementation of a simple and reliable quality assurance tool was accomplished. The end‐to‐end proved efficient, showing excellent agreement between planned and delivered dose evidencing strong consistency of the whole process from imaging through planning to delivery. This test can be used as a first step in beam model acceptance for clinical use.