Feasibility and sensitivity study of helical tomotherapy for dose painting plans

Abstract

Important limitations for dose painting are due to treatment planning and delivery constraints. The purpose of this study was to develop a methodology for creating voxel-based dose painting plans that are deliverable using the clinical TomoTherapy Hi-Art II treatment planning system (TPS). Material and methods. Uptake data from a head and neck patient who underwent a [(61)Cu]Cu-ATSM (hypoxia surrogate) PET/CT scan was retrospectively extracted for planning. Non-uniform voxel-based prescriptions were converted to structured-based prescriptions for compatibility with the Hi-Art II TPS. Optimized plans were generated by varying parameters such as dose level, structure importance, prescription point normalization, DVH volume, min/max dose, and dose penalty. Delivery parameters such as pitch, jaw width and modulation factor were also varied. Isodose distributions, quality volume histograms and planning target volume percentage receiving planned dose within 5% of the prescription (Q(0.95-1.05)) were used to evaluate plan conformity. Results. In general, the conformity of treatment plans to dose prescriptions was found to be adequate for delivery of dose painting plans. The conformity was better as the dose levels increased from three to nine levels (Q(0.95-1.05): 69% to 93%), jaw decreased in width from 5.0cm to 1.05cm (Q(0.95-1.05): 81% to 93%), and modulation factor increased up to 2.0 (Q(0.95-1.05): 36% to 92%). The conformity was invariant to changes in pitch. Plan conformity decreased as the prescription DVH constraint (Q(0.95-1.05): 93% vs. 89%) or the normalization point (Q(0.95-1.05): 93% vs. 90%) deviated from the means. Conclusion. This investigation demonstrated the ability of the Hi-Art II TPS to create voxel-based dose painting plans. Results indicated that agreement in prescription dose and planned dose distributions for all plans were sensitive to physical delivery parameter changes in jaw width and modulation factors, but insensitive to changes in pitch. Tight constraints on target structures also resulted in decreased plan conformity while under a relaxed set of optimization parameters, plan conformity was increased.

Figure 1. Schematic of workflow for dose painting with clinical treatment planning systems

From a fused PET/CT image, PET uptake within the target volume is transformed to a voxel-based prescription via a linear redistribution of dose (prescription function). The prescription is discretized into equi-spaced dose levels (e.g. 5 levels), which form the basis for target substructures (dose discretization). Each substructure is prescribed the mean dose representative of the underlying voxel doses, with a DVH objective given by the fractional volume receiving this mean dose or higher. A clinically deliverable treatment plan is generated from IMRT optimization to substructure objectives, yielding a planned dose that can be compared back to the prescribed dose at every voxel.

Figure 2. Axial planned dose distributions and QVH plots for varying prescription dose levels and permutations in physical delivery parameters in a head and neck cancer patient

Variations in plan conformity are quantified by QVH plots. Note that with increasing dose level, increasing modulation factors (MF) up to 2.0, and decreasing jaw width, plan conformity increases. Plan conformity is invariant to changes in pitch.