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Radiobiological Influence of Mixed Photon Energies for Prostate Cancer Through Tumor Control Probability and Normal Tissue Complication Probability

S Momin1*2, J Grafe2, R Khan3, (1) Washington University School of Medicine, St Louis, MO, USA (2) Ryerson University, Toronto, ON, CA,

Presentations

(Sunday, 7/12/2020)   [Eastern Time (GMT-4)]

Room: AAPM ePoster Library

Purpose: evaluate the response of tumor and normal tissue complication probabilities by studying equivalent uniform dose (EUD), normal tissue complication probability (NTCP) and tumor control probability (TCP) for prostate cancer treated with single energy (6MV or 15MV) versus mixed photon energy (6MV and 15MV) volumetric modulated radiation therapy.


Methods: 15 prostate cancer cases treated with a dose of 79.2Gy in 44 fractions, three VMAT plans (6MV, 15MV, and 6&15MV) were generated using the same optimization parameters and priority weighting. The single energy plans involved 6MV partial-arcs and a 15MV partial-arcs, while the mixed energy plan involved a hybrid of 6MV partial arcs in anterior-posterior direction and 15MV partial-arcs from the lateral directions. The Lymann Kutcher Burman parameters, representing Grade=2 rectal bleeding, were chosen from literature: m=0.14; n=0.24; D50=75.7. For urinary urgency, the LKB parameters included m=0.5; n=1.0; D50=64.2. The TCP parameters were a=-10, TCD50=28.34 and ?_50=1. A paired two-sided student’s t-test was used, with P<0.05 as statistically significant.


Results: For prostate disease control, the average TCP of mixed energy plan was comparable to 6MV (98.8%&98.7%; P=0.45) and 15MV (98.8%&98.7%; P=0.17). Both mixed and single energy plans generated comparable EUD for the femoral heads. In comparison to mixed energy, 6MV and 15MV resulted in a higher average EUD to rectum (48.56Gy&51.78Gy; P=0.001 and 48.56Gy&52Gy; P=0.001), and mixed energy produced lower NTCP (0.8%&1.9%; P=0.001 and 0.8%&1.8%; P=0.002) respectively. The EUD to bladder was higher in 6MV (17.44Gy; P=0.0003) and 15MV (17.41Gy; P=0.0002) plans compared to the mixed energies and the mixed energy technique reduced NTCP by 1% relative to 6MV (7.86% P=0.0014) and 15MV (7.84%; P=0.0017).


Conclusion: both single energy and mixed photon energies produced similar TCP and EUD for prostate, the mixed energy technique has superior radiobiological effects owing to the lower NTCP for rectum and bladder toxicities.

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