We present a new dose delivery scheme utilizing sub-conformal dynamic arc (sub-CD-ARC) with a dynamic multileaf collimator (dMLC) modulation in a single rotation. In sub-conformal delivery, a MLC aperture conforms not to the whole target but to its portion, simultaneously completely avoiding organs at risk (OARs). In CD-ARC therapy, the dose deposition level depends on the target width and distance to the axis of rotation, and therefore the use of multiple arcs is necessary to achieve a uniform dose within the target in 3D. In our delivery scheme, the dose deposition variations symptomatic of non-modulating sub-CD-ARC are compensated for in 3D by a quasi-periodic monotonic dMLC modulation. For this reason, we call this scheme sub-conformal dynamic modulated arc (sub-CD-MARC) therapy. The advantage of using such a modulation is that MLC leaf motion has just a few control points, and that an inverse planning problem is reduced to a linear equation problem with a few unknown parameters, which have clear physical meaning. We show the general dosimetric properties of sub-CD-ARC and sub-CD-MARC for a specific geometry of the target and OARs (rotational symmetry with varying inner and outer radii along the axis of rotation). In addition, we present numerical results of sub-CD-MARC inverse planning optimization.
Quadratic programming is an alternative approach for inverse planning which generates clinically satisfying plans in comparison to the clinical system and constitutes an efficient optimization process characterized by uniqueness and reproducibility of the solution.
The OBI v1.4 system allows for imaging with a larger variety of imaging parameters compared to previous OBI v1.3 systems. The largest doses (up to 4 cGy) were measured in a phantom when OBI v1.4 system was used for imaging with half-fan modes. Using full-fan modes resulted in the doses less than 1.6 cGy. Further decrease in dose may be achieved by reducing mAs while preserving acceptable image quality. Organ specific sparing (e.g., contralateral breast) may be achieved by proper selection of the start and stop angles. For thorax imaging, the use of Low Dose Thorax mode is recommended.
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