Dosimetric Comparison between Bone and Target Matching Considering Interfractional Prostate Motion in Volumetric Modulated Arc Therapy

Abstract

Adequate matching methods are critical for accurate volumetric-modulated arc therapy (VMAT). We investigated the dosimetric differences in the target and organs at risk (OARs) between bone matching and target matching in patients with prostate cancer treated with VMAT. The relationship between the dosimetric differences and interfractional motion of the prostate was also evaluated. Forty patients with prostate cancer classified as intermediate risk were enrolled in a study to assess the differences in dosimetry between two matching methods. These patients were treated with VMAT and prescribed dose was 78 Gy. The dose distribution was calculated using cone-beam computed tomography (CBCT) for this study. We selected clinical target volume (CTV) as the target, and the rectum and bladder as the OARs. The Dmean, D98, D95, and D2 to the target and V10-V70 to the OARs were calculated as different dose from target matching value minus bone matching value. Multiple regression analysis was used to evaluate the effect of interfractional motion of the prostate on the differences in dose. The CTV D95 values differed by −0.22 ± 1.01 Gy (mean ± standard deviation). Rectum and bladder V70 values differed by 4.6% ± 7.2% and −2.6% ± 7.2%, respectively. There was a correlation between interfractional motion of the prostate and the dose differences to OARs (R 2 = 0.73 -0.94). The dose differences to OARs also varied depending on the direction of the prostate's motion. We found that bone matching resulted in an increased rectal dose and high risk of decreasing dose to the CTV.

Probabilistic dose distribution from interfractional motion in carbon ion radiation therapy for prostate cancer shows rectum sparing with moderate target coverage degradation

“…[ 16 ] Another option for volumetric-modulated arc photon therapy margins is to define the CTV as the prostate with proximal 1.5 cm seminal vesicles, and PTV extending this by 8 mm in all directions except posteriorly, extending by 5 mm in this direction. [ 2 ] Of course, from a theoretical perspective, CTV should not depend on radiation modality, but rather the spread of disease.…”

“…The prostate is known to move up to 2 mm during photon irradiation, [ 1 ] and up to 7 mm between fractions. [ 2 ] This study concerns interfractional motion. Intrafractional motion is not a problem because carbon beam treatment time is approximately one minute, but the interfractional motion remains a concern.…”