Real-time Image-guided Adaptive-predictive Prostate Radiotherapy using Rectal Diameter as a Predictor of Motion.

To investigate a relationship between maximum rectal diameter (MRD) on pre-treatment cone beam computed tomography (CBCT) and intra-fraction prostate motion, in the context of an adaptive image-guided radiotherapy (IGRT) method.

The MRD was measured on 2125 CBCTs from 55 retrospective patient datasets and related to prostate displacement from intra-fraction imaging. A linear regression model was developed to determine a threshold MRD associated with a high probability of small prostate displacement. Standard and reduced adaptive margin plans were created to compare rectum and bladder normal tissue complication probability (NTCP) with each method.

A per-protocol analysis carried out on 1910 fractions from 51 patients showed with 90% confidence that for a MRD≤3 cm, prostate displacement will be ≤5 mm and that for a MRD≤3.5 cm, prostate displacement will be ≤5.5 mm. In the first scenario, if adaptive therapy was used instead of standard therapy, median reductions in NTCP for rectum and bladder were 0.5% (from 9.5% to 9%) and 1.3% (from 6.6% to 5.3%), respectively. In the second scenario, the NTCP for rectum and bladder would have median reductions of 1.1% and 2.6%, respectively.

We have identified a potential method for adaptive prostate IGRT based upon predicting small prostate intra-fraction motion by measuring MRD on pre-treatment CBCT.

Clinical oncology (Royal College of Radiologists (Great Britain)). 2016 Oct 22 [Epub ahead of print]

R Oates, A Brown, A Tan, F Foroudi, M Lim Joon, M Schneider, A Herschtal, T Kron

Radiation Therapy Services, Peter MacCallum Cancer Centre, Bendigo, Victoria, Australia; Medical Imaging & Radiation Sciences, Monash University, Melbourne, Victoria, Australia. Electronic address: ., Radiation Oncology, Townsville Cancer Centre, Townsville, Queensland, Australia., Radiation Oncology, Townsville Cancer Centre, Townsville, Queensland, Australia; College of Medicine and Dentistry, James Cook University, Queensland, Australia., Radiation Oncology, Peter MacCallum Cancer Centre, Melbourne, Victoria, Australia., Medical Imaging & Radiation Sciences, Monash University, Melbourne, Victoria, Australia., Biostatistics and Clinical Trials, Peter MacCallum Cancer Centre, Melbourne, Victoria, Australia; Sir Peter MacCallum Department of Oncology, The University of Melbourne, Parkville, Victoria, Australia., Sir Peter MacCallum Department of Oncology, The University of Melbourne, Parkville, Victoria, Australia; Physical Sciences, Peter MacCallum Cancer Centre, Melbourne, Victoria, Australia.