Semester of Graduation

Spring 2024


Master of Science (MS)


Physics & Astronomy (Medical Physics)

Document Type



External beam radiation therapy has been shown to be an effective treatment method for tumors and abnormalities of the spine and vertebral region. Treating the spine using a stereotactic body radiation therapy (SBRT) technique can reduce toxicity to the spinal cord. The 6 MV flattening filter free (6FFF) beam model is currently used to plan and calculate dose for SBRT treatment plans, and the treatment plans are delivered using a linear accelerator (LINAC).

The commissioned beam model represents an invariant component of a LINAC. For volumetric modulated arc therapy (VMAT) treatment plans, the multileaf collimator (MLC) positions are changing throughout the delivery, and variation in these positions can have a large effect on the accuracy of the delivered dose. Since the MLC and jaw positions are patient dependent elements that are implemented as beam modifiers in Elekta Monaco treatment planning system (TPS), post-modeling adjustments to these modifiers can be made to have higher agreement with the delivered dose of the LINAC. With Monaco TPS being implemented as the sole TPS for external beam treatment planning at MBPCC, having high agreement between the TPS beam model calculation and the LINAC delivery is vital to delivering an accurate dose to the patient.

Through the delivery and measurement of control plans, the measured dose can be compared against the calculated dose of the TPS to determine whether the plan can be delivered accurately. This also allows for post-modeling adjustments of the beam model within Monaco TPS to have higher agreement with the measured dose. To compare the level of agreement across treatment plans for quality assurance (QA), the gamma index passing rate (GPR) which includes percent dose difference (%DD) and distance to agreement (DTA) criteria will be calculated to quantify deliverability. After adjusting the beam model parameters in Monaco to match the delivered dose, treatment plans created and calculated in Pinnacle TPS and the re-delivery of the same treatment plans calculated using the 6FFF beam model within Monaco TPS were compared to validate the chosen beam model parameters in Monaco. Additionally, clinically acceptable spine SBRT test patient plans were created in Monaco TPS to verify the deliverability of SBRT plans and further validate the 6FFF beam model in Monaco TPS.

The Express QA plans, control plans, and test patient plans calculated using the adjusted beam modifier parameters for transmission and leaf offset in Monaco TPS, show a higher level of agreement using the GPR at and beyond the clinically acceptable criteria of 3% / 2mm than the default beam modifier parameters. After delivery of the test patient plans using an Elekta Versa HD™ LINAC, the MLC leaf position accuracy was higher across the participating leaves for the Monaco treatment plans compared to the Pinnacle treatment plans.

After conducting post-modeling adjustments to the 6FFF beam model in Monaco TPS, there is higher agreement between the TPS calculated dose and the measured dose distributions for treatment plans in Monaco TPS. The deliverability of the plans was validated through the analysis of the MLC leaf position accuracy. This comprehensive validation of the 6FFF beam model gives confidence that clinically acceptable spine SBRT treatment plans can be created in Monaco TPS and delivered accurately to the patient.



Committee Chair

Stathakis, Sotiri