Conversion Coefficients for Determination of Dispersed Photon Dose during Radiotherapy: NRUrad Input Code for MCNP

Radiotherapy
is a common cancer treatment module, where a certain amount of dose will be delivered
to the targeted organ. This is achieved usually by photons generated by linear accelerator
units. However, radiation scattering within the patient’s body and the
surrounding environment will lead to dose dispersion to healthy tissues which are
not targets of the primary radiation. Determination of the dispersed dose would
be important for assessing the risk and biological consequences in different
organs or tissues. In the present work, the concept of conversion coefficient (F) of the dispersed dose was developed,
in which F = (Dd/Dt),
where Dd was the dispersed
dose in a non-targeted tissue and Dt
is the absorbed dose in the targeted tissue. To quantify Dd and Dt,
a comprehensive model was developed using the Monte Carlo N-Particle (MCNP) package
to simulate the linear accelerator head, the human phantom, the treatment couch
and the radiotherapy treatment room. The present work also demonstrated the feasibility
and power of parallel computing through the use of the Message Passing
Interface (MPI) version of MCNP5.