Optimization of beam weights in conformal radiotherapy planning of stage III non-small cell lung cancer: Effects on therapeutic ratio

Purpose: To evaluate the effects of beam weight optimization for 3D conform al radiotherapy plans, with or without beam intensity modulation, in Stage III non-small cell lung cancer (NSCLC), Methods and Materials: Ten patients with Stage III NSCLC were planned using a conventional 3D technique and a technique involving noncoplanar beam int ensity modulation (BIM), Two planning target volumes (PTVs) were defined: P TV1 included macroscopic tumor volume and PTV2 included macroscopic and mic roscopic tumor volume. Virtual simulation defined the beam shapes and incid ences as well as the wedge orientations (3D) and segment outlines (BIM), We ights of wedged beams, unwedged beams, and segments were determined by huma n trial and error for the SD-plans (3D-manual), by a standard weight table (SWT) for the BIM-plans (BIM-SWT) and by optimization (SD-optimized and BIM -optimized) using an objective function with a biological and a physical co mponent. The resulting non-optimized and optimized dose distributions were compared, using physical endpoints, after normalizing the median dose of PT V1 to 80 Gy, Results: Optimization improved dose homogeneity at the target for 3D- and B IM-plans and the minimum dose at PTV1. The minimum dose at PTV2 was decreas ed by optimization especially in 3D-plans, After optimization, the dose-vol ume histograms (DVHs) of lung and heart were shifted to lower doses for 80- 90% of the organ volume. Since lung is the dose-limiting organ in Stage III NSCLC, an increased minimum dose at PTV1 together with a decreased dose at the main lung volume suggests an improved therapeutic ratio. Optimization allows 10% dose escalation for 3D-plans and 20% for BIM-plans at isotoxicit y levels of lung and spinal cord. Upon dose escalation, esophagus may becom e the dose-limiting structure when PTV1 extends close to the esophagus, Conclusions: Optimization using a biophysical objective function allowed an increase of the therapeutic ratio of radiotherapy planning for Stage III N SCLC. (C) 2000 Elsevier Science Inc.