The purpose of this study was to compare the Pencil Beam (PB) with Monte Carlo (MC) calculated dosimetric results using phantoms for air cavity region. Measurements in Tough water phantom with air gaps were used to verify the calculated dose. The plane-parallel ionization chamber was moved from 2 mm to 20 mm behind air gap. Calculations were performed for various air gaps (1.0, 2.0, 3.0 and 4.0 cm) and field sizes (4.2 × 4.2, 6.0 × 6.0 and 9.8 × 9.8 cm2). The lateral missing tissue measurement was performed using the radiochromic RT-QA film. Dose difference between PB and chamber measurement near an air gap was greater for smaller field size, larger air gap thickness, and shallower depth behind air gap. As the distance from the phantom edge became shorter, the dose differences of the PB calculation and film measurement became larger. MC calculations were found within 3% agreement to the measured dose distributions. Our results demonstrate an excellent agreement between ionization chamber and radiochromic RT-QA film measurements and MC calculations.
 Task Group No. 65, the Radiation Therapy Committee of the American Association of Physicists in Medicine, “Tissue Inhomogeneity Corrections for Megavoltage Photon Beams. Madison,” Medical Physics Publishing, Madison, 2004.
 C. Hurkmans, T. Knöös, P. Nilsson, G. Svahn-Tapper and H. Danielsson, “Limitations of a Pencil Beam Approach to Photon Dose Calculations in the Head and Neck Region,” Radiotherapy & Oncology, Vol. 37, No. 1, 1995, pp. 74-80.
 E. K. Osei, J. Darko, A. Mosseri and J. Jezioranski, “EGSNRC Monte Carlo Study of the Effect of Photon Energy and Field Margin in Phantoms Simulating Small Lung Lesions,” Medical Physics, Vol. 30, No. 10, 2003, pp. 2706-2714. http://dx.doi.org/10.1118/1.1607551
 E. E. Klein, L. M. Chin, R. K. Rice and B. J. Mijnheer, “The Influence of Air Cavities on Interface Doses for Photon Beams,” International Journal of Radiation Oncology*Biology*Physics, Vol. 27, No. 2, 1993, pp. 419-427.
 W. K. Kan, P. M. Wu, H. T. Leung, T. C. Lo, C. W. Chung, D. L. Kwong, et al., “The Effect of the Nasopharyngeal Air Cavity on X-Ray Interface Doses,” Physics in Medicine & Biology, Vol. 43, No. 3, 1998, pp. 529-537. http://dx.doi.org/10.1088/0031-9155/43/3/005
 B. H. Shahine , M. S. A. L. Al-Ghazi and E. El-Khatib, “Experimental Evaluation of Interface Doses in the Presence of Air Cavities Compared with Treatment Planning Algorithms,” Medical Physics, Vol. 26, No. 3, 1999, pp. 350-355. http://dx.doi.org/10.1118/1.598526
 I. Kawrakow, “Accurate Condensed History Monte Carlo Simulation of Electron Transport. I. EGSnrc, the New EGS4 Version,” Medical Physics, Vol. 27, No. 3, 2000, pp. 485-498. http://dx.doi.org/10.1118/1.598917
 M. Fragoso, N. Wen, S. Kumar, D. Liu, S. Ryu, B. Movsas, et al., “Dosimetric Verification and Clinical Evaluation of a New Commercially Available Monte Carlo-Based Dose Algorithm for Application in Stereotactic Body Radiation Therapy (SBRT) Treatment Planning,” Physics in Medicine & Biology, Vol. 55, No. 16, 2010, pp. 4445-4464.
 M. Fippel, F. Haryanto, O. Dohm, F. Nüsslin and S. Kriesen, “A Virtual Photon Energy Fluence Model for Monte Carlo Dose Calculation,” Medical Physics, Vol. 30, No. 3, 2003, pp. 301-311.
 X. A. Li, C. Yu and T. Holmes, “A Systematic Evaluation of Air Cavity Dose Perturbation in Megavoltage X-Ray Beams,” Medical Physics, Vol. 27, No. 5, 2000, pp. 1011-1017. http://dx.doi.org/10.1118/1.598966
 A. L. Petoukhova, K. van Wingerden, R. G. Wiggenraad, P. J. van de Vaart, J. van Egmond, E. M. Franken, et al., “Verification Measurements and Clinical Evaluation of the iPlan RT Monte Carlo Dose Algorithm for 6 MV Photon Energy,” Physics in Medicine & Biology, Vol. 55, No. 16, 2010, pp. 4601-4614.
 C. F. Behrens, “Dose Build-Up behind Air Cavities for Co-60, 4, 6 and 8 MV. Measurements and Monte Carlo Simulations,” Physics in Medicine & Biology, Vol. 51, No. 22, 2006, pp. 5937-5950.
 L. Wang, E. Yorke and C. S. Chui, “Monte Carlo Evaluation of Tissue Inhomogeneity Effects in the Treatment of the Head and Neck,” International Journal of Radiation Oncology*Biology*Physics, Vol. 50, No. 5, 2001, pp. 1339-1349.
 M. Yoon, D. H. Lee, D. Shin, S. B. Lee, S. Y. Park and K. H. Cho, “Accuracy of Inhomogeneity Correction Algorithm in Intensity-Modulated Radiotherapy of Head-and-Neck Tumors,” Medical Dosimetry, Vol. 32, No. 1, 2007, pp. 44-51.
 J. N. Waldron, B. O’Sullivan, P. Warde, P. Gullane, F. F. Lui, D. Payne, et al., “Ethmoid Sinus Cancer: Twenty-Nine Cases Managed with Primary Radiation Therapy,” International Journal of Radiation Oncology*Biology* Physics, Vol. 41, No. 2, 1998, pp. 361-369.
 J. N. Waldron, B. O’Sullivan, P. Gullane, I. J. Witterick, F. F. Liu, D. Payne, et al., “Carcinoma of the Maxillary Antrum: A Retrospective Analysis of 110 Cases,” Radiotherapy & Oncology, Vol. 57, No. 2, 2000, pp. 167-173.