Minimum Planning Target Volume Coverage Necessary for the Delivery of the Prescribed Dose in Lung Radiotherapy

Intensity Modulated Radiation Therapy (IMRT) has been used in lung cancer treatment to deliver dose distributions with steep gradients and exquisite target volume conformality. While many benefits of IMRT exist, significant challenges are involved when unpredictable intrafraction movement is combined with the highly optimized nature of IMRT. The National Cancer Institute (NCI) Guidelines for the Use of IMRT state that there is potential for significant dosimetric consequences if the target volume moves while treatment is being administered. Thus, lung IMRT is especially challenging due to breathing motion. Furthermore, tissue density differences at the lung-tumor interface contribute to disagreements between the administered and planned doses. These inhomogeneities may allow for a relatively large portion of the planning target volume (PTV) to be left without coverage and the target will still receive the prescribed dose during treatment. For example, the Radiation Therapy Oncology Group (RTOG) 0617 study requires that 95% of the PTV is to be covered by 95% (93% is a minor violation) of the prescription dose during planning.This study aimed to establish an IMRT lung planning goal that ensures 99% of the internal target volume (ITV) is covered by 95% of the prescribed dose during treatment. Ten previously generated IMRT plans were analyzed. Each plan was renormalized such that 95% of the PTV is covered by increasingly higher doses, ranging from 93 to 97% of the prescription. For each plan, multiple target movements were simulated by shifting the isocentre of the beams within a range previously measured for this patient population and recomputing the dose. Finally, ITV coverage during simulated treatment was quantified using dose-volume histograms for each planning goal.Statistically, none of the planning goals was significantly different from another based on the number of times the ITV was not covered by 95% of the prescribed dose. Thus, covering 95% of the PTV covered by 93% of the prescription achieves the same result covering 95% of the PTV by 97% of the prescription. The complete data suggest coverage failure rate may decrease with increasing dose but this was not significant. We are currently applying the RTOG 0617 planning goal clinically and will repeat this analysis with additional patient data to ensure optimal delivery of lung IMRT at our institution.