Inverse Planned Conformality And Normal Tissue Complication Probability Effects For High Risk Pelvic Lymph Node Irradiation

To determine the extent of safe dose escalation possible for volume modulated arc therapy (VMAT) for high-risk pelvic lymph-node irradiation. Ten high-risk VMAT prostate cancer patient plans with re-planned 3D-Conformal Radiation therapy (3D-CRT) plans were evaluated using physical dose (PD), radiobiological dose and control probability metrics. Dose-volume histograms (DVH) were used to derive the mean PDs and equivalent to 2Gy per fraction doses (EQD2Gy) of the planning target volume (PTV), PTV-lymph nodes (PTV-LN), bladder, rectum, sigmoid and unspecified normal tissue organs at risk (OAR). Tumor control probability (TCP), normal tissue complication probability (NTCP) and complication-free tumor control probability (P+) were calculated for both techniques from voxel EQD2GyDVH data. A 3DCRT complication equivalent Dose-Escalation (DE) Ratio was applied to the VMAT OARs and then to VMAT targets for calculation of DE-TCP. PTV, PTV-LN PD and EQD2Gy increased an average of 8.5%, 2.7% (+5.6% target avg.) and 8.0%, 2.4% (+5.2% target avg.), respectively. Bladder, rectum, sigmoid and normal tissue PD and EQD2Gyreduced an average of 26.4%, 36.0%, 26.6%, 21.3% (-27.6% OAR avg.) and 32.2%, 42.1%, 32.7%, 26.3% (-33.3% OAR avg.), respectively. Average VMAT TCP (%) increase to PTV and PTV-LN was 7.4 (±2.7) and 17.6 (±12.0) with average NTCP (%) decrease to bladder, rectum and sigmoid of 3.8 (±3.9), 10.7 (±2.5) and 31.6 (±28.4), respectively, resulting in combined P+ (%) increase of 24.0 (±15.9). Average DE ratio for bladder, rectum and sigmoid were 1.04 (±.01), 1.06 (±.02) and 1.10 (±.07), respectively, corresponding to a DE dose per fraction of 187cGy (±3.1). DE VMAT PTV and PTV-LN displayed average TCP increases of 4.0% and 28.8%, respectively; resulting in a +27.6% combined P+ increase. A method for the determination of acceptable complication-equivalent escalation illustrates that a marginal dose per fraction increase may result in large increases in P+, holding constant normal tissue complications.