Effects of different CT value assignment methods on dose calculations in radiotherapy planning for brain metastases

To study the effects of different CT value assignment methods on the dose calculations in radiotherapy plans for brain metastases, this study will provide a reference for radiotherapy planning design based on MR images.

All fifty recruited patients underwent CT and MR simulated localization the same day as, but prior to, three dimensional conformal radiation therapy (3D-CRT) or intensity modulated radiation therapy (IMRT) for brain metastases. After rigid registration of both the CT and MR images, the main tissues and organs were delineated on the CT and MR images. The average CT value of each tissue or organ was calculated. Three groups of pseudo-CT were generated by three CT value assignment methods: (I) the whole tissue was assigned 140 HU; (II) cavity, bone and other tissues were assigned -700, 700 and 20 HU, respectively; (III) tissue- and organ-specific CT values were given. The dose distribution was recalculated based on the three groups of pseudo-CT to obtain Plan2, Plan3 and Plan4, accordingly. The resultant radiotherapy plans were considered the original plan (Plan1). Then, the dosimetric differences between these three plans and Plan1 were compared.

The average pseudo-CT values of bone and cavity were 731.7±69.3 and -725.5±26.1 HU, respectively. The range of average soft-tissue CT values was from -70 to 70 HU. The dose distribution between Plan1 and Plan2, Plan3 or Plan4 showed some differences, and the differences decreased in turn. The differences in the maximum dose of the lenses can reach 5.0%, 1.5% and 1.2%, respectively, while the differences in other dose parameters (maximum dose, mean dose and D _98% to the PTV, D _5% of the brainstem, and maximum dose of the brainstem, corpus callosum, left eye, right eye) were basically less than 2.0%, 1.2% and 0.8%, respectively. This shows that in the CT value assignment method, the dose calculation error can be greatly reduced by assigning the value to the bone and cavity separately, and if the different soft tissues are distinguished, the error of the dose calculation can be further reduced by more than 30%. In the pixel-by-pixel dosimetric comparison, the areas of more than 1% dose difference between Plan1 and Plan3 as well as Plan4 were mainly distributed near skin while those between Plan1 and Plan2 were mainly distributed at the bone, cavity, bone and soft tissues junction, and the skin near the field.

In summary, a scheme for assigning specific CT values to MRI-based radiotherapy is established. The scheme will provide patients with a dose-free radiotherapy plan. Through the calculation of the differences between the new plans and the old plan, it is found that our scheme can basically control the dose error below 0.8% to meet the clinical requirements.