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Photon Fluence Model for Distributed Radiation Sources Using the Convolution Method

S Dhanesar

S Dhanesar*, J Darko, L Schreiner, Queen's University and Cancer Center of Southeastern Ontario, Kingston, Ontario, Canada

SU-E-T-558 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: Sophisticated dose calculation algorithms based on principles of convolution theory are commonly used in commercial treatment planning systems for calculating photon dose from point-like radiation sources. However, not much has been studied regarding the modeling of distributed sources such as a Cobalt-60 (Co-60) source of 2 cm diameter. In this work, we present a convolution based photon fluence model that can be used for dose calculations for any finite size source.

Methods:The photon fluence model proposed in this work is based on two key functions: the source distribution function and the aperture function. The source distribution function, which models primary and scatter radiation independently, was determined using a one-time Monte Carlo (MC) simulation. Since the focus of this work is on Co-60 delivery, the MC modeling was based on a Theratronics T780 Co-60 unit (Best Theratronics). The validity of the MC model was verified with the ion chamber measurements made in a water phantom. The aperture function is a rectangular window function representing the field size. The fluence model was evaluated by comparing the calculated photon fluence to the MC simulated fluence.

Results:The comparisons between the photon fluence calculated using the proposed model and the MC simulation fluences show good agreement, better than 2% in the in-field region of all large and small fields. In the tail and sharp dose gradient regions, this agreement is better than 5% for the large field sizes and 1.5% for the small field sizes. The primary and scatter fluence output factors are within 2% of those obtained with the MC simulations.

Conclusion:The results of our investigations indicate that the proposed photon fluence model can accurately determine fluence for large and small radiation beams from finite size sources. This model has the potential of playing an integral role in Co-60 based IMRT treatment planning.

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