Author: Lars Ewell, Russell J. Hamilton 👨🔬
Affiliation: Banner University Medical Group, Bannerhealth 🌍
Purpose: To estimate the variability and predictability of radiation dose as a function of the planning target volume (PTV) size and the anatomical sites of head & neck, brain and prostate.
Methods: CT scans of fifteen deidentified prostate, brain and head & neck patients were investigated for radiation dose variability and predictability. All patients received between 60Gy and 70Gy of therapeutic radiation to their disease sites in 20 to 35 fractions via volume modulated arc therapy (VMAT). The PTVs of the different regions were contoured by radiation oncologists and the resulting treatment volumes were analyzed in the Raystation treatment planning system (TPS). From the TPS, the average PTV dose as well as the dose that covers the 99% PTV volume (D99) and the highest 1% PTV dose (D1) were determined. The ratio of these values was plotted as a function of the PTV volume. From these plots, a least squares linear fit to the data was calculated as well as the R-squared value associated with the fit.
Results: The slope of the prostate graphs showed the least variation with a slightly negative value for both average/D99 and average/D1. For brain and head & neck however, the slopes of the ratio had a positive value for average/D99 while the average/D1 had negative slopes. In addition, the head & neck slopes had the highest R-squared values followed by brain and finally prostate.
Conclusion: Radiation dose variability was lowest for prostate patients, followed by brain patients and finally head & neck patients. Radiation dose predictability, as measured by R-squared fit value, was highest for head & neck, followed by brain and finally prostate.