Factors that Influence the Need to Start Adaptive Radiotherapy.

INTRODUCTION: Adaptive radiotherapy (ART) is an essential approach to account for anatomical and biological uncertainties. Adaptive radiotherapy is, however, time-consuming, and it is unclear which patients are eligible or when is the best time to start ART.

METHODS: This prospective study was conducted at Kasr El-Aini Center of Clinical Oncology and Nuclear Medicine, Cairo, Egypt from January 2019 to December 2020. Thirty patients with pathologically proven, limited-stage small cell or stage I-II non-small cell lung cancer who were either not fit for or refused surgery or had stage III disease were recruited and underwent treatment planning to receive 60 Gy on a conventional 3D conformal radiation schedule with platinum-based chemotherapy. All patients underwent computed tomography (CT) planning within 2 and 4 weeks of starting radiation therapy to assess the need for adaptation. Pulmonary function test and echocardiography findings were assessed at the end of treatment and at 3 and 6 months after treatment, and were compared to the baseline.

RESULTS: We found a significant reduction in mean value of the planning target volume (PTV) in the CT scans at the second (331 cm3 ) and fourth (257 cm3 ) weeks of treatment as compared to baseline (342 cm3 ) ( p -value < 0.0001). Adaptation decreased the dose to the organ at risk with statistical significance and with improvement of the target coverage. At week 2 of radiotherapy, the need for adaptation was correlated to the conformity index ( p = 0.0473), esophageal V35 ( p = 0.0488), esophageal V50 ( p = 0.0295), and its mean dose ( p = 0.0087). At week 4 it was correlated to forced expiratory volume in 1 second (FEV1) ( p = 0.0303), ratio between the forced expiratory volume in 1 second and the forced vital capacity (FEV1/FVC) ( p = 0.0024), and echocardiography ( p = 0.0183).

CONCLUSIONS: Conformity index and esophageal dose constraints can predict the need for adaptation at week 2, whereas baseline pulmonary function parameters and echocardiography can predict the need for adaptation at week 4 of radiotherapy.