Commissioning of a dedicated commercial Co‐60 total body irradiation unit

Abstract We describe the commissioning of the first dedicated commercial total body irradiation (TBI) unit in clinical operation. The Best Theratronics GammaBeam 500 is a Co‐60 teletherapy unit with extended field size and imaging capabilities. Radiation safety, mechanical and imaging systems, and radiation output are characterized. Beam data collection, calibration, and external dosimetric validation are described. All radiation safety and mechanical tests satisfied relevant requirements and measured dose distributions meet recommendations of American Association of Physicists in Medicine (AAPM) Report #17. At a typical treatment distance, the dose rate in free space per unit source activity using the thick flattening filter is 1.53 × 10−3 cGy*min−1*Ci−1. With a 14,000 Ci source, the resulting dose rate at the midplane of a typical patient is approximately 17 and 30 cGy/min using the thick and thin flattening filters, respectively, using the maximum source to couch distance. The maximum useful field size, defined by the 90% isodose line, at this location is 225 × 78 cm with field flatness within 5% over the central 178 × 73 cm. Measured output agreed with external validation within 0.5%. End‐to‐end testing was performed in a modified Rando phantom. In‐house MATLAB software was developed to calculate patient‐specific dose distributions using DOSXYZnrc, and fabricate custom 3D‐printed forms for creating patient‐specific lung blocks. End‐to‐end OSLD and diode measurements both with and without lung blocks agreed with Monte Carlo calculated doses to within 5% and in‐phantom film measurements validated dose distribution uniformity. Custom lung block transmission measurements agree well with design criteria and provide clinically favorable dose distributions within the lungs. Block placement is easily facilitated using the flat panel imaging system with an exposure time of 0.01 min. In conclusion, a novel dedicated TBI unit has been commissioned and clinically implemented. Its mechanical, dosimetric, and imaging capabilities are suitable to provide state‐of‐the‐art TBI for patients as large as 220 cm in height and 78 cm in width.


| INTRODUCTION
Total body irradiation (TBI) is commonly used as a preparatory regimen for bone marrow transplant for disseminated cancer. American Association of Physicists in Medicine (AAPM) Report #17 "Aspects of Total Body Irradiation" describes the requirements and recommendations for TBI. 1 Since most radiotherapy centers perform relatively few such treatments, typical TBI involves adaptation of conventional radiotherapy equipment and treatment procedures to facilitate irradiation of the whole body. A common example is the use of a conventional linac rotated to deliver a lateral beam irradiating a patient placed a large distance away from the isocenter to create a field large enough to accommodate the entire patient. In many cases, the field size is still not large enough and the patient must be treated in the fetal position. Aside from the complexity of the treatment setup, the use of lateral fields is not optimal since the patient is thicker laterally than anteroposteriorly, resulting in greater dose heterogeneity. In addition, shielding the lungs to mitigate radiation pneumonitis, the most important common toxicity resulting from TBI, is much easier using anterior/posterior fields. Due to the difficulties accommodating TBI treatments using conventional radiotherapy treatment units, and the associated suboptimal treatment characteristics, some facilities performing large numbers of TBI treatments have devised dedicated TBI units by modifying existing clinical equipment to overcome these shortcomings. Some examples of these units are described in the literature using either linear accelerators 2-5 or Co-60 units. 6-8. In 1994, our center modified a commercial Co-60 teletherapy unit (Theratron 780, Atomic Energy of Canada Ltd., Chalk River, Canada) to create a dedicated TBI unit similar to that described by Peters,et al. 9 Modifications included removal of the collimator and treatment couch and the construction of a custom flattening filter. A thin, movable couch with adjustable lung block tray was designed to support the patient a few cm off the floor with a gap underneath large enough to allow the assessment of lung block positioning with radiographic film. The surface of the couch was approximately 190 cm from the source. The removal of the collimator allowed the entire patient length to be included in the treatment fields. Patients were treated with an AP and a PA field using a supine and prone setup, respectively, and the custom flattening filter was designed to produce a uniform dose distribution over the lateral and longitudinal axes of the patient. This unit was well suited to provide uniform total body dose distributions and facilitate appropriate lung blocking and was operated from 1994 to 2016, delivering TBI treatments to over 700 patients in preparation for bone marrow transplant. In 2016, the GammaBeam 500 (Best Theratronics, Inc., Kanata, ON, Canada) became the first FDA approved commercial dedicated TBI unit. Its design is very similar to our previous in-house unit, however, it has greatly improved user interfaces, the height of the treatment head is adjustable, it projects a significantly larger field size, and the treatment couch includes an amorphous silicon flat-panel imaging device. Our in-house developed dedicated Co-60 TBI unit was decommissioned and replaced with this unit in October 2016. This work describes the commissioning processes and results for the first commercial dedicated TBI unit in clinical operation.

