Dosimetric evaluation of 6 MV and 18 MV intensity-modulated radiotherapy plans for treatment of carcinoma of the cervix

target coverage, OAR sparing, integral dose, and monitoring units. Conclusions: The tradeoff of using 6 MV and 18 MV for cervix patients depends on many parameters. Since the same PTV coverage was forced for both energies by having the same optimization constraints, there was little difference in target coverage and conformity index for both energies. for

Low energy produces tighter dose distributions around the target than higher energies but also deposits a higher dose in the surface region near the beam (3-5). In contrast, one of the implied tenets of intensity-modulated radiation therapy (IMRT) has been that energy does not matter or is less important [5,6].
With the advent of Intensity Modulated Radiation Therapy (IMRT), an increase in the number of monitor units (MU) relative to 3DCRT has to lead to a concern about the patient's total body neutron burden incurred during treatment with these high energy photon beams. As a result, the majority of IMRT cervix treatments delivered today are with lower energy (6-10 MV) photons where neutron production in a linear accelerator treatment head is negligible [7-10].
There was no much benefit achieved with 18 MV beams for rectum compared to 6 MV plans ( Figure   8).
The rectum means dose was increased with the use of composite plans compared to 6 MV and 18 MV beam plans. No much difference was observed between 6 MV and18 MV plans (

Discussion
For PTV, Dmax and conformity index was comparable in 6MV, 18MV, and composite plan. In the case of target coverage and minimum dose, there was no significant benefit observed in using higher energies (18 MV) observed in all three plans.
The minimum dose in the target was decreased with 18 MV and composite plans compared to 6 MV plan.
This may be due to lack of target coverage in the posterior of the patient where most of the cervix target extends posteriorly. For the same reason, the dose homogeneity in the target also decreased with higher energies (18 MV).
The OAR sparing was better achieved with 18 MV beams only for Bladder and Bowel. It was observed from these results that the larger volume of OAR was spared with higher energy beams compared to the one which has a smaller volume. The rectal dose was slightly increased with 18 MV and composite plan than the 6 MV plan. This may be due to a higher entrance dose with 6 MV photons from the posterior beam. Also, dose modulation is the key to successful IMRT and this modulation is heavily dependent on the lateral fall off provided by the leaves of the multi-leaf collimator. The ability to modulate is impaired at higher energy because the lateral range of electrons widens the lateral fall off. The lateral range increase is the result of the same fundamental physics that produces a deeper depth of maximum dose for high energy photons. The electrons are being set in motion at higher energies, but not so high that they do not scatter. A typical In motion in the same direction as the photon. This leads to blurring of the lateral boundary and inherently limits the modulation that can be achieved. So high modulation is required when avoidance of the rectum has been assigned high priority in the optimizer. Such high priority demands a very steep gradient between the cervix and rectum. This gradient becomes obviously less steep if a high energy beam is used with its wider penumbra. So irrespective of energy, the dose to the rectum was not much varied between all three plans.
There were no tight dose constraints given for the right and left femoral heads since it is relatively away from the PTV. Since it has no much role in optimization, the dose to femoral heads was not varied much between all three plans irrespective of energy. The total integral dose of the bladder was better achieved with higher energy beams. The integral dose of Body_PTV was also given good results in 18 MV and composite plans compared to lower energies.
In spite of an increase in 10% isodose volume in 18 MV and composite plans, the total integral dose was reduced with 18 MV and composite beams. The volume received by 50% isodose volume was better achieved with 18 MV and composite beams compared to 6 MV beam.
The main disadvantage of using an 18 MV beam is the secondary neutron production in the linac head assembly itself. Neutron is of considerable importance in radiation safety because for any given absorbed dose, neutron irradiation typically yields a much higher biologically effective dose (BED) than photons for practically any biological endpoint.

Limitations
The sample size and specific to one treatment site are limitations of this study.

Conclusion
The tradeoff of using 6 MV and 18 MV for cervix patients depends on many parameters. Since the same PTV coverage was forced for both energies by having the same optimization constraints, there was little difference in target coverage and conformity index for both energies. DVHs for the critical structures showed a little difference between 6 MV, 18 MV, and composite beams. The 6 MV IMRT integral dose was higher than the 18 MV and composite plan. Although integral dose is generally assumed to be improved by the use of higher energy photons, there is concern that the higher neutron contamination with 18 MV beams increases the chance of secondary malignancies.
What does the study add to the existing knowledge?