Poster - Thurs Eve-13: Modeling the effect of organ motion on cumulative rectal dose using EUD

A rigid body model and electronic serial portal imaging (EPI) data were used to generate the cumulative dose distribution for the rectum incorporating organ motion during IMRT. The impact of rectal motion was assessed via NTCP and TCP based on equivalent uniform dose per fraction (EUD_f ). The rectal positional variations were measured fraction-to-fraction from MV EPI for 20 prostate patients implanted with gold seeds. Five-field (5F) and seven-field (7F) IMRT plans for prostate patients were constructed with prescribed dose 78 Gy/39 fractions using a Pinnacle³ treatment planning system. EUD increased in 45% of the patients with greater than 2.5% increase for 5% of the patients. While EUD decreased in 55% of the patients with greater than 2.5% decrease for 10% of the patients. The amplitudes of EUD_f increase and decrease are correlated with the dose gradient. Higher dose gradients lead to higher rectal EUD_f change. The rectal NTCP decreases for half of the patients and increases for the other for both 5F and 7F plans. The NTCP decreased with the increasing of dose gradient between the prostate and rectum for 5F and 7F IMRT plans. The correlation coefficient for the rectal NTCP and the dose gradient is - 0.71. The increase or decrease of rectal cumulative dose depends on the dose gradient, motion amplitude and frequency in AP direction. EUD_f is a useful QA parameter for interpreting the biological impact of geometric uncertainties on the static dose distribution. Rectal NTCP is patient dependent and must be determined individually.