SU-FF-T-511: Effect of Setup Motion and IMRT On the TCP and NTCP for Pediatric Ependymoma

Abstract

Listen

Translate article

Purpose: Compare the TumorControl Probability (TCP), Normal Tissue Complication Probability (NTCP) and Conformity Index (CI) of Intensity Modulated Radiation Therapy(IMRT) versus 3D Conformal Radiation Therapy (3D‐CRT) for Pediatric Ependymoma patients and quantify the effect of set‐up uncertainty (SU) and residual uncertainty (RU) on TCP and NTCP for different PTV margins. Methods and Materials: 20 Ependymoma patients treated between 1998 and 2002 using 3D‐CRT with a 5mm PTV margin were selected for this study. Two IMRT plans for each patient were created, one with identical margins (PTV=5mm) and one with no PTV margin (PTV=0mm). A direct simulation of SU was performed for each plan of each patient based on daily CBCT information obtained from 20 well‐matched patients (age, supine/prone, use of GA) on a localization protocol. A direct simulation of RU, based on post‐treatment CBCT was performed. TCP; NTCP for the cochlea, spinal cord, and brainstem; and CI were calculated for each plan. Also, a predictive IQ formula was used to compare IMRT vs. 3D‐CRT for PTV=5mm. Results:IMRT improved the TCP by 0.023±0.027 vs. 3D‐CRT (p=0.001). A TCP loss of 0.004±0.007 (p=0.02) due to SU and 0.0003±0.001 (p=0.14) due to RU was found for the IMRT with PTV=0mm plan. The NTCP for 3D‐CRT, IMRT with PTV=5mm, and IMRT with PTV=0mm was; Cochlea: 0.66±0.40, 0.29±0.35, and 0.09±0.17; Brainstem: 0.22±0.07, 0.22±0.04, 0.19±0.04; and Spinal Cord: 0.13±0.06, 0.09±0.05, 0.08±0.04; respectively. Mean NTCP change due to SU for IMRT with PTV=5mm was; Cochlea 0.02±0.11, Spinal Cord 0.007±0.01, and Brainstem 0.003±0.01. The CI was 0.64±0.12 for IMRT and 0.4±0.14 for 3D‐CRT. Predictive IQ formula was 9.98±10.3 points higher for the IMRT plan. Conclusions:IMRT improves TCP, reduces NTCP and increases the CI versus 3D‐CRT for pediatric Ependymoma patients. The setup margin required due to SU may be eliminated if daily localization is performed.