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Dose and LET Based Normal Tissue Complication Probability Calculations in Proton Radiotherapy

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A Carabe-Fernandez

A Carabe-Fernandez1*, M Kondrla2, R Dale3, (1) University of Pennsylvania(2) Drexel University, Philadelphia, Pennsylvania, (3) Imperial College London, London, London

SU-E-T-298 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: Normal tissue complication probability (NTCP) calculations have traditionally been based on dose distributions only. Recent interests in accounting for LET distributions in proton radiotherapy suggest that NTCP values should also be based on LET distributions, as dose and LET have equal weight of influence on proton effectiveness to induce cell kill. We will present an NTCP model that accounts for proton dose and LET distributions.

Methods: Higher LET radiations induce larger amount of complex DNA damage that requires longer time to repair. The normal tissue at the distal edge of a proton beam is always exposed to the largest LET components of the beam. If full repair between fractions is not accomplished, DNA damage accumulation from each fraction will increase the NTCP. The NTCP is calculated in two ways: 1) using a new model that is formulated in terms of repair kinetics to account for the increase of unrepaired damage at the high LET section of the beam; and 2) using just the dose distribution. In both cases, the NTCP is calculated voxel-by-voxel and then integrated in order to provide an overall NTCP value for a given organ. The resultant NTCP from (1) and (2) are compared.

Results: The NTCP of different organs in a brain treatment is calculated as a function of (1) dose and LET or (2) just accounting for dose. The model shows that the NTCP of critical organs at the distal edge of the beam calculated with method 1 is larger than with method 2. Also, when the time between fractions increases, the NTCP decreases even for organs receiving higher-LET proton doses.

Conclusion: NTCP calculations in proton radiotherapy need to account for the LET distribution within the beam, especially in cases where a critical organ is unavoidably at the distal edge of the beam.

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