This work aimed at
evaluating the effect of 6- and 10-MV photon energies on intensity-modulated radiation therapy (IMRT) treatment plan outcome in different
selected diagnostic cases. For such purpose, 19 patients,
with different types of
non CNS solid tumers, were selected. Clinical step-and-shoot IMRT treatment
plans were designed for delivery on a Siemens
Oncor accelerator with 82 leafs; multi-leaf collimators (MLCs). To ensure that
the similarity or difference among the plans is due to energy alone, the
same optimization constraints were applied for both energy plans. All the parameters like beam angles,
number of beams, were kept constant to achieve the same clinical objectives.
The Comparative evaluation was based on dose-volumetric analysis of both energy
IMRT plans. Both qualitative and quantitative methods were used. Several
physical indices for Planning Target Volume (PTV), the relevant Organs at Risk
(OARs) as mean dose (Dmean), maximum dose (Dmax), 95% dose (D95), integral
dose, total number of segments, and the number of MU were applied. Homogeneity
index and conformation number were two other evaluation parameters that were
considered in this study. Collectively, the use of 6 MV photons was
dosimetrically comparable with 10 MV photons in terms of target coverage,
homogeneity, conformity, and OAR savings. While 10-MV plans showed a
significant reduction in the number of MUs that varied between 4.2% and 16.6%
(P-value = 0.0001) for the different cases compared to 6-MV. The percentage volumes
of each patient receiving 2 Gy and 5 Gy
were compared for the two energies. The general trend was that 6-MV plans had
the highest percentage volume, (P-value = 0.0001, P-value = 0.006)
respectively. 10-MV beams actually decreased the integral dose (from average 183.27 ± 152.38 Gy-Kg to 178.08 ± 147.71 Gy-Kg, P-value = 0.004)
compared with 6-MV. In general, comparison of the above parameters showed
statistically significant differences between 6-MV and 10-MV groups. Based on
the present results, the 10-MV is the optimal energy for IMRT, regardless of
the concerns about a potential risk of radiation-induced malignancies. It is
recommended that the choice to treat at 10 MV be taken as a risk vs. benefit as the clinical significance remains to
be determined on case by case basis.