| MATERIALS AND METHODS
The GammaBeam 500 Total Body Irradiator is a Co-60 teletherapy unit designed to deliver a large field size at extended distance for TBI. The treatment beam points toward the floor and the head moves vertically with a minimum and maximum distance of 74 and 250 cm from the source to the floor, respectively. As such, both the field size and dose rate are variable. Patients are treated with AP and PA fields on a movable treatment couch with built-in imaging capabilities. While the unit allows rotation of the treatment head, we have disabled beam operation if the head is not locked such that the beam points vertically downward. A blocking tray which attaches to the couch can hold custom lung blocks at three different heights above the treatment couch. The couch has motorized vertical motion to allow easy patient access prior to lowering the couch to the treatment position, and a 41 9 41 cm a-Si flat panel imager with motorized longitudinal motion which can be used for positional verification of the patient and/or blocks. The couch is interlocked such that treatment can only be performed when the couch is at its lowest position. The manufacturer specified treatment field at a distance of 220 cm from the source is 70 9 200 cm. A photograph of the unit is shown in Fig. 1.
While comprehensive recommendations for commissioning and clinical implementation are regularly issued for conventional radiotherapy treatment units and techniques, this is not the case for unique or specialized treatment units such as that described here.
Indeed, AAPM Report #17 on TBI was published in 1986 and no updated AAPM recommendations have since been issued. As a result, limited guidance exists for the commissioning, clinical implementation, and quality assurance of treatment units such as this. Our first goal in the commissioning process was to assure the safe use of the treatment unit and confirm the proper operation of all treatment unit functions. This included a radiation survey; testing of all safety interlocks, emergency systems, and radiation indicators; development of quality assurance tests and frequencies; development of staff training documentation; and a failure mode and effects analysis.
Following the development and testing of all safety processes, we evaluated the operation and accuracy of all mechanical systems within the treatment unit, couch, and imaging system. Accuracy and reproducibility of the treatment head rotation and locking mechanism and flattening filter position and reproducibility were evaluated along with their resulting effects on the characteristics of the treatment field. Source translation accuracy and reproducibility testing included repeated calibration measurements for a fixed treatment time, measurement of timer linearity and magnitude and reproducibility of timer error, and reproducibility of measured time for each phase of source translation. These phases represent the time to trigger successive microswitches in the source translation processes and are referred to as the "fully shielded", "just shielded", and "fully exposed" phases of both the "exposure" and "return" processes. The Absolute calibration was performed using a Farmer chamber within a 60 9 60 cm 2 water phantom filled to a depth of 30 cm with 40 9 40 9 30 cm 3 solid water placed on either side of the water phantom in the longitudinal direction. Since the field size is not adjustable we cannot create a 10 9 10 cm field size, and since we don't have a phantom large enough to cover the single fixed field size, we are not able to achieve full scatter. As such, we are not able to perform calibration per AAPM TG-51 guidelines, 10 however, the formalism of the TG-51 report was followed as closely as possible.  In addition, we followed the recommendations from AAPM Report #17 section 3.1 which state that one should (1) perform absolute calibration using large field geometry and the largest phantom possible, (2) correct for (a) dose that would be obtained for a phantom that covers the entire beam, and (b) dose that would be obtained within a deep phantom (full scatter), and (3) correct for patient dimensions in terms of both area and thickness when performing calculations for patient treatment. The unit has only one field size and this phantom is the largest we could create with existing equipment. We performed the recommended corrections to our measurements using Tables 2 and 3 from AAPM Report #17. Since Table 2 is limited to a field size of 75 9 75 cm 2 and a phantom size of 50 9 50 cm 2 it was necessary to extrapolate to accommodate our setup. Our equivalent field size as defined by the 50% isodose line in air of 250 9 85 cm is approximately 125 9 125 cm 2 . Our equivalent phantom size for the 120 9 30 9~55 cm is approximately 75 9 75 cm 2 .
