Potential of Adaptive Radiotherapy to Escalate the Radiation Dose in Combined Radiochemotherapy for Locally Advanced Non-Small Cell Lung Cancer

Purpose: To evaluate the potential of adaptive radiotherapy (ART) for advanced-stage non-small cell lung cancer (NSCLC) in terms of lung sparing and dose escalation. Methods and Materials: In 13 patients with locally advanced NSCLC, weekly CT images were acquired during radio- (n = 1) or radiochemotherapy (n = 12) for simulation of ART. Three-dimensional (3D) conformal treatment plans were generated: conventionally fractionated doses of 66 Gy were prescribed to the planning target volume without elective lymph node irradiation (Plan3D). Using a surface-based algorithm of deformable image registration, accumulated doses were calculated in the CT images acquired during the treatment course (Plan4D). Field sizes were adapted to tumor shrinkage once in week 3 or 5 and twice in weeks 3 and 5. Results: A continuous tumor regression of 1.2% per day resulted in a residual gross tumor volume (GTV) of 49% ± 15% after six weeks of treatment. No systematic differences between Plan3D and Plan4D were observed regarding doses to the GTV, lung, and spinal cord. Plan adaptation to tumor shrinkage resulted in significantly decreased lung doses without compromising GTV coverage: single-plan adaptation in Week 3 or 5 and twice-plan adaptation in Weeks 3 and 5 reduced the mean lung dose by 5.0% ± 4.4%, 5.6% ± 2.9% and 7.9% ± 4.8%, respectively. This lung sparing with twice ART allowed an iso-mean lung dose escalation of the GTV dose from 66.8 Gy ± 0.8 Gy to 73.6 Gy ± 3.8 Gy. Conclusions: Adaptation of radiotherapy to continuous tumor shrinkage during the treatment course reduced doses to the lung, allowed significant dose escalation and has the potential of increased local control.