PurDose/Obieclive: For treatment of lung cancer, dose heterogeneity corrections and subsequent prescription alteration remain controversial.Previous dosimetry studies based on slab geometry and single beam geometry may not adequately represent the clinical situation of a circumscribed tumor within lung.Energy choice also remains a controversy.The objective of this study was to perform dose measurements for a tumor in lung in an anthropomorphic phantom using a clinically relevant beam arrangement for both 6 and 18 MV photons.Measured and calculated dose distributions were compared, using several different dose calculation algorithms.Methods &Materials: An anthropomorphic phantom was modified by replacing lung cylinders (2.5 and 5.0 cm diameter) with muscle-equivalent cylinders.The phantom was scanned on a CT simulator.Gross, clinical, and planning target volumes (GTV, CTV, PTV1 -tumor and regional nodes plus one cm margin, PTV2 -tumor only plus one cm margin) were delineated slice-by-slice.3D planning was performed with large fields (AP/PA/RPO) covering PTV1 and boost fields optimized for each PTV2 for 6 and 18 MV photons.Ratio-TAR (RTAR) both with and without heterogeneity corrections, convolution adapted RTAR (CARTAR), and superposition convolution dose calculation algorithms were tested.Film was placed in between phantom slices at the 'tumor' levels.The phantom was irradiated using homogeneous monitor unit calculations.Measured and calculated dose distributions were compared by isodoses and dose volume histograms.One test case (2.5 cm.cylinder) compared film and TLD dose measurements with similar results.Lung tissue ratio (LTR) measurements with an ion chamber imbedded in a 3 X 3 cm 2 muscle-equivalent rectangular solid, surrounded by either lung or muscle-equivalent material, were also performed.Results: The three heterogenei W correction algorithms, compared with the measured iscdoses, overpredicted the minimum dose to PTV2 by 11-18% for the smaller tumor, and by 5-8% for the larger tumor.None of the algorithms predicted the diffuse penumbra associated with 18 MV photons in lung.For the 2.5 cm diameter tumor, the measured minimum dose covering GTV and PTV2 was 98% and 95% of the homogeneous prescription dose, respectively, for 6 MV photons.GTV and PTV2 received a minimum dose of 95% and 89% of the homogeneous prescription dose, respectively, for 18 MV.For the 5.0 cm tumor, both the 6 and 18 MV beams provided approximately 100% of the homogeneous prescription dose to both GTV and PTV2.The measured LTR for 6 MV was 1.11 versus 1.17 calculated by RTAR with heterogeneity corrections.The 18 MV measured LTR was 1.01 compared with a calculated value of 1.09.Conclusion: Our studies indicate strong caution must be exercised before reducing dose prescriptions for lung cancer based on simplistic dose calculation algorithms An institution must validate their algorithms' accuracy by performing realistic phantom studies.The LTR value of unity for 18 MV photons indicates that increased transmission is offset by reduced local scatter.High energy beams provide deficient coverage of small target volumes within lung and 6 MV photons may be preferable despite slightly increased hot spots. 2035