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Tumor Control Probability Calculation for Short and Long Course of Rectum Cancer

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G Mora

D Mateus12 , O Pares2 , G Mora3*, M Ferreira3 , I Espinoza4 , (1)Champalimaud Foundation, (2) Mercurius Health, Lisboa, Portugal, (3) IBEB, Faculdade Ciencias, Universidade Lisboa, Portugal (4) Pontificia Universidad Catolica de Chile, Santiago, Chile


SU-I-GPD-T-652 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: Purpose: The aim of this study is to calculate the radiobiological response of rectum cancer patients using an analytical published model of tumor control probability (TCP) for two different schemes: short-course of 5 fractions (25Gy in pelvis region and 30Gy in region with integrated boost) and long-course of 25 fractions (45Gy in pelvis region and 54 or 56Gy in region with integrated boost).

Methods: To compute the TCP, the Poisson model considering repopulation and dose volume histograms obtained from a treatment planning system was implemented. A clonogenic cells density of ρ=1×10⁵ cells/mm³ and a kick-off time of Tk=7 days were assumed. The TCP was calculated using α⁄β=10Gy and γ⁄α=0.54 Gy/day for both schemes. As suggested in other works, alternative values of α⁄β=5Gy (for short-course), α⁄β=11Gy and γ⁄α=0.153Gy/day (for long-course) were also considered. The results were compared with the observed clinical response of 17 patients receiving either short- or long-course schemes.

Results: For both short- and long-course schemes, for α⁄β=10Gy the calculated TCP for all cases was very close to zero. When α⁄β=5Gy for short-course and α⁄β=11Gy for long-course, reasonable TCP values were obtained (up to 0.75): in average 0.35 for short-course and 0.13 for long-course. However, no correlation was found between clinical outcome and TCP calculation

Conclusion: Our preliminar results indicate that the calculated radiobiological response of the studied schemes do not differ significantly. The TCP calculations are not able to explain the observed clinical results for these two options of rectal cancer treatment, even when using different published parameters values. These results motivate a deeper study of the applicability of simple TCP models to the case of radiotherapy studied in this work

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