The Impact of the CT Dataset Used for Dose Accumulation on Target and Normal Tissue Doses in Adaptive Radiation Therapy for Lung Cancer

Purpose/Objective(s): Current practice in adaptive planning for NSCLC considers changes in the tumor volume during RT while assuming the OARs remain constant. RTOG 1106, for example, adapts to the residual PET target during RT, but uses the pre-RT CT for OAR definition and dose accumulation. This study aimed to investigate the dosimetric differences observed when accumulating dose either rigidly or deformably on the during-RT CT compared to dose accumulation on the pre-RT CT. Materials/Methods: Pts enrolled onto a prospective study of adaptive RT with pre-RT and during-RTCT scans were eligible for this study. Adaptive RT planning aimed to maximize tumor dose subject to lung NTCP <17.2% and standard thoracic OAR dose constraints. Three composite plans were studied: A) The clinical composite, which consisted ofw50 Gy to the preRT PTV followed by an adaptive plan delivering an additional w20-35 Gy to the during-RT PTV; OARs were defined on and dose was calculated on the pre-RT CT; B) The same plans as in A, but dose is calculated and accumulated on the during-RT CT using rigid methods; C)The same plans as in A, but the dose is accumulated on the during-RT CT using deformable methods. Dose and volume metrics for B and C were compared to A. Results: Fourteen cases with satisfactory deformable registration were studied. The mean change in lung volume during RTwas 164.6 cc ( 186 to 496 cc). In A vs B, the pre-RT PTV gEUD and D95 changed by mean absolute magnitudes of 1.4 Gy ( 5.2 to 2.5) and 1.7 Gy ( 6.0 to 4.5). The during-RT PTV gEUD and D95 changed by 2.6 Gy ( 6.6 to 5.2) and 1.6 Gy ( 3.9 to 3.5). The change in mean lung dose and NTCP ranged from 3.5 to 1.4 Gy and 6.4 to 2.4%. The change in mean heart dose ranged from 1.2 to 2.4 Gy. The change in mean and max cord dose ranged from 5.0 to 4.1 Gy and 3.1 to 3.0 Gy. The change in mean esophagus dose and NTCP ranged from 5.0 to 4.1 Gy and 9.3 to 17.4%. For Avs C: gEUD and D95 doses in pre-RT PTvs changed by mean absolute magnitudes of 1.9 Gy ( 9.6 to 3.1 Gy) and 1.5 Gy ( 6.5 to 3.0 Gy). For the during-RT PTV, the gEUD and D95 changed by a mean absolute magnitude of 2.1 Gy ( 2.9 to 7.2Gy) and 1.2Gy ( 6.8 to 0.8 Gy). Themean lung dose andNTCP changes ranged from 3.5 to 1.4 Gy and 6.3 to 1.7%. Themean heart dose changes ranged from 0.5 to 0.8 Gy. The change in mean and max cord dose ranged from 0.2 to 0.1 Gy and 1.5 to 1.5 Gy. The change in mean esophageal dose and NTCP ranged from 3.5 to 3.8 Gy and 6.5 to 11.4%. Conclusions: The CT dataset used for adaptive planning and dose accumulation in adaptive lung RT can have a considerable impact on the dosimetry of targets and OARs. The largest changes were seen inmean lung and esophagus doses. Studies are ongoing in our institution to determine which dose accumulation method is more accurate in predicting clinical outcome. Author Disclosure: W. Chen: None. M.M. Matuszak: None. K. Vineberg: None. D. Jarema: None. R. Ten Haken: None. F. Kong: None.