Devices for dosimetric measurements and quality assurance of the Xstrahl 300 orthovoltage unit

Abstract The Xstrahl 300 orthovoltage unit is designed to deliver kilovoltage radiation therapy using the appositional technique. However, it is not equipped with some typical linear accelerator features, such as mechanical distance indicator and crosshair projection, which are useful for facilitating equipment setup during various quality assurance (QA) and research activities. Therefore, we designed and constructed slip‐in devices to facilitate QA for dosimetric measurements of our Xstrahl 300 unit. These include: (a) an ion chamber positioning system for dosimetric measurements, (b) a mechanical pointer for setting dosimeter distance to a nominal 50 cm, and (c) a crosshair projector with built‐in light to facilitate alignment of dosimeter to the center of the radiation field. These devices provide a high degree of setup reproducibility thereby minimizing setup errors. We used these devices to perform QA of the Xstrahl 300 orthovoltage unit. One of the QA tests we perform is a constancy check of beam output and energy. Our data since start of clinical use of this unit (approximately 2.5 yr) show dose outputs to be remarkably reproducible (2σ = ±0.4%) for all three clinical beams (75, 125, and 250 kVp). These devices have provided both convenience and high‐precision during the unit’s commissioning, and continue to provide the same for various QA activities on the Xstrahl 300 orthovoltage unit.


| INTRODUCTION
Xstrahl 300 (Xstrahl Ltd., Surrey, UK) is a commercial orthovoltage unit used for radiation therapy. It is capable of delivering kilovoltage x-ray beams of 40-300 kVp. 1 Radiation beams with kilovoltage x-ray energies deposit most of the dose close to the skin. 2,3 Consequently, kilovoltage x-ray beams are suited for treatment of shallow lesions, such as nonmelanoma skin cancer. [3][4][5] For treatment using the appositional technique, the ends of fixed-length applicators are placed against the patient's skin. The applicators can have square or round apertures of varying sizes, and their ends may be open or closed. 6 The clinically desired depth of skin penetration dictates the choice of beam energy.
To ensure safe and consistent radiation therapy, routine quality assurance (QA) tests are essential. The Xstrahl 300 orthovoltage unit does not come equipped with mechanical distance indicators, crosshair projection light field, or wall lasers. These are common devices or features of linear accelerators in radiation oncology clinics that are used for equipment positioning during QA. Precision in dosimeter positioning during any dosimetry measurement is an important factor in reducing measurement uncertainty, thereby enhancing identification of machine-related problems or dosimetry issues. Kilovoltage x-ray beam dosimetry is also highly sensitive to measurement distances, 7  To achieve precise dosimeter positioning during QA measurements of our Xstrahl 300 orthovoltage unit (output, energy, cone factor, etc.), an ion chamber positioning system was designed and constructed. A couple of other slip-on devices, namely a mechanical distance pointer and crosshair projector, were also designed and constructed to facilitate QA measurements in general. Herein, these QA devices used with our Xstrahl 300 orthovoltage unit are described, and the results of QA tests performed with them are reported. The devices can aid radiation oncology clinics in complying with national and international standards for QA of kilovoltage x-ray radiotherapy equipment. While the American Association of Physicists in Medicine (AAPM) has not published QA recommendations for kilovoltage x-ray machines, the report by Task Group 61 (TG-61) states that "the consistency of the x-ray output shall be checked routinely". 8 The ion chamber positioning system can be used to perform routine checks of the x-ray output. Output checks are also recommended in the comprehensive technical quality control guidelines for kilovoltage x-ray radiotherapy machines published by the Canadian Partnership for quality radiotherapy (CPQR). 9 In addition to the typical radiation oncology clinic setting, the devices described here can be adapted to facilitate other types of QA or research in various settings, such as outpatient dermatology clinics or settings with limited resources. A recently published consensus dermatology guidelines described the renaissance of kilovoltage x-ray radiotherapy for the treatment of nonmelanoma skin cancers and recurrent keloids. 5 This may be because kilovoltage xray radiotherapy has produced better cosmesis for certain anatomical locations than has surgery, is more cost-effective than electronic brachytherapy, 10 and has a sharper lateral penumbra than does electron beam therapy. 5 Furthermore, in low-income countries, the frequent trade-off in determining the appropriate level of radiotherapy is that between evidence of clinical efficacy and treatment complexity, the latter of which gives rise to personnel training issues, high machine cost, maintenance challenges, etc. 11 Thus, kilovoltage x-ray treatment, which does not require complex treatment plans, is especially relevant for low-income countries, as it is one of the few radiotherapy techniques for which clinical efficacy 4,5 aligns with costeffectiveness and ease of use. 1,10 Moreover, the use of kilovoltage x-ray beams for radiobiology research has been rising. 3,[12][13][14] 25.0 % of radiobiology studies used kilovoltage x-rays as the radiation source, according to a large-scale systematic review of radiobiology preclinical and translational research literature over the last two decades. 13 In recent years, kilovoltage x-rays have even outpaced Co-60 and Cs-137 gamma rays to become the most prevalent radiation type used in murine research. 14 To facilitate QA or research in different settings, structural or procedural modifications to the use of the devices presented in this paper can be made. For instance, the ion chamber positioning device was designed to hold a Farmer chamber, but can be used with any cylindrical ion chamber with a diameter <1.5 cm, and furthermore, the user may even fashion and attach different holders, such as a platform to hold small animals or cells for preclinical research. On the other hand, the mechanical distance pointer can be used whenever positioning at 50 cm distance is desired, such as during transmission measurements of an eye shield to navigate around the shield's suture hole knob, while the crosshair projector can be used to check centering of objects in most situations. This report may inspire others to adapt the designs described here and construct similar devices to suit their specific QA or research activities on their kilovoltage x-ray units. Other supporting devices that we devised to facilitate QA of the Xstrahl 300 orthovoltage unit were a mechanical distance pointer and crosshair projector. The mechanical distance pointer consists of | 153 a brass rod in a plastic disk that can be mounted onto the unit as shown in Fig. 2. To assemble the pointer, the rod was force-fitted into the tight hole at the center of the disk. In use, the tip of the rod indicated 50 cm FSD when mounted in the Xstrahl 300 orthovoltage unit's sub-tube assembly. The crosshair projector consisted of two thin metal wires attached perpendicular to each other on the base of a transparent plastic cylinder, an attached light-emitting diode, a battery for the light-emitting diode, and a plastic disk that can be mounted onto the orthovoltage unit (Fig. 3). This device effectively mimicked the built-in light field with crosshair projection on a linear accelerator. It is useful in aiding the positioning of phantoms and dosimeters, such as ion chambers, film, thermoluminescent dosimeters, and optically stimulated luminescent dosimeters. The room temperature was measured using a thermometer, and the room's atmospheric pressure was measured using a barometer. At our institution, three orthovoltage energies were commissioned: 75, 125, and 250 kVp. These energies were chosen based on clinical experience and physician preference. Two output readings were obtained for each energy. The measured outputs, corrected for temperature and pressure variations, were compared with baseline outputs to determine the output deviations. Finally, the monthly output deviations over 2.5 yr were plotted. This output check was performed more than once a month in the initial months following commissioning of the Xstrahl 300 orthovoltage unit, yielding 35 data points in total.

