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A Phantom Study of Proton Range Uncertainty From the Measured Exit Dose in Bilateral HN Treatment

W Yao1*, M Zhu2 , S Chen3 , K Langen4 , (1) University of Maryland School of Medicine, Baltimore, MD, (2) University of Maryland School of Medicine, Baltimore, MD, (3) University of Maryland School of Medicine, Baltimore, MD, (4) Emory University, Atlanta, GA


(Wednesday, 7/17/2019) 10:30 AM - 11:00 AM

Room: Exhibit Hall | Forum 1

Purpose: The accuracy of stopping power ratio converted from the patient CT is limited. To take into account this limitation, robust planning is commonly used in proton plans, in which the range uncertainty is set to be around 3% of the range. The robust planning, however, compromises the dose on organs at risk. Particularly, for bilateral head-and-neck (HN) cases, robust planning results in un-wanted dose into the opposite shoulder. We propose to examine the range uncertainty by using the exit dose from our bilateral HN treatment.

Methods: In the phantom study, we made robust IMPT plans with 0%, 1%, 2% and 3.5% range uncertainty for an HN phantom. The plans were delivered to the phantom positioned with cone beam CT image guidance. In order to estimate the range uncertainty, we measured the exit dose distribution by a 2D array detectors (MatriXX PT) with 0 mm, 5 mm, 10 mm and 15 mm extra buildups. The measured doses were compared to the planned ones by their mean value and patterns. From the depth doses, we calculated the depth of 80% dose normalized to the dose with 0 mm extra buildup.

Results: The dose distribution pattern showed the measured close to the planned at their corresponding range uncertainty and depth, however, the mean doses from the measurement was about 14% hotter than the planned. The depth of the 80% dose was about 1.3 mm deeper in the measured than in the planned.

Conclusion: In this phantom study, the treatment planning system predicted well the exit dose pattern. However, both the predicted exit dose and energy were lower than the measured. This may indicate that the stopping power ratio was interpreted slightly high from the phantom CT numbers. We will examine patient cases in which the stopping power ratio of human tissues can be intricate.


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