An in-silico comparison of proton beam and IMRT for postoperative radiotherapy in completely resected stage IIIA non-small cell lung cancer

Abstract

Introduction: Post-operative radiotherapy (PORT) for stage IIIA completely-resected non-small cell lung cancer (CR-NSCLC) has been shown to improve local control; however, it is unclear that this translates into a survival benefit. One explanation is that the detrimental effect of PORT on critical organs at risk (OARs) negates its benefit. This study reports an in-silico comparative analysis of passive scattering proton therapy (PSPT)- and intensity modulated proton therapy (IMPT) with intensity modulated photon beam radiotherapy (IMRT) PORT.

Methods: The computed tomography treatment planning scans of ten patients with pathologic stage IIIA CR-NSCLC treated with IMRT were used. IMRT, PSPT, and IMPT plans were generated and analyzed for dosimetric endpoints. The proton plans were constructed with two or three beams. All plans were optimized to deliver 50.4 Gy(RBE) in 1.8 Gy(RBE) fractions to the target volume.

Results: IMPT leads to statistically significant reductions in maximum spinal cord, mean lung dose, lung volumes treated to 5, 10, 20, and 30 Gy (V5, V10, V20, V30), mean heart dose, and heart volume treated to 40 Gy (V40), when compared with IMRT or PSPT. PSPT reduced lung V5 but increased lung V20, V30, and heart and esophagus V40.

Conclusions: IMPT demonstrates a large decrease in dose to all OARs. PSPT, while reducing the low-dose lung bath, increases the volume of lung receiving high dose. Reductions are seen in dosimetric parameters predictive of radiation pneumonitis and cardiac morbidity and mortality. This reduction may correlate with a decrease in dose-limiting toxicity and improve the therapeutic ratio.

Figure 1

Average DVHs of 10 patients comparing IMRT (blue), PSPT (red), and IMPT(green), (A) heart, (B) ipsilateral lung, (C) contralateral lung, (D) total lung-PTV, (E) esophagus.

Figure 2

Average DVHs of 10 patients comparing IMRT (blue), PSPT (red), and IMPT (green) for the target structures: (A) PTV and (B) CTV.

Figure 3

Patient with stage IIIA NSCLC status post right upper lobe lobectomy with positive lymph nodes in station 7. Representative isodose distributions for (A) IMRT (B) PSPT (C) IMPT plans in the axial view. Key demonstrates corresponding dose to colorwash.

Figure 4

Patient with stage IIIA NSCLC status post left lower lobe lobectomy with positive lymph nodes in stations 5 and 12. Representative isodose distributions for (A) IMRT (B) PSPT (C) IMPT plans in the axial view. Key demonstrates corresponding dose to colorwash.

Figure 5

DVH bands of the targets (A), (B) and organ at risks (C)-(E). The nominal DVH is shown as the black line while dotted lines are results from lateral, vertical and longitudinal setup errors of 3mm. The worst case DVH results from lateral setup errors.