Purpose: The GammaPod is a novel device for stereotactic breast treatments that employs 25 rotating Co-60 sources while the patient is continuously translated in three axes to deliver a highly conformal dose to the target. There is no commercial software available for independent second calculations. The purpose of this study is to determine an efficient way to estimate GammaPod treatment times based on target volume and use it as a second calculation for patient-specific QA.
Methods: Data was extracted from the plan reports of 65 breast cancer patient treatments. Similar to the Curie-seconds concept in brachytherapy, we considered dose-rate*time/(prescribed dose) as a function of target volumes. Using a MATLAB script, we generated linear (with 95% confidence interval) and quadratic fits and tested the resulting equations in an additional set of 18 patients.
Results: We found a strong correlation between the dose-rate*time/(prescribed dose) and a patient’s target volume for both the linear and quadratic models. The linear fit was selected for use and using the polyval function in MATLAB, a 95% confidence interval (CI) graph was created to depict the accuracy of the prediction for treatment times. Testing the model in 18 additional patients with target volumes ranging from 29cc to 193cc yielded treatment times from 10 to 25 minutes that in all cases were within the predicted CI. The average absolute difference between the predicted and actual treatment times was 1.2 minutes (range 0-4 minutes). The average percent difference was 7.3% (range 0-20%).
Conclusion: This work has resulted in a viable independent calculation for GammaPod treatment times. After multiple tests, the spreadsheets are ready for clinical use to predict and verify the accuracy of breast cancer treatment times.