SU‐E‐T‐525: Dose Volume Histograms (DVH) Analysis and Comparison with ICRU Point Doses in MRI Guided HDR Brachytherapy for Cervical Cancer

Abstract

Purpose:Brachytherapy plays a crucial role in management of cervix cancer. MRI compatible applicators have made it possible to accurately delineate gross‐target‐volume(GTV) and organs‐at‐risk(OAR) volumes, as well as directly plan, optimize and adapt dose‐distribution for each insertion. We sought to compare DVH of tumor‐coverage and OARs to traditional Point‐A, ICRU‐38 bladder and rectum point‐doses for four different planning‐techniques.Methods:MRI based 3D‐planning was performed on Nucletron‐Oncentra‐TPS for 3 selected patients with varying tumor‐sizes and anatomy.GTV,high‐risk‐clinical‐target‐volume(HR‐CTV),intermediate‐risk‐clinical‐target‐volume(IR‐CTV) and OARs: rectum, bladder, sigmoid‐colon, vaginal‐mucosa were delineated. Three conventionally used techniques: mg‐Radium‐equivalent(RaEq),equal‐dwell‐weights(EDW),Medical‐College‐of‐Wisconsin proposed points‐optimization(MCWO) and a manual‐graphical‐optimization(MGO) volume‐coverage based technique were applied for each patient. Prescription was 6Gy delivered to point‐A in Conventional techniques (RaEq, EDW, MCWO). For MGO, goal was to achieve 90%‐coverage (D90) to HR‐CTV with prescription‐dose. ICRU point doses for rectum and bladder, point‐A doses, DVH‐doses for HR‐CTV‐D90,0.1cc‐volume(D0.1),1ccvolume(D1),2cc‐volume(D2) were collected for all plans and analyzed. Results:Mean D90 for HR‐CTV normalized to MGO were 0.89,0.84,0.9,1.0 for EDW, RaEq, MCWO, MGO respectively. Mean point‐A doses were 21.7% higher for MGO. Conventional techniques with Point‐A prescriptions under covered HR‐CTV‐D90 by average of 12% as compared to MGO. Rectum, bladder and sigmoid doses were highest in MGO‐plans for ICRU points as well as D0.1,D1 and D2 doses. Among conventional‐techniques, rectum and bladder ICRU and DVH doses(0.1,1,2cc) were not significantly different (within 7%).Rectum D0.1 provided good estimation of ICRU‐rectum‐point doses (within 3.9%),rectum D0.1 were higher from 0.8 to 3.9% while bladder D0.1 overestimated the bladder ICRU point dose up to 43% for conventional‐techniques.Bladder‐D2 provided a good estimation of ICRU bladder point‐doses(within 3.6%) for conventional‐techniques. This correlation is not observed for MGO plans perhaps due to steering of isodose line, leading to unpredictable dwell‐weighting.Conclusion:MRI based HDR‐planning provides accurate delineation of tumor volumes and normal structures, and optimized tumor‐coverage can be achieved with acceptable normal‐tissue doses. This study showed that for conventional techniques D0.1 rectum dose and D2 bladder dose are good representation of ICRU‐reference‐point doses.