Proceedings of the 50th Annual ASTRO Meeting S447

considered such as smaller peripheral tumors, larger tumors adjacent to chest wall/mediastinum, nodal involvement, at most onereplan, less than partial response (\50 % GTV shrinkage), asymmetrical shrinkage, wider margins, respiratory movement, setuperrors, or oblique beams then ART results in even less lung sparing. Our analysis suggests that the expected improvement in MLDto be no more than 0.7 Gy in a radically treated lung patient with 1 replan. Therefore, ART efforts directed to lung sparing may be oflimited clinical benefit.

Author Disclosure: B.J. Cho, None; A. Bezjak, None; A. Brade, None; A. Hope, None; A. Sun, None.

2621 A Nomogram for Predicting the Survival Benefit of Post-operative Radiotherapy for Patients with Non-

small Cell Lung Cancer

S. J. Wang1, J. Kalpathy-Cramer1, B. E. Lally2, J. Kim3, C. D. Fuller4, C. R. Thomas1

1Oregon Health & Science University, Portland, OR, 2Fox Chase Cancer Center, Philadelphia, PA, 3Portland State University,Portland, OR, 4University of Texas Health Science Center at San Antonio, San Antonio, TX

Purpose/Objective(s): The survival benefit of post-operative radiotherapy (PORT) for non-small cell lung cancer (NSCLC) re-mains controversial, since published clinical trial data have shown conflicting results. The purpose of this study was to constructa survival prediction model to enable individualized predictions of the net survival benefit of PORT for NSCLC cancer patientsbased on specific patient and tumor characteristics.

Materials/Methods: A Cox proportional hazards (CPH) multivariate regression model was constructed using data from 51,423patients with resected NSCLC diagnosed from 1988-2004 from the NCI’s Surveillance, Epidemiology, and End Results (SEER 17)database. Patient and tumor characteristics selected as covariates included age, gender, TNM stage, and histology. The primaryendpoint measured was overall survival (OS) with and without adjuvant radiotherapy. The model was internally validated forboth discrimination and calibration using bootstrap resampling.

Results: The CPH model showed that age, gender, histology, TNM stage, and PORT were all significant independent predictors ofOS. The survival prediction model demonstrated reasonable calibration and discrimination with a bootstrap-corrected C-index of0.66. A nomogram and a browser-based software tool were built from the model that can calculate individualized estimates of pre-dicted net survival gain attributable to PORT. While the model shows that PORT gives a survival benefit for most patients with N2disease, the actual expected benefit from PORT for any individual patient can vary. For example, a 55 year old female with T3-T4N2 squamous cell carcinoma would be predicted to obtain a 5-year OS improvement from 20% to 39% with the addition of PORT,while a 50 year old male with T1N2 lung adenocarcinoma would only expect an improvement from 43% to 47% with PORT.

Conclusions: A nomogram based on historical SEER data can be used to make individualized predictions of the expected survivalbenefit from PORT for patients with resected NSCLC.

Author Disclosure: S.J. Wang, None; J. Kalpathy-Cramer, None; B.E. Lally, None; J. Kim, None; C.D. Fuller, None; C.R. Thomas,None.

2622 Reduced Normal Tissue Dose and Individualized Radical Radiotherapy by Intensity Modulated Proton

Therapy Compared with Intensity Modulated Photon Therapy in NSCLC

X. Zhang, Y. Li, X. Pan, R. Mohan, R. Komaki, J. D. Cox, J. Y. Chang

M.D. Anderson Cancer Center, Houston, TX

Purpose/Objective(s): Conventional photon radiotherapy is associated with 50% local failure in stage III NSCLC. Dose escalationhas been shown to improve local control and survival. However, for very complicated clinical cases, dose escalation is impossibleeven with intensity modulated photon radiation therapy (IMXT) due to exit photon dose. We compared dose volume histograms(DVHs) of intensity modulated proton therapy (IMPT) with IMXT in stage IIIB NSCLC and explored the possibility of individ-ualized radical radiotherapy.

Materials/Methods: DVHs for IMPT and IMXT plans, designed to deliver standard dose of 60-63 Gy for 11 patients with exten-sive stage IIIB NSCLC, were compared. All patients underwent 4-D CT simulation. Gross tumor volume (GTV) was defined as theenvelop of GTVs extracted from the component images in 10 breath phases. Clinical target volume (CTV) was defined as the GTVplus 8 mm margin and planning target volume (PTV) was CTV plus 5-10 mm margin. These patients were selected based on bulkyprimary with extensive involvement of contra-lateral mediastinal/hilar, supra clavicle lymph nodes and were considered to be un-able or borderline to tolerate IMXT with 60-63 Gy based on normal tissue dose volume constraints (lung V20 \35%, total meanlung dose\20 Gy, cord dose\45 Gy). All patients were actually treated with IMXT. For some the prescription dose could not bereached due to normal tissue constraints. The possibility of increasing the total tumor dose (TTD) with IMPT for each patient with-out exceeding normal tissue constraints was investigated.

Results: The lung V5, V10, V20, V30 and mean lung dose were 36.3% (25.0%-48.2%), 32.8% (22.7%-44.7%), 27.0% (19.3%-40.0%), 21.6% (14.1%-35.9%) and 14.3 Gy (10.3-21.9 Gy) for IMPT plans, whereas they were 58.3% (38.7%-73.3%), 45.3%(31.6%-63.5%), 34.7% (23.7%-52.9%), 28.4% (19.4%-46.2%) and 19.8 Gy (14.1-29.2 Gy) for IMXT plan respectively. The spinalcord maximum dose, heart V40, esophagus V55 were 26.0 Gy (1.0-47.2 Gy), 3.7 % (0.0%-8.4%), 28.3% (0.2%-49.6%) for IMPTplans, whereas they were 43.4 Gy (35.7-48.1 Gy), 12.2 % (0.0%-33.3%) and 33.5% (8.8%-53.5%) respectively for IMXT plans.IMPT converted all cases to be treatable to definitive dose. The maximum tolerable PTV dose based on the normal tissue constraintswas 93.0 Gy (65.0 -127.8 Gy) using IMPT while it was below 63 Gy using IMXT.

Conclusions: IMPT reduces the normal tissue dose and allows dose escalation for extensive stage IIIB NSCLC who can not tol-erate or are borderline for standard dose of radiotherapy using IMXT. Individualized maximum tolerable dose irradiation based onnormal tissue dose constraints is feasible using IMPT in this group of patients that may potentially lead to the better local tumorcontrol.

Author Disclosure: X. Zhang, NCI CA74043, B. Research Grant; Y. Li, None; X. Pan, None; R. Mohan, NCI CA74043, B. Re-search Grant; R. Komaki, None; J.D. Cox, None; J.Y. Chang, None.