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Investigation of Proton Beam Range Uncertainty in Breast Treatment Using Real Tissue Samples

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X Cui

X Cui1,3*, E Bentefour2 , K Jee1 , Y Song3 , H Wang3 , H Lu1 , (1) Massachusetts General Hospital, Boston, MA, (2) IBA, Philadelphia, PA, (1) Massachusetts General Hospital, Boston, MA, (3) CASHIPS, Hefei, Anhui, (3) CASHIPS, Hefei, Anhui, (1) Massachusetts General Hospital, Boston, MA


SU-K-108-10 (Sunday, July 30, 2017) 4:00 PM - 6:00 PM Room: 108

Purpose: In principle, proton therapy after breast-conserving surgery (BCS) could substantially reduce dose to lung and cardiac structures. However, these dosimetric benefits are subject to beam range uncertainty in the patient. We performed water equivalent path length (WEPL) measurements with real tissue samples in treatment configurations to evaluate range uncertainty for proton beam treatment of BCS.

Methods: The breast tissue CT-Hounsfield unit (HU) values were sampled for 10 patients. Real animal tissue samples including muscle, adipose tissue and mixtures of both were prepared to match the HU distribution. The WEPL of the real tissue samples were measured both in homogeneous conditions and in treatment configurations using the 12-diode detector array of our time-resolved in-vivo WEPL measurement system (IVWMS). The measured WEPL and relative stopping power (RSP) values were compared with treatment planning calculations (Xio, Elekta, Crawley, UK) based on the standard stoichiometric CT-HU calibration.

Results: The mean of the sampled HU values for breast tissue ranged from -100 to -80, significantly lower than -35 for the plastic insert representing the breast tissue in the Gammex CT calibration phantom. The relative differences of WEPL between measurements and calculations for water, muscle and adipose tissue were -0.29%, -1.31%, -5.65% respectively.

Conclusion: The 5.65% error of WEPL calculation for adipose tissue by the planning system is substantially larger than the testing limit of ±3.5% for beam range robustness in current clinical practice. Because the main component of breast tissue is adipose tissue (about 77% on average), proton treatment of BCS could be undershooting if proper measures were not taken against this specific uncertainty. The standard stoichiometric CT HU calibration technique relying on theoretical tissue substitute compositions may not represent the physical properties of real tissues from various individual patients.

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