Poster Presentation Clinical Oncology Society of Australia Annual Scientific Meeting 2019

Retrospective evaluation of radiation therapy planning margins for patients undergoing radical radiation therapy treatment for bladder cancer using Volumetric Modulated Arc Therapy (#185)

Kath Dower 1 , Andriana Ford 2 , Michael Sandford 2 , Andrew Doherty 2 , Stuart Greenham 2 , Luke Kerin 3 , Patrick Dwyer 1 , Carmen Hansen 3 , Justin Westhuyzen 2 , Tom Shakespeare 2
  1. Radiation Therapy, North Coast Cancer Institute , Lismore, NSW, Australia
  2. Radiation Therapy, Mid-North Coast Cancer Institute, Coffs Harbour, NSW, Australia
  3. Radiation Therapy, Mid North Coast Cancer Institute, Port Macquarie, NSW, Australia

Introduction: Current contouring guidelines for curative radiotherapy for muscle-invasive bladder cancer (MIBC) recommend margins of 1.5-2.0cm to account for variability of bladder filling and the limitations of planar image verification.  The aim of this study is to assess whether the use of Volumetric Modulated Arc Therapy (VMAT) and cone beam CT (CBCT) would allow for a smaller PTV expansion.

Method: The daily CBCT images (382 in total) for 12 patients treated on protocol across three integrated cancer centres were retrospectively reviewed.  Isotropic expansions of 0.5cm, 1.0cm and 1.5cm were applied to the CTV on the planning study-set to create PTV0.5, PTV1.0 and PTV1.5, respectively.  An expansion of 1.5cm superiorly and 1.0cm inferiorly, anteriorly and posteriorly was applied to the CTV to create an anisotopic PTV. These margins were exported with the reference scan to XVI. After acquisition, the CBCTs were visually compared to the reference planning scan to determine the smallest margin which encapsulated the entire bladder. In total, 382 CBCTs were assessed.

Results: A 0.5cm isotropic margin (IM) covered the CTV in 46.1% of CBCTs; a 1.0cm IM covered the CTV in 96.9% of CBCTs. Our anisotropic margins (ANIM) covered the CTV in 100% of the CBCTs.

Conclusion:  We endorse an anisotropic PTV margin of 1.0-1.5cm superiorly and 1.0cm in all other dimensions for the radiotherapy treatment of radical whole bladder cases using a strict bladder preparation protocol, VMAT and pre-treatment CBCTs.