Validation of a modern second‐check dosimetry system using a novel verification phantom

Abstract

PurposeTo evaluate the Mobius second‐check dosimetry system by comparing it to ionization‐chamber dose measurements collected in the recently released Mobius Verification Phantom™ (MVP). For reference, a comparison of these measurements to dose calculated in the primary treatment planning system (TPS), Varian Eclipse with the AcurosXB dose algorithm, is also provided. Finally, patient dose calculated in Mobius is compared directly to Eclipse to demonstrate typical expected results during clinical use of the Mobius system.MethodsSeventeen anonymized intensity‐modulated clinical treatment plans were selected for analysis. Dose was recalculated on the MVP in both Eclipse and Mobius. These calculated doses were compared to doses measured using an A1SL ionization‐chamber in the MVP. Dose was measured and analyzed at two different chamber positions for each treatment plan. Mobius calculated dose was then compared directly to Eclipse using the following metrics; target mean dose, target D95%, global 3D gamma pass rate, and target gamma pass rate. Finally, these same metrics were used to analyze the first 36 intensity modulated cases, following clinical implementation of the Mobius system.ResultsThe average difference between Mobius and measurement was 0.3 ± 1.3%. Differences ranged from −3.3 to + 2.2%. The average difference between Eclipse and measurement was −1.2 ± 0.7%. Eclipse vs. measurement differences ranged from −3.0 to −0.1%. For the 17 anonymized pre‐clinical cases, the average target mean dose difference between Mobius and Eclipse was 1.0 ± 1.1%. Average target D95% difference was ‐0.9 ± 2.0%. Average global gamma pass rate, using a criteria of 3%, 2 mm, was 94.4 ± 3.3%, and average gamma pass rate for the target volume only was 80.2 ± 12.3%. Results of the first 36 intensity‐modulated cases, post‐clinical implementation of Mobius, were similar to those seen for the 17 pre‐clinical test cases.ConclusionMobius correctly calculated dose for each tested intensity modulated treatment plan, agreeing with measurement to within 3.5% for all cases analyzed. The dose calculation accuracy and independence of the Mobius system is sufficient to provide a rigorous second‐check of a modern TPS.

Report of AAPM Task Group 219 on independent calculation‐based dose/MU verification for IMRT

“…Reports and scientific publications that describe the accuracy of independent IMRT dose or monitor unit calculation software, whether developed in‐house or commercial products, are scarce. Most publications deal with static (step‐and shoot) or dynamic IMRT delivery based on a multileaf collimator or VMAT, 43‐45 and only a few of them with advanced treatment techniques performed with a robotic linac 46 and TomoTherapy 47 . Nevertheless, the commercial vendor list (Table 2) reveals that these tools are clinically applied for verifying IMRT or VMAT.…”

Development of Automated Delivery Quality Assurance Analysis Software for Helical Tomotherapy

“…[8][9][10] Many studies have evaluated the feasibility of a log-based PSQA for tomotherapy. 8,9,[11][12][13] However, for the tomotherapy PSQA, since no log-based commercial software is currently available, the cylindrical Virtual Water™ phantom (Gammex RMI, Middleton, WI; called the "Cheese" phantom), which is available upon the purchase of a tomotherapy machine, is widely used.…”

Is it still necessary to perform measured based pre‐treatment patient‐specific QA for SRS HyperArc treatments?

“…It performs a full recalculation of dose on the patient CT and allows for quality assurance of the treatment plan by offering a “delivered dose” calculation, generated using the M3D model and treatment machine's log files. 18 , 19 …”