N Singh - Department of Radiotherapy, King George Medical University, Lucknow, India
N K Painuly - M.S. Patel Cancer Center, Shree Krishna Hospital and Research Centre, Karamsad, Gujarat, India
L N Chaudhari - M.S. Patel Cancer Center, Shree Krishna Hospital and Research Centre, Karamsad, Gujarat, India
A Chairmadurai - Department of Radiotherapy, Jaypee Hospital, Noida, Uttar Pradesh, India
T Verma - Department of Radiotherapy, King George Medical University, Lucknow, India
D Shrotiya - Department of Radiotherapy, J.K.Cancer Hospital, Kanpur, Uttar Pradesh, India
C Bhatt - Department of Radiation Oncology, Batra Hospital, New Delhi, India

Aims: The aims of the present work are (۱) to evaluate dose calculation accuracy of two commonly used algorithms for ۱۵ MV small photon fields in a medium encompassing heterogeneity and (۲) to compare them with measured results obtained from gafchromic film EBT۲.Materials and Methods: Authors employed kailwood (Pinus Wallichiana) to mimic lung. Briefly, seven Kailwood plates, each measuring ۲۵x۲۵ cm۲ of varying thicknesses totaling ۱۳ cm equivalent to the mean thickness of an adult human lung, were sandwiched between ۵ cm tissue equivalent material from top and ۱۰ cm below. Physical measurements were performed using Radiochromic film EBT۲. The field sizes of ۱x۱, ۲x۲, ۵x۵ and ۱۰x۱۰ cm۲ were selected at ۱۰۰ cm SSD. Simulations were performed using EGSnrc/DOSRZnrc Monte Carlo code. The dose variation inside the inhomogeneity and near the interface was calculated using AAA & XVMC algorithm. Results: Preliminary results show that there is large variation of dose inside inhomogeneity. The maximum variation of dose inside the inhomogeneity for ۱x۱ cm۲ was found ۴۰% by AAA and ۴.۵% by XVMC compared to measured/simulated results. For the field size of ۲x۲ cm۲, these figures were ۲۷% by AAA & ۳.۵% by XVMC. For ۵x۵ cm۲ field size, the variation is small which becomes insignificant for larger fields. Conclusion: The results presented in this work indicate that for smaller fields, XVMC algorithm gives more realistic prediction, while there is the need for caution on using AAA algorithm for dose calculations involving small area irradiation encompassing heterogeneities and low-density media.

EGSnrc, Monte Carlo Simulation, Gafchromic Film, AAA, XVMC