2.A | Construction of the QA devices
Another QA test performed was the energy consistency check.
Designated filters were placed in the filter slot of the orthovoltage unit, and additional attenuators were placed on the shelf of the ion chamber positioning device [ Fig. 1(h)]. Three metal sheets-1. In addition, we obtained an independent output check of the Xstrahl 300 orthovoltage unit from The MD Anderson Radiation Dosimetry Services (RDS). 16 After placing the thermoluminescent detectors (TLDs) on the folding platform provided by RDS, the crosshair projector was mounted such that the TLDs could be centered.
Next, the mechanical distance pointer was mounted to guide positioning of the TLDs at 50 cm FSD. The slender profile of the mechanical distance pointer allowed for it to then be removed without disturbing the TLDs. After dose delivery with 250 kVp, the TLDs were sent back to RDS. RDS later provided us with a report of the independent dose measurement.

| RESULTS
We used the ion chamber positioning system, mechanical distance pointer, and crosshair projector to achieve precise positioning of the chamber during QA tests of our Xstrahl 300 orthovoltage unit. For the independent output check using TLDs from RDS set up using the mechanical distance pointer and crosshair projector, we obtained a ratio of output measurement performed by RDS to that performed by our institution of 0.98 (−2%), which was within the ±10% considered to be a satisfactory output check for orthovoltage energies by RDS. 16

