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Session Item

Thursday
January 21
17:00 - 18:55
SBRT for oligometastatic NSCLC
0100
Module
00:00 - 00:00
Effect of pseudoCT methods on dose-derived rectal toxicity prediction in MR-only prostate RT
PO-1751

Abstract

Effect of pseudoCT methods on dose-derived rectal toxicity prediction in MR-only prostate RT
Authors: Thomas|, Christopher(1,2)*[christopher.thomas@gstt.nhs.uk];Dregely|, Isabel(2);Oksuz|, Ilkay(2);Guerrero-Urbano|, Teresa(2,3);Greener|, Antony(1);King|, Andrew(2);Barrington|, Sally(2);
(1)Guy's and St. Thomas' NHS Foundation Trust, Medical Physics, London, United Kingdom;(2)King's College London, School of Biomedical Engineering and Imaging Sciences, London, United Kingdom;(3)Guy's and St. Thomas' NHS Foundation Trust, Clinical Oncology, London, United Kingdom;
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Purpose or Objective

MR-planning for radiotherapy (RT) is increasingly popular and strategies for toxicity-based dose adjustment are emerging. MR-only RT requires pseudoCT generation for dose calculation. This study evaluates the effect of 4 pseudoCT methods on rectal toxicity predictors in an MR-only RT pathway for prostate cancer.

Material and Methods

Twelve patients were recruited to an investigational study (*****). Patients underwent a large field of view diagnostic quality T2-weighted MRI scan on the same day as a standard-of-care (SoC) RT planning CT scan for prostate RT. All patients were planned for radical RT with 60Gy in 20 fractions. Four pseudoCTs were created per patient: “CT0” (bulk-density of zero Hounsfield Units (HU); 2) “CTpop” (bulk-density equal to population average HU); “CTstrat” (replicating a commercial system of density-stratification into 4 tissue classes); 4) “SynCT” (voxel-based HU predicted from T2tse MRI using a 2D neural network based on U-Net architecture). Treatment plans were calculated using each pseudoCT. Rectal dose volume histograms (DVH) and rectal dose surface maps (DSM) were extracted for each dose distribution and used to assess grade-2 rectal bleeding risk. Errors introduced into rectal DVH- and DSM-derived grade 2 rectal bleeding risk were evaluated compared with the gold-standard SoC CT.

Results

Mean (± 1 standard deviation) HU error for the SynCTs compared to the SoC CT was -5.7HU (±6.1), with mean absolute error of 40.1HU (±4.4). CT0, CTpop, CTstrat and SynCT introduced maximum absolute errors of 0.8%, 0.5%, 0.2% and 0.3% into DVH-based rectal bleeding risk prediction respectively. DSM-derived risk predictions were unaffected in all cases.



Fig 1. SoC CT, and pseudoCTs used for dose calculation



Fig 2. Box plot of errors in DVH-derived G2 rectal bleeding toxicity. Full range of errors (whiskers), interquartile range (boxes), median (line) and mean (cross) are shown. Median for CTstrat is zero.
Conclusion

Tissue-stratification (CTstrat) and voxel based methods (SynCT) offer an improvement in accuracy over bulk density approaches, but all four pseudoCT methods offer acceptable resolution of rectal dose and G2 rectal bleeding risk prediction based on DVH and DSM. The voxel-based technique (SynCT) allows increased HU resolution but appears unnecessary for assessment of acute toxicity risk in prostate cancer beyond that offered by CTstrat. However, the assessment of pseudoCT-derived errors should also be carried out in other treatment sites where dose to critical structures will guide dose escalation and adaptive RT strategies, such as isotoxic escalation in lung and head and neck RT.