Purpose: Tissue inhomogeneity such as lung affects tumor dose as well as transmission dose in new concept of on-line dosimetry which estimates tumor dose from transmission dose using the new algorithm. This study was carried out to confirm accuracy of correction by tissue density in tumor dose estimation utilizing transmission dose. Method: Cork phantom (CP, density $0.202;gm/cm^3$) having similar density with lung parenchyme and polystyrene phantom (PP, density $1.040;gm/cm^3$) having similar density with soft tissue were used. Dose measurement was carried out under condition simulating human chest. On simulating AP-PA irradiation, PPs with 3 cm thickness were placed above and below CP, which had thickness of 5, 10, and 20 cm. On simulating lateral irradiation, 6 cm thickness of PP was placed between two 10 cm thickness CPs additional 3 cm thick PP was placed to both lateral sides. 4, 6, and 10 MV x-ray were used. Field size was in the range of $3{times}3$ cm through $20{times}20$ cm, and phantom-chamber distance (PCD) was 10 to 50 cm. Above result was compared with another sets of data with equivalent thickness of PP which was corrected by density. Result: When transmission dose of PP was compared with equivalent thickness of CP which was corrected with density, the average error was 0.18 (${pm}0.27$) % for 4 MV, 0.10 (${pm}0.43$) % for 6 MV, and 0.33 (${pm}0.30$) % for 10 MV with CP having thickness of 5 cm. When CP was 10 cm thick, the error was 0.23 (${pm}0.73$) %, 0.05 (${pm}0.57$) %, and 0.04 (${pm}0.40$) %, while for 20 cm, error was 0.55 (${pm}0.36$) %, 0.34 (${pm}0.27$) %, and 0.34 (${pm}0.18$) % for corresponding energy. With lateral irradiation model, difference was 1.15 (${pm}1.86$) %, 0.90 (${pm}1.43$) %, and 0.86 (${pm}1.01$) % for corresponding energy. Relatively large difference was found in case of PCD having value of 10 cm. Omitting PCD with 10 cm, the difference was reduced to 0.47 (${pm}$1.17) %, 0.42 (${pm}$0.96) %, and 0.55 (${pm}$0.77) % for corresponding energy. Conclusion When tissue inhomogeneity such as lung is in tract of x-ray beam, tumor dose could be calculated from transmission dose after correction utilizing tissue density.
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