A first-time survey across 15 cancer centers in Ontario, Canada, on the current practice of patient-specific quality assurance (PSQA) for intensity modulated radiation therapy (IMRT) and volumetric modulated arc therapy (VMAT) delivery was conducted. The objectives were to assess the current state of PSQA practice, identify areas for potential improvement, and facilitate the continued improvement in standardization, consistency, efficacy, and efficiency of PSQA regionally. The survey asked 40 questions related to PSQA practice for IMRT/VMAT delivery. The questions addressed PSQA policy and procedure, delivery log evaluation, instrumentation, measurement setup and methodology, data analysis and interpretation, documentation, process, failure modes, and feedback. The focus of this survey was on PSQA activities related to routine IMRT/ VMAT treatments on conventional linacs, including stereotactic body radiation therapy but excluding stereotactic radiosurgery. The participating centers were instructed to submit answers that reflected the collective view or opinion of their department and represented the most typical process practiced. The results of the survey provided a snapshot of the current state of PSQA practice in Ontario and demonstrated considerable variations in the practice. A large majority (80%) of centers performed PSQA measurements on all VMAT plans. Most employed pseudo-3D array detectors with a true composite (TC) geometry. No standard approach was found for stopping or reducing frequency of measurements. The sole use of delivery log evaluation was not widely implemented, though most centers expressed interest in adopting this technology. All used the Gamma evaluation method for analyzing PSQA measurements; however, no universal approach was reported on how Gamma evaluation and pass determination criteria were determined. All or some PSQA results were reviewed regularly in twothirds of the centers. Planning related issues were considered the most frequent source for PSQA failures (40%), whereas the most frequent course of action for a failed PSQA was to review the result and decide whether to proceed to treatment.
Respiratory motion may lead to dose errors when treating thoracic and abdominal tumours with radiotherapy. The interplay between complex multileaf collimator patterns and patient respiratory motion could result in unintuitive dose changes. We have developed a treatment reconstruction simulation computer code that accounts for interplay effects by combining multileaf collimator controller log files, respiratory trace log files, 4DCT images and a Monte Carlo dose calculator. Two three-dimensional (3D) IMRT step-and-shoot plans, a concave target and integrated boost were delivered to a 1D rigid motion phantom. Three sets of experiments were performed with 100%, 50% and 25% duty cycle gating. The log files were collected, and five simulation types were performed on each data set: continuous isocentre shift, discrete isocentre shift, 4DCT, 4DCT delivery average and 4DCT plan average. Analysis was performed using 3D gamma analysis with passing criteria of 2%, 2 mm. The simulation framework was able to demonstrate that a single fraction of the integrated boost plan was more sensitive to interplay effects than the concave target. Gating was shown to reduce the interplay effects. We have developed a 4DCT Monte Carlo simulation method that accounts for IMRT interplay effects with respiratory motion by utilizing delivery log files.
The impact of intrafractional prostate motion on dose delivery is sensitive to different motion patterns but not to different delivery techniques. Dose escalation to DIL using either sequential or simultaneous boost plans with 7 mm PTV margin is achievable in the presence of intrafractional prostate motion, even if the severe motion comprised 8.6% (3 out of the 35) treatment fractions.
Purpose: To study the feasibility of boosting the dose delivered to a dominant intraprostatic lesion (DIL), identified by functional imaging, requiring higher dose for better tumor control using Intensity‐Modulated Radiation Therapy (IMRT) and Volumetric Modulated Arc Therapy (VMAT). Materials and Methods: Treatment planning was performed on 2 patient CT‐scans with hypothetical DIL (1/8 the size of the prostate) in the inferior‐posterior‐left side of the prostate using Pinnacle treatment planning system (Philips Medical Systems). Three types of plan were generated, (A1) 10 Gy 5‐field IMRT boost to the DIL‐PTV followed by 76 Gy 5‐field IMRT to the Prostate‐PTV, (A2) 10 Gy VMAT boost to the DIL‐PTV followed by 76 Gy 5‐field IMRT to the Prostate‐PTV and (A3) 10 Gy VMAT boost to the DIL‐PTV followed by 76 Gy VMAT to the Prostate‐PTV. Plans were delivered using Varian clinac and verified using MapCheck and MapPhan (Sun Nuclear). Results: For plans A1, A2 and A3, the DIL‐PTV received a mean dose of 87.0±0.7 Gy, 86.4±0.4 Gy and 87.5±0.8 Gy respectively, and the Prostate‐PTV — DIL‐PTV received a mean dose of 77.9±1.1 Gy, 77.7±0.7 Gy and 78.1±1.3 Gy respectively. All three plans did not exceed the normal tissue tolerances from the guidelines of the RTOG 0126. Plan A2 had a slightly lower dose to the rectum and bladder. All the plans were delivered and verified successfully. The plan A3 had better dose conformity to DIL‐PTV. Conclusions: This study demonstrates the feasibility of using any of the three treatment methods in treating a DIL without exceeding normal tissue tolerances. However, considering the speed of VMAT delivery over the 5‐field IMRT and better conformality of the dose distribution, a VMAT boost to a DIL followed by a VMAT to the Prostate‐PTV is a preferable treatment option for DIL dose‐escalation. Research sponsored by CIHR.
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