Using daily diagnostic quality images to validate planning margins for prostate interfractional variations

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 false(n=126false) 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 false(normalV65Gyfalse) and rectum normalV70Gy were also analyzed. After interfractional correction, using CTV normalD99>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 normalV70Gy and bladder normalV65Gy 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

Dosimetric Impact of Interfractional Variations in Prostate Cancer Radiotherapy—Implications for Imaging Frequency and Treatment Adaptation

“…A previous analysis based on CBCTs from 20 patients demonstrated improved target coverage and reductions of the V 50 , V 65 , and D mean to the rectum with daily imaging, although no voxel-wise dose accumulation could be performed due to limitations in image quality and registration (27). Similar improvements of target coverage by daily imaging have been reported in other publications, and it has been suggested that PTV margins should be adjusted according to the imaging frequency (28). In our dataset, daily image-guided repositioning resulted in better target volume conformity and target volume coverage and a trend toward increased rectal doses.…”

Dosimetric Impact of Interfractional Variations for Post-prostatectomy Radiotherapy to the Prostatic Fossa—Relevance for the Frequency of Position Verification Imaging and Treatment Adaptation

“…While no data are available on this topic for the treatment of the prostatic fossa, a previous study using CBCTs of 20 patients undergoing definitive prostate radiotherapy suggested an improved coverage of the CTV and reduced doses to the rectum by daily positional imaging, although the data quality was limited by the low imaging contrast and the lack of a deformable registration algorithm (33). Similar analyses have suggested that daily imaging may result in better target coverage, allowing a reduction of the PTV margins for daily imaging (34). While these suggestions are mainly derived from definitive treatment concepts, the data presented here back up this suggestion also for postoperative radiotherapy: In our dataset, daily CT-guided repositioning resulted in no significant deviations of the applied fractional doses as compared to the treatment plan for both the CTV and the PTV and led to a better PTV coverage and dose homogeneity than weekly repositioning.…”

Dosimetric Impact of the Positional Imaging Frequency for Hypofractionated Prostate Radiotherapy – A Voxel-by-Voxel Analysis

“…A previous publication reported improvements in target volume coverage and a reduction in high doses to the rectum for daily CBCT imaging and repositioning based on registration ( 35 ). It has been suggested that the imaging frequency should direct the choice of PTV margins, although the relevance of the observed dosimetric improvements for the treated patients remains unclear ( 36 , 37 ). So far, several randomized trials have addressed the clinical relevance of daily CBCT regarding treatment-related toxicities and patient outcomes: A French trial enrolled 470 patients and reported improved progression-free survival and reduced late rectal toxicity for the cohort receiving daily CBCT.…”