Dosimetric Evaluation of Different Algorithms on Heterogeneous Slab Phantom Using CMS XiO and MONACO Treatment Planning System for 4MV, 6MV and 15MV Beam Energy: An Institutional Study.

Aim: To study the dosimetric behavior of dose computational algorithms in inhomogeneous medium using CMS XiO and MONACO treatment planning system (TPS) for 4 megavoltage (MV), 6 MV and 15 MV photon beam energies.

Material and methods: Styrofoam blocks of thickness 1.90 cm, 3.8 cm and 5.70 cm was used to introduce inhomogeneity in a slab phantom. Wipro GE computed tomography (CT) scanner was used to scan the phantom. Doses were computed on the central axis of the beam using convolution (C), superposition (S), fast superposition (FS), collapsed cone convolution (CCC) and monte carlo (MC) algorithms for field geometries of 5x5 cm2 and 10x10 cm2 for above said photon beam energies, respectively. An Ion chamber (IC) of 0.6 cc volume was used for the dose measurements. The deviation between measured and TPS computed doses were recorded.

Results: The PDD (Percentage depth dose) data obtained from the TPS (calculated data) and LINAC (measured data) was used for comparison based on different algorithms in order to calculate the percentage of maximum deviation (PMD). The PMD in MC algorithm were calculated for field sizes of 5x5 cm2 and 10x10 cm2 are found to be in ranging from 0.73% to -4.49% for 4MV,  1.62% to -2.42% for 6MV and 4.53% to -1.47%  for 15 MV for 1.90 cm air gap, 2.21% to -3.75% for 4MV, 3.87% to -2.88% for 6 MV and 4.87% to -3.46%  for 15 MV for 3.80 cm air gap, 2.77% to -4.66% for 4MV, 3.87% to -2.86% for 6 MV and 5.66% to -4.92% for 15 MV for 5.70cm air gap which is less as compared to CCC, C, FS, and S algorithms.

Conclusion: The comparison of C, S, FS, CCC and MC algorithms demonstrated that MC having better agreement with IC measurements. In conclusion, MC is a superior option for dose computation, particularly in the presence of low-density heterogeneities.