20 Contour assessment and influence on dose at target volumes (TV) and organ at risk (OAR) in the case of prostate cancer

Abstract

Introduction In the treatment of prostate cancer, the target volumes are the prostate (HR PTV) and the seminal vesicles (LR PTV) and the main OARs are the rectum, bladder, femoral heads and penile bulb. This study seeks to evaluate the quality of the delineation of volumes according to HAS recommendations [1] by showing the existence or not of discrepancies in terms of volumes and the influence on the doses delivered with different treatment planning techniques. Methods In a first step, the structures obtained by delineation by 8 radiotherapists with different levels of experience were evaluated. Operators (op) were anonymized, the most experienced radiotherapist in prostate cancer at the time of the study is referenced (op2). The kappa index (KI) was then calculated, measuring the degree of agreement between the reference operator and an operator x. In a second step, the dosimetry was realized on the previous delineations for a treatment technique by Tomotherapy with 80 Gy for the HR PTV and 56 Gy for the LR PTV in 40 sessions. In particular, the study focused on volume disparity extremes for HR PTV, LR PTV and rectum. In each case, the dosimetry performed from the contours of an operator A was carried over to the contours of an operator B. Not respecting the constraints to VC and OAR [2] by the previous method, the dosimetry was optimized for the contours of each operator. The doses received at TVs and OARs were compared for the two operators concerned by the extreme volumes. Results At first, there was a great agreement (KI: 0.81–1.00) on the delineations of the prostate, femoral heads and bladder, a medium agreement (KI: 0.41–0.60) for the seminal vesicles and the rectum, a weak agreement (KI: 0.00–0.20) for the penile bulb. We have found that the extremes are not obtained by the same operators. In a second step, the dosimetry was performed on the following extremes: [42.06 cc (op1) VS 61.64 cc (op2)] for the prostate, [11.72 cc (op3) VS 78.38 cc (op5)] for the seminal vesicles, [21.62 cc (op2) VS 105.27 cc (op4)] for the rectum. The observed volume differences are not related to noted dose differences (for rectal extremes, 0.7% and 8.2% differences for Dmax and V50, respectively). Conclusions At the end of this study, it was shown a disparity in the delineation of volumes in relation to medical experience. In all cases, the optimized dosimetry met the criteria for coverage of target volumes and savings of organs at risk. The results allowed us to evaluate the quality of our dosimetric technique in Tomotherapy. Subsequently, this method will be repeated to evaluate our VMAT dosimetry technique.