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BackgroundTo evaluate the impact of image-guided radiation therapy (IGRT) versus non-image-guided radiation therapy (non-IGRT) on the dose to the clinical target volume (CTV) and the cervical spinal cord during fractionated intensity-modulated radiation therapy (IMRT) for head-and-neck cancer (HNC) patients.Material and MethodsFor detailed investigation, 4 exemplary patients with daily control-CT scans (total 118 CT scans) were analyzed. For the IGRT approach a target point correction (TPC) derived from a rigid registration focused to the high-dose region was used. In the non-IGRT setting, instead of a TPC, an additional cohort-based safety margin was applied. The dose distributions of the CTV and spinal cord were calculated on each control-CT and the resulting dose volume histograms (DVHs) were compared with the planned ones fraction by fraction. The D50 and D98 values for the CTV and the D5 values of the spinal cord were additionally reported.ResultsIn general, the D50 and D98 histograms show no remarkable difference between both strategies. Yet, our detailed analysis also reveals differences in individual dose coverage worth inspection. Using IGRT, the D5 histograms show that the spinal cord less frequently receives a higher dose than planned compared to the non-IGRT setting. This effect is even more pronounced when looking at the curve progressions of the respective DVHs.ConclusionsBoth approaches are equally effective in maintaining CTV coverage. However, IGRT is beneficial in spinal cord sparing. The use of an additional margin in the non-IGRT approach frequently results in a higher dose to the spinal cord than originally planned. This implies that a margin reduction combined with an IGRT correction helps to maintain spinal cord dose sparing best as possible. Yet, a detailed analysis of the dosimetric consequences dependent on the used strategy is required, to detect single fractions with unacceptable dosimetric deviations.

Purpose: To study potential advantages and disadvantages of a multileaf collimator (MLC) relative to iris variable aperture collimation for prostate cancer treatment with a CyberKnife robotic radiosurgery system. Methods: Six treatment plans for three prostate cases are generated using target conformal segments with a hypothetical MLC vs. circular beams with an iris variable aperture collimator. Conformality, homogeneity, number of MUs and segments are used as plan quality indicators. Quasi Newton Gradient descent optimization is applied as optimization strategy. MU reduction during the optimization process is achieved with two‐dimensional median fluence smoothing. The final sequencing is performed at sequencing levels 2 and 3 to reduce the number of segments. Iris baseline plans were generated using the vendorˈs TPS. Treatment plans A and B are conventionally fractionated IMRT with homogeneous PTV coverage. Plans C and D are IMRT with homogeneous PTV coverage (−/+ integrated boosts). Plans E and F are hyperfractionated SBRT with homogeneous PTV coverage (−/+ integrated boosts, and stricter OAR dose constraints). Results: With identical number of CyberKnife nodes for an iris and MLC treatment plan, an improvement in dose conformality is achieved with a MLC for all treatment plans and dose homogeneity is only slightly degraded. In all cases, the total number of MUs with multileaf collimated fields is reduced in comparison to circular collimated iris variable aperture fields (avg. reduction 28%). The number of segments for a comparable MLC treatment plan with an iris plan is also reduced for 4 cases and slightly increased for 2 of them (avg. reduction 17%). Conclusions: This investigation shows that a MLC could provide several benefits for intensity modulated prostate radiosurgery with a CyberKnife robotic radiosurgy system. Multileaf collimated beams for radiosurgery allow a highly conformal radiation. Their target conformal segments in addition reduce the required total number of MUs and segments.

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