Effects of defining realistic compositions of the ocular melanoma on proton therapy

Abstract

Background: Recent studies in eye plaque brachytherapy have shown a considerable difference between the dosimetric results using water phantom and a model of human eye containing realistic materials. In spite of this fact, there is a lack of simulation studies based on such a model in proton therapy literatures. In the presented work, the effect of utilizing an eye model with ocular media on proton therapy is investigated using the MCNPX Monte Carlo Code.

Methods: Two different eye models are proposed to study the effect of defining realistic materials on dose deposition due to utilizing pencil beam scanning (PBS) method for proton therapy of ocular melanoma. The first model is filled with water, and the second one contains the realistic materials of tumor and vitreous. Spread out Bragg peaks (SOBP) are created to cover a typical tumor volume. Moreover, isodose curves are figured in order to evaluate planar variations of absorbed dose in two models.

Results: The results show that the maximum delivered dose in ocular media is approximately 12-32% more than in water phantom. Also it is found that using the optimized weighted beams in water phantom leads to disturbance of uniformity of SOBP in ocular media.

Conclusion: Similar to the results reported in eye brachytherapy published papers, considering the ocular media in simulation studies leads to a more realistic assessment of sufficiency of the designed proton beam in tissue. This effect is of special importance in creating SOBP, as well as in delivered dose in the tumor boundaries in proton pencil beam scanning method.

Keywords: Dosimetry; MCNPX; Proton therapy; SOBP; Uveal melanoma.

Figure 1

A schematic figure of the simulated eye phantom with a tumor located at the corner of the eye. The rectangular voxels are considered for dose calculation in depth direction.

Figure 2

A typical created SOBP with subplot weighted Bragg peaks.

Figure 3

A profile of depth-dose curves due to the irradiation of two typical proton pencil beams to the water phantom (solid lines) and ocular media (dashed lines) which are described in Materials and Methods. The 31 MeV and 23 MeV proton beams are corresponding to the peaks which are formed in the more and less depths, respectively.

Figure 4

The created spread out Bragg peaks (SOBP) inside instance depths in water phantom and ocular media based on the optimized weighting factors reported in table 2.

Figure 5

The created SOBP in instance depth of ocular media using the weighting factors which are optimized for water phantom (solid line), and the SOBP created in water phantom using the appropriate weighting factors for water (dashed line). See table 2.

Figure 6

Relative isodose curves due to irradiation of a 32 MeV proton pencil beam to the water phantom (red curves) and ocular media (black curves). For better resolution, the horizontal axis is limited between 0.4 to 1.1 cm.