Can Stereotactic Body Radiation Therapy Be Used With Mediastinal Chemoradiation for Stage III Non-Small Cell Lung Cancer?: A Dosimetric Evaluation

Abstract

Purpose/Objective(s): Current therapy of stage 3 NSCLC is unsatisfactory. One strategy is to treat the mediastinum with conventional chemoradiation (CRT) together with SBRT to primary tumor. This could improve local control of the primary tumor as well as decease normal tissue toxicity, particularly in between primary and nodes. This strategy also allows assessment of primary tumor response to chemotherapeutics in the absence of radiation, which may influence subsequent chemotherapy. Finally the PTV for SBRT may be decreased if the primary responds to chemotherapy. However SBRT with mediastinal CRT may exceed normal tissue tolerance. We analyzed seven stage 3 NSCLC plans and asked a) whether the mediastinal CRT influenced the ability to meet RTOG dosimetric constraints for SBRT and b) whether normal tissue dose was increased by adding SBRT to mediastinal CRT. Materials/Methods: Seven stage 3 NSCLC patients treated with standard CRT were identified. Plans were created for mediastinal CRT to 60 Gy in 30 fractions to PTV-mediastinum (GTV + 15-20 mm). SBRT to 50 Gy in 5 fractions to PTV-primary (GTV + axial 5 mm, craniocaudal 10 mm) was also planned. A composite plan was created with linear-quadratic equivalent dose (LQED) calculation complying with typical constraints for both fractionations. The LQED contribution from CRT vs SBRT was compared for pertinent structures. Results: SBRT without CRT met all RTOG 0813 constraints. Composite plans complied with both SBRT and CRT constraints for three patients. Two patients met all constraints except SBRT lung V20 <10% and CRT lung V20 <25% (see Table). The last two patients met neither SBRT nor CRT lung V20, cumulatively or with CRT alone, but had small lung volumes or large tumor burden. Conclusions: RTOG dosimetric constraints can be met when combining mediastinal CRT with SBRT of the primary lung tumor, as per cumulative LQED, although CRT can triple the lung V20 from SBRT alone. Addition of SBRT to mediastinal CRT increased V20 by about a third but the cumulative V20 was still under 25% for a third of patients. For appropriate stage III lung cancer patients with small volume disease or large lung capacity, this therapy may improve local control and normal tissue tolerance as well as aid assessment of chemotherapeutic effect. These findings support development of a clinical trial. Author Disclosure: N. Jenkins: None. K. Wijesooriya: None. C.E. Geesey: None. J.M. Larner: None.