CT–MRI image fusion for delineation of volumes in three-dimensional conformal radiation therapy in the treatment of localized prostate cancer

Abstract

The objective of this study was to assess the utility of CT-MRI image fusion software and compare both prostate volume and localization with CT and MRI studies. We evaluated the differences in clinical volumes in patients undergoing three-dimensional conformal radiation therapy for localized prostate cancer. After several tests performed to ensure the quality of image fusion software, eight patients suffering from prostate adenocarcinoma were submitted to CT and MRI studies in the treatment position within an immobilization device before the start of radiotherapy. The clinical target volume (CTV) (prostate plus seminal vesicles) was delineated on CT and MRI studies and image fusion was obtained from the superimposition of anatomical fiducial markers. A comparison of dose-volume histograms relative to CTV, rectum, bladder and femoral heads was performed for both studies. Image fusion showed a mean overestimation of CTV of 34% with CT compared with MRI. Along the anterior-posterior and superior-inferior direction, CTV was a mean 5 mm larger with CT study compared with MRI. The dose-volume histograms resulting from CT and MRI comparison showed that it is possible to spare a mean 10% of rectal volume and approximately 5% of bladder and femoral heads, respectively. This study confirmed an overestimation of CTV with CT images compared with MRI. Because this finding only allows a minimal sparing of organs at risk, considering the organ motion during each radiotherapy session and the excellent outcomes of prostate cancer treatment with CT based target identification, we are still reluctant to reduce the CTV to that identified by MRI.

The influence of MRI scan position on image registration accuracy, target delineation and calculated dose in prostatic radiotherapy

“…The changes in target volume in this study are in agreement with several other studies [2][3][4][5][6][7], which have demonstrated that outlining prostate and seminal vesicles using MRI results in smaller volumes and anatomically more accurate delineation. One study [7] has shown that outlining with MRI results in a smaller volume of rectal wall being included, potentially leading to a reduced risk of late toxicity.…”

“…MRI has reduced interference from metal implants such as prosthetic hips [2] and gold seed implants and therefore has the potential to provide improved target volume delineation over CT. Changes in anatomical and tumour definition as a result of using MRI data compared with CT have been reported for prostate [2][3][4][5][6][7], head and neck [8][9][10][11] and brain cancer [12,13] patients.…”

Simultaneous integrated boost to intraprostatic lesions using different energy levels of intensity-modulated radiotherapy and volumetric-arc therapy

“…The CT and MRI fusion was done by automated computerized fusion and then checked manually, as described in other IPL boost studies. 14,21,22 Clinical target volume (CTV) included the prostate and the entire seminal vesicles. The planning target volume for 78 Gy (PTV78) was defined as CTV with a margin of 5 mm posterior and 8 mm in other directions.…”

Magnetic resonance imaging in prostate cancer

“…New developments including fusing MRI with other imaging modalities may help direct radiotherapy planning. [33][34][35][36] More accurate targeting may decrease morbidity by reducing dose to erectile tissue and rectum. 37,38 MRI can also be helpful in the post-treatment follow-up of …”