Author: Indra J. Das, Ahtesham U. Khan, Andrew J. White, Poonam Yadav 👨🔬
Affiliation: Northwestern Memorial Hospital, Northwestern University Feinberg School of Medicine 🌍
Purpose: During intensity modulated radiotherapy (IMRT) planning, low MU segments (< 3 MU) are commonly avoided due to documented dosimetric nonlinearity and poor reproducibility. This work aims to explain the poor reproducibility by analyzing individual linac pulses acquired from an efficient, high signal plastic scintillation detector (PSD).
Methods: A Blue Physics (Tampa, FL) Model 11 PSD was used to resolve individual linac pulses on a Varian TrueBeam (Palo Alto, CA). To determine the macroscopic stability in the number of pulses and dose per pulse during standard linac operation, data was first acquired by delivering 100 MU at multiple energies (6X, 6FFF, 10X, 10FFF) with a constant dose rate of 400 MU/min. Additionally, data were collected to investigate low MU deliveries. For these measurements, 1, 2, and 3 MUs were delivered at the same four energies, but with varying repetition rates of 5 to 2400 MU/min. An in-house MATLAB script was used to extract and analyze dose per pulse and the number of pulses for each sequence. A PTW 30010 farmer ion chamber was placed adjacent to the PSD and served as the reference dose for evaluating reproducibility.
Results: For 100 MU deliveries, the average dose per pulse remained constant across multiple trials (less than 0.33% COV), however the dose per pulse within an individual sequence was variable (up to 13.3% COV). Low MU deliveries resulted in an “overshoot” effect for 10FFF beams, where an additional pulse was observed in some sequences. The increase in signal was consistent across both detectors.
Conclusion: Pulse-wise analysis was used to extract information about MU substructure. Variation in dose per pulse and total number of pulses was observed, particularly with high energy FFF beams, that leads to additional uncertainty for low MU deliveries.