Optimization of beam weights in conformal radiotherapy planning of stage III non-small cell lung cancer: effects on therapeutic ratio.
OBJECTIVE
To evaluate the effects of beam weight optimization for 3D conformal radiotherapy plans, with or without beam intensity modulation, in Stage III non-small cell lung cancer (NSCLC).


METHODS
Ten patients with Stage III NSCLC were planned using a conventional 3D technique and a technique involving noncoplanar beam intensity modulation (BIM). Two planning target volumes (PTVs) were defined: PTV1 included macroscopic tumor volume and PTV2 included macroscopic and microscopic tumor volume. Virtual simulation defined the beam shapes and incidences as well as the wedge orientations (3D) and segment outlines (BIM). Weights of wedged beams, unwedged beams, and segments were determined by human trial and error for the 3D-plans (3D-manual), by a standard weight table (SWT) for the BIM-plans (BIM-SWT) and by optimization (3D-optimized and BIM-optimized) using an objective function with a biological and a physical component. The resulting non-optimized and optimized dose distributions were compared, using physical endpoints, after normalizing the median dose of PTV1 to 80 Gy.


RESULTS
Optimization improved dose homogeneity at the target for 3D- and BIM-plans and the minimum dose at PTV1. The minimum dose at PTV2 was decreased by optimization especially in 3D-plans. After optimization, the dose-volume 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 isotoxicity levels of lung and spinal cord. Upon dose escalation, esophagus may become 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 NSCLC.
De Gersem
W R
WR
Division of Radiotherapy, University Hospital, Gent, Belgium.
Derycke
S
S
De Wagter
C
C
De Neve
W C
WC
D000465:Algorithms; D002289:Carcinoma, Non-Small-Cell Lung / Q000473:pathology / Q000532:radiotherapy*; D003198:Computer Simulation; D004947:Esophagus; D006321:Heart; D006801:Humans; D008168:Lung; D008175:Lung Neoplasms / Q000473:pathology / Q000532:radiotherapy*; D055585:Physical Phenomena; D010825:Physics; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods / Q000592:standards*; D020266:Radiotherapy, Conformal / Q000592:standards*; D013116:Spinal Cord