[Show abstract][Hide abstract] ABSTRACT: Tomotherapy is an image-guided, intensity-modulated radiation therapy system that delivers highly conformal dose distributions in a helical fashion. This system is also capable of acquiring megavoltage computed-tomography images and registering them to the planning kVCT images for accurate target localization. Quality assurance (QA) of this device is time intensive, but can be expedited by improved QA tools and procedures. A custom-designed phantom was fabricated to improve the efficiency of daily QA of our Tomotherapy machine. The phantom incorporates ionization chamber measurement points, plugs of different densities and slide-out film cartridges. The QA procedure was designed to verify in less than 30 min the vital components of the tomotherapy system: static beam quality and output, image quality, correctness of image registration and energy of the helical dose delivery. Machine output, percent depth dose and off-axis factors are simultaneously evaluated using a static 5 x 40 cm(2) open field. A single phantom scan is used to evaluate image quality and registration accuracy. The phantom can also be used for patient plan-specific QA. The QA results over a period of 6 months are reported in this paper. The QA process was found to be simple, efficient and capable of simultaneously verifying several important parameters.
Full-text · Article · Oct 2009 · Physics in Medicine and Biology
[Show abstract][Hide abstract] ABSTRACT: Purpose: To verify the patient‐specific dosimetric accuracy of the helical tomotherapy system. Methods and Materials: Tomotherapy is a new and complex delivery system. Varying parameters such as pitch and modulating the beam at various angles during fan‐beam delivery can produce highly conformal dose distributions. Patient‐specific dosimetric verification is thus critical. This study uses a custom‐designed 18×18×18 cm3 phantom made from water‐equivalent plastic and Exradin A1SL ionization chambers to perform patient‐specific quality assurance (QA) procedures. During treatment, proper positioning of the patient is critical to avoid compromising treatment delivery. Tomotherapy allows roll correction to compensate for patient positioning errors. The roll correction was tested for 5°, 10°, 20°, and 30° using radiographic film dosimetry, the “cheese” phantom and the custom‐designed cuboid phantom. Results: Average ionization chamber correction factors for all patients treated on Tomotherapy were within 1.2%. Film dosimetry for every patient was also performed prior to treatment. Gamma and isodose overlay profiles were analyzed using commercial film analysis software. Results showed no significant dose delivery errors, and all patients passed within 5%. Gamma analysis was performed and showed excellent agreement by comparison with plans without phantom rotation.. Gamma values were within 3.3% at 3mm and 5% distance to agreement. A custom leaf‐control file, or sinogram, is created for each patient's plan, and replicated each time the patient plan is to be delivered. Dosimetric verification for three patient plans was performed to verify the integrity of the sinogram replication process. Results for each tested plan agreement within 1% for each patient fraction. Conclusion: Tomotherapy allows for accurate delivery, and accurately applies the roll correction as shown by direct dose distribution measurements. Conflict of Interest: This work supported in part by a grant from Tomotherapy, Inc.
[Show abstract][Hide abstract] ABSTRACT: Purpose: Tomotherapy is a form of intensity modulated radiation therapy(IMRT) that utilizes rotating fan beams modulated by a binary‐multileaf collimator. The radiation is delivered either serially or helically as the patient is moved in a craniocaudal sequence for tumor coverage. While Tomotherapy can deliver highly conformal dose distributions, it yields the lowest delivery efficiency (tumordose per MU) of current IMRT‐delivery options. This relatively low efficiency has the potential for delivering high total‐body doses due to head leakage, so a quantitative evaluation of the whole‐body dose is warranted. Methods and Materials: We conducted this evaluation for a dedicated helical Tomotherapy delivery device (Hi‐Art System®, Tomotherapy Inc) and compared the results against the previously published serial Tomotherapy system (Corvus, NOMOS Corporation) and traditional IMRT whole‐body data. A typical head‐and‐neck treatment plan (2Gy per fraction, 6622MU) was prepared and delivered to a large water‐equivalent phantom. An ADCL‐calibrated large‐volume ionization chamber (A17 Exradin) was used to measure the low doses. The dose was measured at both 1.5cm (dmax) and at the center of the phantom. Results: From 10cm to 48cm from the inferior target edge (the most proximal serial tomotherapy point was at 10 cm), the helical tomotherapy dose was less than 0.5% of the target dose, and was between 20% and 30% of the serial tomotherapy leakage dose. This study showed that the whole‐body dose for the 70Gy is approximately 140mSv. This dose is less than the 560mSv for a 70Gy treatment as published by Followill, et al. (Int.J.Radiat.Oncol.Biol.Phys. 38, 667). Conclusions: This study indicates that the commercial helical Tomotherapy system provides less whole‐body dose than serial Tomotherapy or conventional IMRT. This is probably due to the internal linear accelerator shielding design and the use of 6 MV photons. This work was supported in part by funding from Tomotherapy, Inc.