Cervix brachytherapy: determining the optimal bladder volume using empty and full bladder MRI scans
Sharon Oultram,
Australia
PO-2141
Abstract
Cervix brachytherapy: determining the optimal bladder volume using empty and full bladder MRI scans
Authors: Sharon Oultram1,2, Guneet Kaur3, Bradley Beeksma1, Geetha Govindarajulu1,4, Swetha Sridharan1,4, Jodi Waugh1, Stephanie Wrightson1, Paul Simpson1, Claire Dempsey1,5,6
1Calvary Mater Newcastle, Radiation Oncology, Newcastle, Australia; 2University of Newcastle, School of Health Sciences, Newcastle , Australia; 3GenesisCare, Radiation Oncology Crows Nest, Sydney, Australia; 4University of Newcastle, School of Medicine and Public Health, Newcastle, Australia; 5University of Newcastle, School of Health Sciences, Newcastle, Australia; 6University of Washington, Radiation Oncology, Seattle, USA
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Purpose or Objective
There is no consensus regarding optimal bladder volume (50cc to full) for brachytherapy planning and treatment. Most bladder filling protocols use an in-dwelling catheter (IDC) to produce a consistent volume in the bladder for treatment planning imaging and treatment. It is speculated that filling the bladder with fluid will extend the bladder wall away from the cervix, reducing bladder wall dose. The aim of this study was to investigate the impact of bladder filling on bladder volume and dose to the bladder and bladder wall.
Material and Methods
All patients in this study had T2-MRI scans acquired per fraction. Straight sagittal and para-axial scans were taken with the bladder IDC open to allow a free-draining bladder and with the IDC tube clamped and ≤60 ml sterile water injected in the bladder to achieve a consistent 100ml bladder volume.
Images were imported to the Oncentra brachytherapy treatment planning system and bladder, rectum, sigmoid and high-risk clinical target volumes (HR-CTV) were contoured on para-axial scans for free-draining and filled bladder datasets. Optimised plans were created on both datasets, focusing on HR-CTV coverage and limiting OAR dose. Dose-volume histogram data for each plan was analysed.
Results
This study analysed 25 paired scans. There were deviations in free-draining and filled bladder volumes both inter-fraction and inter-patient. For a single patient, free-draining and filled bladder volumes varied by up to 210% and 171% inter-fraction respectively. The average filled bladder volume across all patients and fractions was 128.8 (±27.6) mL.
Optimised plans for free-draining and filled bladder datasets had average HR-CTV D90 dose differences of 2.8%. Two thirds (66%) of cases had < 5% D2cc bladder dose difference between filled and free-draining bladder dataset. For cases with > 5% dose difference, 91% had a higher filled bladder than free-draining bladder D2cc dose. Bladder wall D0.1cc doses correlated loosely with bladder D2cc doses, with 52% of cases having < 5% D0.1cc bladder wall dose differences. For those cases with > 5% dose differences, 62% had higher filled bladder than free-draining bladder wall D0.1cc dose.
Average D2cc rectum and sigmoid differences for rectum and sigmoid were –1.0% and 0.0% respectively between free-draining and filled bladder cases.
Conclusion
There is variation in bladder volumes from patient-to-patient and fraction-to-fraction despite using a bladder filling protocol. This may have major implications for patient dosimetry if patients do not have individual plans created for each treatment fraction. There appears to be no dosimetric benefit to the bladder or bladder wall to fill the bladder against leaving the bladder free draining, nor does bladder filling appear to impact on rectal or sigmoid dose. The increased risk of infection and patient preparation time would suggest that there is also no systematic benefit to using a bladder filling protocol.