Background: Worldwide, lung cancer is the most common cancer in incidence and mortality, Radiotherapy is very important in treatment of lung cancer, required for almost 40% of patients with lung cancer, technologic advances in radiation therapy have contributed to improvements in local control, decreased toxicity. Patients and methods: 40 Patient were simulated using CT simulator,2 plans were done: a}Two dimensions(2D.) plan:Two parallel opposed fields by isocentric technique (virtual plan) up to 40Gy (phase1) then 3d conformal for 20-26 Gy(phase2)B}Three dimensions (3D.) plan: conformal plan by three to five fields. Plan was evaluated by max.,min. and mean dose using dose volume histogram(DVH) as well as: 1) Lung :V20 (volume of lungs receiving 20 Gy ):<35%-40%,3)Heart:V30 (volume of heart receiving 30 Gy):<46%,4)Spinal cord:<46 Gy. Results: For the 40 cases PTV dose was min. 56.5% , 53.6% ,max.113.8%, 112.9% mean. 101.5% , 101.8%,for 2d and 3d respectively(P value =0.75), Both lungs average V20 was 42%,38 %for 2d and 3d respectively (P values = 0.09), Spinal cord average dose was max.85%,74% for 2d&3d respectively (P value = 0.073), Heart averageV30 was18%,12% for 2d&3d respective.(P values = 0.14).cases then was divided into 3 groups according to PTV vol.:Group(A)[300cc-800cc]:average PTV mean 100.8%, 101% for 2d&3d respective.(P value =0.08), V20 of lungs was 38.5%, 30% for 2d and 3d respective.(P value =0.04) , V30 of heart was 26%,15% for 2d&3d respective.(P value =0.046), spinal cord max. 96.5%, 74% for 2d and 3d respective (Pvalue =0.039) , Group (B) [800cc-1300cc] : average PTV mean 102.5%,101.5% for 2d &3d respective.(P value =0.16), V20 of lungs was35.32%,35.62% for 2d&3d respective.(P value =0.23), V30 of heart was26.75%,24.68% for 2d&3d respective. (P value =0.076), spinal cord max. 81%, 79% for 2d and 3d respective. (P value =0.269) Group(C)[1300cc-1800cc] : average PTV mean 102.8%,102% for 2d &3d respective.(P value =0.19), V20 of lungs was 39%, 37% for 2d and 3d respective.(P value =0.374), V30 of heart was28%,26% for 2d&3d respective. (P value =0.267), spinal cord max. 93.5%, 90.5% for 2d and 3d respective. (P value =0.139), conclusion: there is significant dosemetric diff. between 2d and 3d in group (a) patients [i.e. smallest volume of the 3 groups].