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Abstract

PURPOSE:

This work aims at investigating intensity corrected cone-beam x-ray computed tomography (CBCT) images for accurate dose calculation in adaptive intensity modulated proton therapy (IMPT) for prostate and head and neck (H&N) cancer. A deformable image registration (DIR)-based method and a scatter correction approach using the image data obtained from DIR as prior are characterized and compared on the basis of the same clinical patient cohort for the first time.

METHODS:

Planning CT (pCT) and daily CBCT data (reconstructed images and measured projections) of four H&N and four prostate cancer patients have been considered in this study. A previously validated Morphons algorithm was used for DIR of the planning CT to the current CBCT image, yielding a so-called virtual CT (vCT). For the first time, this approach was translated from H&N to prostate cancer cases in the scope of proton therapy. The warped pCT images were also used as prior for scatter correction of the CBCT projections for both tumor sites. Single field uniform dose and IMPT (only for H&N cases) treatment plans have been generated with a research version of a commercial planning system. Dose calculations on vCT and scatter corrected CBCT (CBCTcor) were compared by means of the proton range and a gamma-index analysis. For the H&N cases, an additional diagnostic replanning CT (rpCT) acquired within three days of the CBCT served as additional reference. For the prostate patients, a comprehensive contour comparison of CBCT and vCT, using a trained physician's delineation, was performed.

RESULTS:

A high agreement of vCT and CBCTcor was found in terms of the proton range and gamma-index analysis. For all patients and indications between 95% and 100% of the proton dose profiles in beam's eye view showed a range agreement of better than 3 mm. The pass rate in a (2%,2 mm) gamma-comparison was between 96% and 100%. For H&N patients, an equivalent agreement of vCT and CBCTcor to the reference rpCT was observed. However, for the prostate cases, an insufficient accuracy of the vCT contours retrieved from DIR was found, while the CBCTcor contours showed very high agreement to the contours delineated on the raw CBCT.

CONCLUSIONS:

For H&N patients, no considerable differences of vCT and CBCTcor were found. For prostate cases, despite the high dosimetric agreement, the DIR yields incorrect contours, probably due to the more pronounced anatomical changes in the abdomen and the reduced soft-tissue contrast in the CBCT. Using the vCT as prior, these inaccuracies can be overcome and images suitable for accurate delineation and dose calculation in CBCT-based adaptive IMPT can be retrieved from scatter correction of the CBCT projections.

Author information​

Kurz C1, Kamp F2, Park YK3, Zöllner C4, Rit S5, Hansen D6, Podesta M7, Sharp GC3, Li M2, Reiner M2, Hofmaier J2, Neppl S2, Thieke C2, Nijhuis R2, Ganswindt U2, Belka C2, Winey BA3, Parodi K4, Landry G4.

1Department of Radiation Oncology, LMU Munich, Munich 81377, Germany and Department of Medical Physics, Ludwig-Maximilians-Universität München, Garching bei München 85748, Germany.

2Department of Radiation Oncology, LMU Munich, Munich 81377, Germany.

3Department of Radiation Oncology, Massachusetts General Hospital and Harvard Medical School, Boston, Massachusetts 02114.

4Department of Medical Physics, Ludwig-Maximilians-Universität München, Garching bei München 85748, Germany.

5Université de Lyon, CREATIS, CNRS UMR5220m Inserm U1044, INSA-Lyon, Université Lyon 1, Lyon F69373, France.

6Department of Oncology, Aarhus University Hospital, Aarhus 8000, Denmark.

7Department of Radiation Oncology (MAASTRO), GROW-School for Oncology and Developmental Biology, Maastricht University Medical Centre, Maastricht 6229 ET, The Netherlands.