Radiation oncology departments expect their linear accelerators to delivery radiation doses accurately as programmed. While it's possible to check planned dose delivery after treatment, safety for patients is increased if dose data can be monitored in real-time.
The medical physicists at the department of radiation oncology at William Beaumont Hospital in Royal Oak, MI, have developed a real-time dose monitoring and dose reconstruction tool for volumetric-modulated arc therapy (VMAT) called the Linac Data Monitor, or LDM. The researchers described their development of the tool and its capabilities in a presentation at the American Association of Physicists in Medicine (AAPM) annual meeting held earlier this year in Charlotte, NC.
The LDM, developed by study co-author David Gersten, connects to a linac in clinical mode and displays, records, and compares real-time machine parameters to the planned parameters. The authors have defined a new quantity called integral error that keeps a running total of leaf overshoot and undershoot errors in each leaf pair, multiplied by leaf width and the amount of time during which error exists in monitor unit delivery.
The software tool was developed to independently monitor the performance of a new linear accelerator (Elekta) while it was being commissioned in 2007 and as the radiation oncology department was transitioning from intensity-modulated radiation therapy (IMRT) to VMAT, explained senior author and medical physicist Neelam Tyagi, PhD. The tool was easy to use and helped them evaluate the mechanical accuracy of VMAT deliveries. This added to the clinical staff's confidence that patients were receiving the treatment dose they had been prescribed.
How it works
Before a patient's treatment, a medical physicist loads the planned parameters from the treatment planning system as the reference, and also sets up each specific mechanical and dosimetric tolerance into the linac's console. During the treatment, the LDM interfaces to the linac via Elekta's iCom client interface and an Ethernet connection, and collects real-time machine parameters. It then compares the actual data to the planned reference data in real-time. If any parameter goes out of tolerance, such as the gantry position or the multileaf collimator, the discrepancy will instantly be displayed in red.
Currently, the red error bars shown in the tool are used primarily for monitoring purposes. The Beaumont team believes the tool will detect all the transient errors that are allowed by the tolerance values set by the linac control system. They are in the process of creating a decision tree where no action is taken if the errors persist only for a split second. If the leaves or gantry angles are persistently out of tolerance, the delivery is stopped and the cause is investigated.
"VMAT delivery is very complex and intricate, in the sense that the leaf position and gantry angle and dose rate change dynamically and are not controlled by a radiation therapist once the treatment begins," Tyagi explained. "A dose error in one fraction of conventional treatment may be of negligible impact to a patient, but if the fraction is delivered during stereotactic radiosurgery [SRS] or stereotactic body radiation therapy [SBRT], the impact will be more significant."
"Not only does the LDM data help the physician to make a decision to continue the treatment or stop it when the overall discrepancy is clinically relevant, but it will enable the medical physicist to better understand the complex VMAT delivery and prepare a better delivery plan in the future," medical physics resident Kai Yang said.
Optimal planning parameters and delivery accuracy benchmarks have been established by Beaumont's radiation oncology department for different treatment sites through a series of VMAT plan evaluations using the LDM tool.
At the AAPM presentation, the Beaumont team presented a study that quantified the delivery characteristics of various standard and hypofractionation VMAT plans delivered on Elekta Axesse and Synergy linacs. The multileaf collimator and gantry errors for all treatment sites were 0.00 ± 0.59 mm and 0.05 ± 0.31°, indicating good multileaf collimation calibration.
Standard fractional plans had a larger gantry error than hypofractionation plans due to frequent dose-rate changes. On average, the multileaf collimation errors were negligible, but larger errors of 4 to 6 mm and 2.5° were seen when the dose rate varied frequently. Large gantry errors occurred during the acceleration and deceleration process and correlated well with the multileaf collimation errors.
"We are pleased with the performance we have monitored from the linacs over the past five years, but we continue to use the tool to maintain the high standards of patient safety and quality assurance of our department," Tyagi said.
Tyagi told AuntMinnie.com that the researchers have shared the real-time dose delivery tool with other Elekta customers, whom she believes have used it for commissioning and research purposes. "I don't know if anyone else uses it for routine patient radiotherapy treatment, but a lot of people asked about it after the AAPM presentation."
The hope is to commercialize the tool: "We want every large hospital to be able to use it," she said. "From a safety perspective, the tool is invaluable."
