Volume correlation of bladder, rectum and intestines in gynecological pelvis radiotherapy

Abstract

Introduction. One the greatest challenges in gynecological pelvis radiotherapy are internal movements of organs at risk (OAR) in target volume close proximity. Since there is a direct correlation in between OAR volume, thus obtaining relatively constant bladder volume during the whole radiotherapy treatment is one of the key factors for reducing irradiated intestine volume. This is provided with sustainable bladder filling protocol. Cone beam computed tomography (CBCT) provides needed quality assurance of absorbed doses in OAR. Materials and methods. 30 patients with diagnosed gynecological malignancies were included and their data analyzed. Median age was 57 years (30-75) at University clinic of radiotherapy and oncology – Skopje. All patients received three-dimensional conformal radiotherapy (3D-CRT) in post-operative setting (25 fractions, daily dose of 2Gy with total dose of 50Gy) with adequate bladder filling protocol (500ml of liquids, 1hour prior irradiation) expected to fill the bladder with 200- 300cc. CBCT was used twice a week in comparison with simulation computed tomography (CT). OAR volumes compared were bladder, rectal and intestine volumes. Dosimetric volumes constrains for bladder were less than cumulative 60Gy, for rectum less or equal to 46.3Gy (50% organ volume) and intestines constrains used were less of equal to 42Gy for 25%, 37Gy for 50% and 33Gy for 67% organ volume. Quantitative Analyses of Normal Tissue Effects in the Clinic (QUANTEC) constraines of 45Gy for 195cc were used for intestines as well. Results. Pre-treatment average CT volume for OAR was 283cc, 75cc and 1084cc for bladder, rectum and intestines respectively. CBCT control average OAR volumes were bladder 198cc, rectum 68cc and intestines 940cc (inside CBCT range). QUANTEC intestines volume averaged at 245cc. Conclusion. For rectum bladder filling protocol did not have significant impact. Bladder and intestinal absorbed doses directly correlated with bladder filling protocol. However, considering the absence of uterus and parametria, even with sufficient bladder filling protocol, intestines can not be sufficiently protected due to retrovesical loop placement. Thus increasing bladder filling protocol volume may be needed.