ABSTRACT
OBJECTIVE: The objective of this study was to evaluate the dosimetric impact on hypofractionated prostate radiation therapy of two geometric uncertainty sources: rectum and bladder filling and intrafractional prostate motion. MATERIALS AND METHODS: This prospective study included 544 images (375 pre-treatment cone-beam CT [CBCT] and 169 post-treatment CBCT) from 15 prostate adenocarcinoma patients. We recalculated the dose on each pre-treatment CBCT once the positioning errors were corrected. We also recalculated two dose distributions on each post-treatment CBCT, either using or not intrafractional motion correction. A correlation analysis was performed between CBCT-based dose and rectum and bladder filling as well as intrafraction prostate displacements. RESULTS: No significant differences were found between administered and planned rectal doses. However, we observed an increase in bladder dose due to a lower bladder filling in 66% of treatment fractions. These differences were reduced at the end of the fraction since the lower bladder volume was compensated by the filling during the treatment session. A statistically significant reduction in target volume coverage was observed in 27% of treatment sessions and was correlated with intrafractional prostate motion in sagittal plane > 4 mm. CONCLUSIONS: A better control of bladder filling is recommended to minimize the number of fractions in which the bladder volume is lower than planned. Fiducial mark tracking with a displacement threshold of 5 mm in any direction is recommended to ensure that the prescribed dose criteria are met.
Subject(s)
Adenocarcinoma/radiotherapy , Organ Motion , Prostatic Neoplasms/radiotherapy , Rectum/anatomy & histology , Urinary Bladder/anatomy & histology , Adenocarcinoma/diagnostic imaging , Cone-Beam Computed Tomography , Fiducial Markers , Humans , Male , Organ Size , Organs at Risk/anatomy & histology , Organs at Risk/diagnostic imaging , Organs at Risk/radiation effects , Prospective Studies , Prostate/diagnostic imaging , Prostatic Neoplasms/diagnostic imaging , Radiation Dose Hypofractionation , Radiation Tolerance , Radiotherapy Setup Errors , Radiotherapy, Intensity-Modulated , Rectum/diagnostic imaging , Rectum/radiation effects , Urinary Bladder/diagnostic imaging , Urinary Bladder/radiation effectsABSTRACT
BACKGROUND: The objective was to determine the magnitude of the prostate intrafractional motion relative to bony pelvis anatomy, and to evaluate the relationship between this displacement and some clinical and anatomical variables. MATERIALS AND METHODS: The prospective study consisted of 544 images (375 pre-treatment CBCT and 169 post-treatment CBCT) from 15 prostate adenocarcinoma patients that were used for intrafractional prostate motion determination. In addition, two radiation oncologists re-contoured the bladder and rectum on each CBCT according to the patient's anatomy of the day. ANOVA and correlation analysis followed by linear regression analysis were performed to identify clinical or anatomical variables that predict large prostate intrafractional motion. RESULTS: Prostate shift increased in patients with hormone therapy (p ≤ 0.02). The regression analysis showed that patients with large bladder intrafractional filling (p < 0.01) and a large bladder volume difference from planning CT were more likely to experience bigger longitudinal prostate motion (> 3 mm). Recommended bladder size values: anterior-posterior size ≤ 10 cm and anterior-posterior/cranio-caudal ratio ≤ 1.7, both parameters measured in the midsagittal prostate plane, were defined. CONCLUSIONS: The treatment margin should not be reduced for those patients who were treated with hormone therapy and/or whose rectum or bladder was far from complying the preparation protocol conditions.