ABSTRACT
PURPOSE: Robotic radiosurgery requires plan delivery quality assurance (DQA) but there has never been a published comprehensive analysis of a patient-specific DQA process in a clinic. We proposed to evaluate 350 consecutive film-based patient-specific DQAs using statistical process control. We evaluated the performance of the process to propose achievable tolerance criteria for DQA validation and we sought to identify suboptimal DQA using control charts. METHODS AND MATERIAL: DQAs were performed on a CyberKnife-M6 using Gafchromic-EBT3 films. The signal-to-dose conversion was performed using a multichannel-correction and a scanning protocol that combined measurement and calibration in a single scan. The DQA analysis comprised a gamma-index analysis at 3%/1.5mm and a separate evaluation of spatial and dosimetric accuracy of the plan delivery. Each parameter was plotted on a control chart and control limits were calculated. A capability index (Cpm) was calculated to evaluate the ability of the process to produce results within specifications. RESULTS: The analysis of capability showed that a gamma pass rate of 85% at 3%/1.5mm was highly achievable as acceptance criteria for DQA validation using a film-based protocol (Cpm>1.33). 3.4% of DQA were outside a control limit of 88% for gamma pass-rate. The analysis of the out-of-control DQA helped identify a dosimetric error in our institute for a specific treatment type. CONCLUSION: We have defined initial tolerance criteria for DQA validations. We have shown that the implementation of a film-based patient-specific DQA protocol with the use of control charts is an effective method to improve patient treatment safety on CyberKnife.
Subject(s)
Film Dosimetry , Radiosurgery , Radiotherapy Planning, Computer-Assisted , Robotic Surgical Procedures , Calibration , Humans , Phantoms, Imaging , Quality Assurance, Health Care , Radiometry , Radiotherapy DosageABSTRACT
PURPOSE: This study evaluates the peripheral dose (PD) delivered to healthy tissues for brain stereotactic radiotherapy treatments (SRT) performed with a CyberKnife M6™ Robotic Radiosurgery System and proposes a model to estimate PD before treatment. METHOD: PD was measured with thermoluminescent dosimeters. Measurements were performed to evaluate the influence of distance, collimator type (fixed or Iris™) and aperture size on PD for typical brain treatment plans simulated on an anthropomorphic phantom. A model to estimate PD was defined by fitting functions to these measurements. In vivo measurements were subsequently performed on 30 patients and compared to the model-predicted PD. RESULTS: PD (in cGy) was about 0.06% of MU at 15cm for a 20mm fixed collimator and 0.04% of MU for the same aperture with Iris™ collimator. In vivo measurements showed an average thyroid dose of 55mGy (σ=18.8mGy). Computed dose for thyroid, breast, umbilicus and gonads showed on average a relative difference of 3.4% with the in vivo dose (σ=12.4%). CONCLUSION: PD at the thyroid with Iris™ was about a third lower than with a fixed collimator in case of brain SRT. Despite uncertainties (use of anthropomorphic PD to estimate patient specific PD, surface PD to estimate OAR PD) the model allows PD to be estimated without in vivo measurements. This method could be used to optimise PD with different planning strategies.
Subject(s)
Radiosurgery , Radiotherapy Dosage , Radiotherapy Planning, Computer-Assisted , Brain Neoplasms/radiotherapy , Female , Humans , Male , Phantoms, Imaging , Pituitary Neoplasms/radiotherapyABSTRACT
Intensity-modulated radiation therapy (IMRT) is essential to have a dose distribution matching with the planning target volume (PTV) in case of concave-shape target. Today IMRT delivery techniques with linear accelerator can be divided into two classes: techniques with fixed gantry, called "step and shoot" (S&S) and "sliding window" (SW), and rotational techniques, called intensity modulated arc therapy (IMAT) and volumetric modulated arc therapy (VMAT). We discuss about constraints for IMRT implementation from dosimetric planning to treatment delivery. We compare S&S and VMAT performances concerning dose distribution quality, efficiency and delivery time. We describe quality controls that must be implemented and the methods for analysis and follow-up performances. VMAT tends to yield similar dose distribution to MRT with fixed gantry. VMAT also decreases monitor units as well as treatment delivery time to less than 5 minutes. However, VMAT is an IMRT technique more difficult to master than S&S technique because there are more variable parameters.
Subject(s)
Health Physics , Radiotherapy Planning, Computer-Assisted/methods , Radiotherapy, Intensity-Modulated/methods , Cone-Beam Computed Tomography , Dose-Response Relationship, Radiation , Equipment Design , Humans , Neoplasms/diagnostic imaging , Neoplasms/pathology , Neoplasms/radiotherapy , Particle Accelerators , Photons , Quality Control , Radiotherapy Dosage , Radiotherapy, Intensity-Modulated/instrumentation , Radiotherapy, Intensity-Modulated/standards , Reproducibility of Results , RotationABSTRACT
PURPOSE: To compare two Intensity Modulated Radiation Therapy (IMRT) techniques for prostate cancer: the Volumetric Modulated Arc Therapy (VMAT) and the "Step and Shoot" technique (S&S). MATERIALS AND METHODS: VMAT and S&S plans (RX 18MV) were created and compared (Wilcoxon test) for 10 patients. The dosimetric goal of both treatments was to deliver 46 Gy to the seminal vesicles and 80 Gy to the prostate, while respecting the dose constrains in the organs at risk of toxicity. For one patient, the two techniques were compared for dose painting and escalation in target volumes defined on MRI and registered thanks to intraprostatic fiducials. RESULTS: VMAT, compared to S&S, offered: an increase of the PTV2s (prostate) volume receiving 77 to 80 Gy and a decrease of V(82) and V(83); a decrease of V(4) to V(6), V(16) to V(23), and V(69) to V(73) for the rectal wall; a decrease of V(25) for the bladder wall; a decrease of V(21) to V(43) for the femoral heads; a decrease of V(26) to V(44) and V(72) to V(80) but an increase of V(1) to V(21) and V(49) to V(60) for the healthy tissues. The Conformal Index "COIN" was better with VMAT than S&S (0.60 to 0.66). The delivered MU were significantly reduced with VMAT (8% mean) as well as the delivery time (4 min to 1.5 min). VMAT allowed delivering theorically 90Gy in the peripheral zone and 100 Gy in the tumor. CONCLUSION: In case of prostate irradiation, VMAT shows improvement compared with S&S. In particular, organs at risk are better spared, the delivery time is shortened and the number of delivered UM is decreased.