Tissue-air ratio data for our TBI program have traditionally been based on percent depth dose (PDD) data from british journal of radiology Supplement 11 11 modified to include larger field sizes and to better match in-phantom measurements. These TAR values match very well (within 0.3% for typical field sizes) with values from Table 1 from van Dyk et al. 12 Since our largest phantom is not sufficient to provide infinite scatter for this field size, we are unable to directly measure TARs or PDDs for this treatment unit. Instead, we performed a set of representative measurements to determine whether the previously used TAR values are accurate for this treatment unit. This was performed for phantom geometries smaller than the total field size and similar to those of typical patients. PDDs Detectors (OSLDs) were irradiated using the standard IROC acrylic irradiation block and setup. In addition, we made measurements using an identical acrylic calibration block with a hole drilled for a 0.053 cm 3 ionization chamber (Exradin A1SL, Standard Imaging, Middleton, WI, USA). The dose to muscle tissue in the calibration setup was calculated from the ionization chamber measurement and compared to the dose to muscle tissue calculated from the OSLD measurement.
In-house processes were developed for treatment planning and for the creation of lung blocks 13   image quality using a contrast-detail phantom, 15 and then used to image the Rando phantom with and without lung blocks to qualitatively evaluate the accuracy of lung block placement.
Finally, we developed a set of routine quality assurance tests associated with the dosimetric, mechanical, and radiation safety aspects of the unit and its operation. These tests  All TAR values calculated from measurements made at depths of 0.2-20 cm within the 50 9 50 cm 2 , 60 9 60 cm 2 , and 75 9 75 cm 2 equivalent square phantoms were within 2.0% of those used for our previous in-house modified treatment unit and from Table 1  End-to-end testing was performed initially without lung blocking.   are approximately 9 Gy for the entirety of both lungs. Figure 6 shows a photograph of the lung blocks created for the end-to-end testing with and without the PLA form. The forms can be printed to and blocks poured to any desired thickness in the case that we End-to-end testing was repeated with the 2.5 cm thick lung blocks described above and Table 2 Figure 7 shows the AP view of the end-to-end phantom along with the projected shapes of the lung blocks contoured within the Eclipse   other design feature that could be improved is the manual backup operation of the emergency shutter. The emergency shutter is designed to deploy automatically in the case that the source is stuck in the exposed position. However, in the event that the shutter does not deploy automatically, manual operation must be performed from the treatment unit gantry stand. Ideally, users could be given the option to manually close the emergency shutter without entering the room and coming in close proximity to the active treatment beam.

CONCLUSI ONS
A novel dedicated TBI unit has been commissioned and clinically implemented. Its mechanical, dosimetric, and imaging capabilities are suitable to provide state-of-the-art TBI for patients as large as 225 cm in height and 78 cm in width. The uniformity of this treatment field is within 10% over this stated field size and within 5% over the central 178 cm and 73 cm in the longitudinal and lateral directions, respectively. All characteristics of this unit provide adequate capability to deliver TBI in accordance with tolerances recommended by AAPM Report #17.

CONF LICTS OF INTEREST
The authors have no actual or potential conflicts of interest for the work presented here.