Toward truly optimal IMRT dose distribution: inverse planning with voxel-specific penalty

Abstract

Purpose: To establish an inverse planning framework with adjustable voxel penalty for more conformal IMRT dose distribution as well as improved interactive controllability over the regional dose distribution of the resultant plan.

Materials and method: In the proposed coarse-to-fine planning scheme, a conventional inverse planning with organ specific parameters is first performed. The voxel penalty scheme is then "switched on" by allowing the prescription dose to change on an individual voxel scale according to the deviation of the actual voxel dose from the ideally desired dose. The rationale here is intuitive: when the dose at a voxel does not meet its ideal dose, it simply implies that this voxel is not competitive enough when compared with the ones that have met their planning goal. In this case, increasing the penalty of the voxel by varying the prescription can boost its competitiveness and thus improve its dose. After the prescription adjustment, the plan is re-optimized. The dose adjustment/re-optimization procedure is repeated until the resultant dose distribution cannot be improved anymore. The prescription adjustment on a finer scale can be accomplished either automatically or manually. In the latter case, the regions/voxels where a dose improvement is needed are selected visually, unlike in the automatic case where the selection is done purely based on the difference of the actual dose at a given voxel and its ideal prescription. The performance of the proposed method is evaluated using a head and neck and a prostate case.

Results: An inverse planning framework with the voxel-specific penalty is established. By adjusting voxel prescriptions iteratively to boost the region where large mismatch between the actual calculated and desired doses occurs, substantial improvements can be achieved in the final dose distribution. The proposed method is applied to a head and neck case and a prostate case. For the former case, a significant reduction in the maximum dose to the brainstem is achieved while the PTV dose coverage is greatly improved. The doses to other organs at risk are also reduced, ranging from 10% to 30%. For the prostate case, the use of the voxel penalty scheme also results in vast improvements to the final dose distribution. The PTV experiences improved dose uniformity and the mean dose to the rectum and bladder is reduced by as much as 15%.

Conclusion: Introduction of the spatially non-uniform and adjustable prescription provides room for further improvements of currently achievable dose distributions and equips the planner with an effective tool to modify IMRT dose distributions interactively. The technique is easily implementable in any existing inverse planning platform, which should facilitate clinical IMRT planning process and, in future, off-line/on-line adaptive IMRT.

Figure 1

DVHs of the head and neck IMRT plans obtained using the conventional approach with the fixed prescription (solid curves) and the proposed strategy of automated voxel prescription adjustment (dashed curves).

Figure 2

Head and neck plan dose distributions before (left) and after voxel prescription adjustment (right). The PTV and the sensitive structures (brainstem, left and right parotids) are shown in different colors. The isodose lines correspond to 95%, 65% and 30% of the prescribed dose (66 Gy). Hotspots are marked with red crosses.

Figure 3

Head and neck plan dose distributions for the manual voxel prescription adjustment method. A (top left): before the prescription adjustment. B (top right): after the adjustment of the brainstem prescription. C (bottom center): after the adjustment of the brainstem and PTV prescriptions. The PTV and the sensitive structures (brainstem, left and right parotids) are shown in different colors. The iso-dose lines correspond to 95%, 65% and 30% of the prescribed dose (66 Gy). Hotspots are marked with red crosses.

Figure 4

DVHs of the prostate IMRT plans obtained using the conventional approach with the fixed prescription (solid curves) and the proposed strategy of automated voxel prescription adjustment (dashed curves).

Figure 5

The difference between the ideal prescription and the calculated dose for selected voxels in the PTV and the cord for the head and neck case as a function of iteration number.

Figure 6

The value of the spinal cord objective as a function of iteration number.

Figure 7

The automated prescription adjustment method applied only to the PTV for the head and neck case. Solid lines represent the DVHs for the conventional IMRT plan with the fixed ideal prescription. Dashed lines correspond to the plan obtained with automated prescription adjustment.