A method to evaluate dose errors introduced by dose mapping processes for mass conserving deformations.

PURPOSE: To present a method to evaluate the dose mapping error introduced by the dose mapping process. In addition, apply the method to evaluate the dose mapping error introduced by the 4D dose calculation process implemented in a research version of commercial treatment planning system for a patient case. METHODS: The average dose accumulated in a finite volume should be unchanged when the dose delivered to one anatomic instance of that volume is mapped to a different anatomic instance-provided that the tissue deformation between the anatomic instances is mass conserving. The average dose to a finite volume on image S is defined as d(S)=e(s)/m(S), where e(S) is the energy deposited in the mass m(S) contained in the volume. Since mass and energy should be conserved, when d(S) is mapped to an image R(d(S→R)=d(R)), the mean dose mapping error is defined as Δd(m)=|d(R)-d(S)|=|e(R)/m(R)-e(S)/m(S)|, where the e(R) and e(S) are integral doses (energy deposited), and m(R) and m(S) are the masses within the region of interest (ROI) on image R and the corresponding ROI on image S, where R and S are the two anatomic instances from the same patient. Alternatively, application of simple differential propagation yields the differential dose mapping error, Δd(d)=|∂d∂e*Δe+∂d∂m*Δm|=|(e(S)-e(R))m(R)-(m(S)-m(R))m(R) (2)*e(R)|=α|d(R)-d(S)| with α=m(S)/m(R). A 4D treatment plan on a ten-phase 4D-CT lung patient is used to demonstrate the dose mapping error evaluations for a patient case, in which the accumulated dose, D(R)=∑(S=0) (9)d(S→R), and associated error values (ΔD(m) and ΔD(d)) are calculated for a uniformly spaced set of ROIs. RESULTS: For the single sample patient dose distribution, the average accumulated differential dose mapping error is 4.3%, the average absolute differential dose mapping error is 10.8%, and the average accumulated mean dose mapping error is 5.0%. Accumulated differential dose mapping errors within the gross tumor volume (GTV) and planning target volume (PTV) are lower, 0.73% and 2.33%, respectively. CONCLUSIONS: A method has been presented to evaluate the dose mapping error introduced by the dose mapping process. This method has been applied to evaluate the 4D dose calculation process implemented in a commercial treatment planning system. The method could potentially be developed as a fully-automatic QA method in image guided adaptive radiation therapy (IGART).

SU-E-T-526: Evaluation of Dose Mapping Errors via Use of a Volume-Based Dose Mapping Method.

PURPOSE: To quantify dose mapping errors (DMEs) of a point-based dose mapping method for 4D lung treatment plans. METHODS: Point-based dose mapping methods utilize deformation vector fields (DVFS) to interpolate dose from a deformed image. Volume-based dose mapping methods consider the volume overlap between deformed and reference voxels; defining dose as the integral energy divided by the integral mass of the voxel, and conserving integral dose . DME is defined as the dose differences between volume-based and point-based mapped dose (DME=(DpointBased-DvolumeBased)/DRx). The DME for a 4D lung case is compared with a bitmap DME method, both using a Pinnacle research version 8.1y DVF. DME is computed for ten 4D lung cases (five 10 phases, five 3 phase) with Pinnacle research version 9.100 DVFs. Multi-phase accumulated 4D DMEs are also evaluated. RESULTS: For all cases, the largest DMEs are located in the dose/density gradient regions. With Pinnacle 8.1y DVF, mapping dose from phase 9 to phase 0, results in a DME=-0.2%±6.1% (range of -76%∼112%). The same case with Pinnacle 9.100 DVFs, DME=0.3%±4.8%(-41%∼32%). Locations of large DME are consistent with those from the bitmap method. For the ten 4D lung cases, accumulated mean DME are within ±0.07% (std. deviations: 1∼5%, range -102%∼64%). Maximum tumor DMEs are less than 30cGy (DRx=7200cGy) for all patients. CONCLUSIONS: Due to its inherent integral dose conservation, volume-based dose mapping methods can quantify errors in point-based dose mapping methods. While mean DME values are small for the cases tested, standard deviations near 5% indicate that a substantial number of voxels have ∼5% dose mapping errors, however these dose errors do not occur in the target structures. Work supported by NIH P01CA116602.