Abstract
The single-isocenter technique in linear accelerator-based stereotactic radiosurgery/stereotactic body radiotherapy (SRS/SBRT) has been broadly used to treat multiple lesions. However, quantitative study to verify that the mechanical field center coincides with the radiation field center when both are off from the isocenter has never been performed. We developed an innovative method to measure this accuracy, called the off-isocenter Winston-Lutz test, and here we provided a practical clinical guideline to implement this technique. We used ImagePro V.6 to analyze images of a Winston-Lutz phantom obtained using a Varian 21EX linear accelerator with an electronic portal imaging device, set up as for single-isocenter SRS/SBRT for multiple lesions. We investigated asymmetry field centers that were 3 cm and 5 cm away from the isocenter, as well as performing the standard Winston-Lutz test. We used a special beam configuration to acquire images while avoiding collision, and we investigated both jaw and multileaf collimation. For the jaw collimator setting, at 3 cm off-isocenter, the mechanical field deviated from the radiation field by about 2.5 mm; at 5 cm, the deviation was above 3 mm, up to 4.27 mm. For the multileaf collimator setting, at 3 cm off- isocenter, the deviation was below 1 mm; at 5 cm, the deviation was above 1 mm, up to 1.72 mm, which was 72% higher than the tolerance threshold. These results indicated that the further the asymmetry field center is from the machine isocenter, the larger the deviation of the mechanical field from the radiation field, and the distance between the center of the asymmetry field and the isocenter should not exceed 3 cm in our clinic. We recommend that every clinic that uses linear accelerator, multileaf collimator-based SRS/SBRT perform the off-isocenter Winston-Lutz test in addition to the standard Winston-Lutz test and use their own deviation data to create planning guideline.
Highlights
With the rapid development of highly accurate image guidance technology for both hardware and software and multiple published reports such as American Association of Physicists in Medicine Task Groups (AAPM TG) [1]-[3] and the American Society for Radiation Oncology (ASTRO) [4] [5] in the past decade, stereotactic radiosurgery/stereotactic body radiotherapy (SRS/SBRT) has advanced from a specialty procedure performed only at comprehensive university medical centers to a widely accepted treatment regimen adopted by most radiation oncology clinics worldwide
Our results indicated that use of the single-isocenter technique to treat multiple lesions is efficient and accurate only when the maximum distance from the center of the mechanical field to the machine isocenter is within 3 cm for the machine in our clinic
Our results illustrate that in our clinic, the use of the single-isocenter technique to treat multiple lesions is efficient and accurate only when the maximum distance from the center of the mechanical field to the machine isocenter is within 3 cm
Summary
With the rapid development of highly accurate image guidance technology for both hardware and software and multiple published reports such as American Association of Physicists in Medicine Task Groups (AAPM TG) [1]-[3] and the American Society for Radiation Oncology (ASTRO) [4] [5] in the past decade, stereotactic radiosurgery/stereotactic body radiotherapy (SRS/SBRT) has advanced from a specialty procedure performed only at comprehensive university medical centers to a widely accepted treatment regimen adopted by most radiation oncology clinics worldwide. Among all topics relating to SRS/SBRT, the precision of treatment delivery is always the most heavily discussed and researched This precision depends on the treatment machine and image-guided radiotherapy tools used, and the location and number of lesions. About half of patients who receive SRS/SBRT for metastatic brain lesions are being treated for more than one lesion [6] This creates a big challenge for treatment delivered on isocentric units such as linear accelerators. It is unclear whether linear accelerator-based systems should deliver SRS/SBRT to multiple lesions using a single isocenter or multiple isocenters, and this has become an important topic of discussion in some publications and student theses [7] [8]. No quantitative medical physics study has been performed to generate guidelines for the appropriate criteria to maintain the accuracy of this technique
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