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

Authors


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.

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