| DISCUSSION
Various devices to aid reproducible QA testing of the Xstrahl 300 orthovoltage unit was constructed. We chose the construction materials for these QA devices based on availability, ease of construction, resistance to impact, and cost. For the ion chamber positioning device, the main reason for choosing polycarbonate was practicality.
Polycarbonate can better withstand repeated handling over time than can acrylic, which may crack if mishandled. In addition, the transparency of polycarbonate makes the ion chamber easily visible during setup. The material also must be transparent to the radiation beam to allow the radiation beam to pass through it as well to prevent additional scatter. We used a minimum amount of polycarbonate material to reduce dose interference while maintaining solidity.
Similarly, for the crosshairs, instead of metallic wires, we used thin plastic threads to prevent additional scatter. We initially used human hairs for the crosshairs, but keeping those fine crosshairs in focus via the camera viewport was difficult. We could consistently obtain well-focused images with the thin plastic threads. As for the mechanical distance pointer, we considered stainless steel but Nevertheless, this precision of our QA setup minimizes measurement uncertainties and allows us to better isolate machine-specific issues, such that we can set a limit tighter than a typical action level of 3% deviation 8 to trigger investigation of the accuracy of the tube potential and filament settings. Moving forward, we will continually monitor the orthovoltage unit's output with the aforementioned QA devices over many years to ensure accurate delivery of radiation therapy.
Other devices could have been used to perform the tests described herein. For instance, a vendor-provided fixed-length applicator can be used instead of the ion chamber positioning device for the output measurements. In that event, because the effective point of measurement is at the center of the ion chamber's sensitive volume 8 and intersecting the applicator end into the chamber is not physically possible, correction factors may need to be applied to account for the added distance of the effective point of measurement to the end of the applicator and end-plate scattering contribution if using closed-ended applicators. 17 In addition, our ion chamber positioning device has a shelf that can accept any custom attenuator for tailored transmission measurements. Due to these advantages, we designed these devices specifically to facilitate our QA needs.
Similarly, a 50 cm fixed-length applicator can be used for distance indications instead of the mechanical distance pointer described herein. From our experience, however, the light weight and slim profile of the mechanical distance pointer make for much easier handling and maneuvering around measurement systems.
The design of the ion chamber positioning device has some limitations. The ion chamber positioning device does not allow for concurrent use of an applicator, hence, any measurements performed with it will not account for the presence of an applicator. For instance, the output constancy measurements described here do not account for any changes to the applicator over time. Most clinics will use multiple applicators and measurements in reference conditions performed with a reference applicator can only monitor changes to one reference applicator. In our clinic, we use three sets of applicators with our Xstrahl 300 unit: closed-ended square applicators provided by Xstrahl Ltd., as well as shorter open-ended circular flat applicators and beveled applicators that used to be part of a decommissioned RT250 unit (Philips, Amsterdam, Netherlands). The latter ones are still used due to long-standing clinical experience of our physicians and a slide-in adaptor is used to mount these applicators.
Given the three different sets of applicators, the initial motivation for designing the ion chamber positioning device was to be able to efficiently perform QA of the beam independent of the choice of applicator set. Clinical dosimetry for any kilovoltage x-ray beam primarily involves the in-air dose output in reference geometry, cone factor, and backscatter factor, among other factors. 3,6,18 The cone factors and backscatter factors are not expected to change as long as the applicators do not get damaged. To monitor the integrity of all our applicators over time, we measure the cone factor with respect to our open reference geometry for each of our applicators annually. The backscatter factors are established based on data from TG-61. 8 Thus, the remaining variable is the in-air dose output in reference geometry that reflects the machine performance. With the assistance of the described devices, we check this monthly via the described output and energy constancy tests as well as annually following the TG-61 protocol 8 for dose output and HVL measurements for beam energy. Again, we stress that since the ion chamber positioning device does not incorporate an applicator, it should serve as a reminder to separately account for the different applicators in clinical calculations to avoid incidents such as the Ottawa Orthovoltage Incident 19 where backscatter factors for field sizes other than 10 × 10 cm 2 were not applied, resulting in under-and over-doses to patients. Another limitation of the ion chamber positioning device is that the described energy constancy check is done in the broad F I G . 5. Radiation transmission measurements using the ion chamber positioning system with the Xstrahl 300 orthovoltage unit over 2.5 yr. beam geometry. For measurement of definitive HVL, narrow beam geometry should be employed to minimize detection of radiation scattered from the attenuators. 3,8 While the ion chamber positioning device is not used during our annual check of the HVL, the mechanical distance pointer and crosshair projector are helpful to ensure correct chamber positioning in the radiation field under the scatter-free narrow beam geometry.

| CONCLUSIONS
Devices that facilitate QA tests of the Xstrahl 300 orthovoltage unit were designed and constructed. The detailed descriptions of these devices may inspire other clinics with kilovoltage x-ray machines to construct similar devices of their own to support their various QA or research objectives.