Abstract
The purpose of this study is to use the same diagnostic‐quality verification and planning CTs to validate planning margin account for residual interfractional variations with image‐guided soft tissue alignment of the prostate. For nine prostate cancer patients treated with IMRT to 78 Gy in 39 fractions, daily verification CT‐on‐rails images of the first seven and last seven fractions (n=126) were retrospectively selected for this study. On these images, prostate, bladder, and rectum were delineated by the same attending physician. Clinical plans were created with a margin of 8 mm except for 5 mm posteriorly, referred to as 8/5 mm. Three additional plans were created for each patient with the margins of 6/4 mm, 4/2 mm, and 2 mm uniform. These plans were subsequently applied to daily images and radiation doses were recalculated. The isocenters of these plans were placed according to clinical online shifts, which were based on soft tissue alignment to the prostate. Retrospective offline shifts by aligning prostate contours were compared to online shifts. The resultant daily target dose was analyzed using D99, the percentage of the prescription dose received by 99% of CTV. The percent of bladder volume receiving 65 Gy (V65Gy) and rectum V70Gy were also analyzed. After interfractional correction, using CTV D99>97%% criteria, 8/5 mm, 6/4 mm, 4/2 mm, and 2 mm planning margins met the CTV dose coverage in 95%, 91%, 65%, and 53% of the 126 fractions with online shifts, and 99%, 98%, 85%, and 68% with offline shifts. The rectum V70Gy and bladder V65Gy were significantly decreased at each level of margin reduction (p<0.05). With daily diagnostic quality imaging‐guidance, the interfractional planning margin may be reduced from 8/5 mm to 6/4 mm. The residual interfractional uncertainties most likely stem from prostate rotation and deformation.PACS number(s): 87.53.‐j
Highlights
62 Li et al Planning margin validation treated using intensity-modulated radiation therapy (IMRT) for prostate cancer showed a grade 2 or above late gastrointestinal (GI)toxicity of 15% and genitourinary (GU) toxicity of 17%.(2) This suggests planning target volume (PTV) reduction should decrease the toxicity
Techniques, PTV reduction deserves further examination.[3]. This is important for stereotactic body radiation therapy (SBRT)(4) as the effect of interfraction dose variation may be greater with fewer fractions.[5]
The main clinical practice to compensate for the interfractional variations is to reposition patients prior to treatment according to the registration of the daily image and planning image
Summary
62 Li et al Planning margin validation treated using IMRT for prostate cancer showed a grade 2 or above late gastrointestinal (GI)toxicity of 15% and genitourinary (GU) toxicity of 17%.(2) This suggests PTV reduction should decrease the toxicity. The main clinical practice to compensate for the interfractional variations is to reposition patients prior to treatment according to the registration of the daily image and planning image. Both implanted markers and soft tissue are effective for the image registration.[6,7,8] Markerbased registration, requires an invasive implantation procedure, which may cause discomfort, bleeding, and infection.[9] marker-based registration provides little information about the rotation or deformation of the prostate, and marker migration may pose issues with accurate target localization.[10] It can be difficult to localize the seminal vesicles and to detect changes in the surrounding normal anatomy.[11]. In the era of SBRT, with IGRT image quality the same as planning CTs, one may expect near zero interfractional planning margins.
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