A Feasibility Study and Optimization of Dosimetric Objective with Heterogeneity Corrections for RTOG Studies with Hypo-fractionated Stereotactic Radiotherapy of Lung Cancers

RTOG-0236 is a protocol that uses hypo-fractionated stereotactic radiotherapy (SBRT) to treat early stage non-small cell lung cancer. Dose calculations for this protocol did not consider tissue density variations, but heterogeneity corrected plans with the same monitor units were collected for each case. We previously reported on the use of corrected plans to establish dose constraints for future SBRT lung protocols that require heterogeneity corrections. This research reports on the re-optimization of the dose distributions for each case by performing the treatment planning process with heterogeneity corrections. The results of this study could determine if the planning dose constraints established in our previous study need revision. Fifteen RTOG-0236 study sets were recalculated on CMS XIO using pixel by pixel density corrections with the superposition algorithm. The plans for these cases with unit density originally met the 0236 objective criteria. New plans were devised to meet objectives as recommended by Xiao et al1 for heterogeneity corrections. The plans were compared to RTOG 0813 which used constraints that were modified slightly from 0236 and they were also evaluated against the paper by Hurkmans et al2 for the ROSEL study. Generating heterogeneity corrected lung SBRT treatment plans using dose constraints from the Xiao et al paper resulted in 12 of 15 cases that were acceptable. Three of the cases had major deviations (MDs) for 50% isodose line volume to planning target volume (50%IDL to PTV), and two of three also had MDs for maximum dose 2cm from the PTV as a percent of prescription dose. Comparing these same plans to the dose constraints for the RTOG-0813 protocol resulted in 12 of 15 acceptable cases. One case had a MD for D_2cm and 50%IDL to PTV, one case had a MD for D_2cm, and the third case had a MD for 50%IDL to PTV. Comparing the constraints for the ROSEL study resulted in 11 of 15 acceptable cases. Two of the MDs were the ratio of prescription isodose volume to PTV on the R100% values, and two MDs were for D_2cm. For RTOG-0813 and Rosel criteria, where minor deviations are specified, minor deviations occurred in approximately 50% of the plans. We found achieving D_2cm and 50%IDL to PTV first, then optimizing the R100% values rendered the highest quality plan. These three sets of criteria are comparable. Therefore, dose constraints for the planning situation discussed here must be viewed as guidelines for achieving optimized dose distributions.