SU‐E‐J‐97: Pretreatment Test and Post‐Treatment Evaluation for Iso‐NTCP Dose Guided Adapive Radiotherapy (DGART), Experience with Prostate Cancer Patients Treated with Rectal Balloons

Abstract

Purpose:To explore the feasibility of pretreatment test for iso‐NTCP DGART and to compare the pretreatment test results with post‐treatment evaluations.Methods:NTCP here refers to late rectal wall toxicity only and is calculated with the ring rectal wall DVH. Simulation for one time iso‐ NTCP DGART starts after half of the total dose was done for 10 patients to investigate if TCP gains could be achieved. Six patients were treated using a 12‐fraction 4.3Gy technique and four using 16‐fraction 3.63Gy technique. For each of the 12‐fraction cases a VMAT plan was generated in Pinnacle3™ using the daily CT obtained prior to the 6th fraction. A pretreatment simulation was performed using only the first 6 daily CTs. The idea is to add the 6 original plan delivered doses with 6 DGART plan delivered doses by deformable dose accumulation (DDA) on each of the first 6 CTs, resulting in 6 rectal wall doses (RWDs) and NTCPs. The 95% confidence interval (95%CI) for the 6 NTCPs were computed. The posttreatment evaluation was done by: a) copy the DGART plan to 6 CTs for fraction 7–12 and calculate the 6 actual DGART delivered fractional doses; b) sum the 6 actual DGART doses with the 6 original plan delivered doses by DDA on each of the 12 CTs resulting in 12 post‐treatment RWDs and NTCPs; c) boxplot the 12 post‐treatment NTCPs.Results:Target dose gain is 0.76–1.93 Gy. The 95%CI widths of the pretreatment tests NTCPs were 1.1–2.7%. For 5 patients, the planned NTCP fell within the 95%CI. For 4 patients, the planned NTCP was lower than the 95%CI lines. Post‐treatment results show that for 7 patients, the upper quartile was within the 95%CI; for 2 patients, the upper quartile were higher than the 95%CI.Conclusion:The pretreatment test yields conservative prediction of the actual delivered NTCP.