Plan delivery quality assurance for CyberKnife: Statistical process control analysis of 350 film-based patient-specific QAs.

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.

CyberKnife® M6™: Peripheral dose evaluation for brain treatments.

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.

[Expected benefit of lymph node and seminal vesical dissection to decrease high-risk prostate cancer radiotherapy]. / Bénéfice attendu du curage ganglionnaire et de l'exérèse des vésicules séminales pour diminuer la toxicité de la radiothérapie des tumeurs prostatiques de haut risque.

PURPOSE: In case of pelvic lymph node and seminal vesicle dissection followed by prostate cancer intensity-modulated radiotherapy, the objective of the study was to evaluate the dosimetric benefit of reducing the target volume. PATIENTS AND METHODS: A total of 25 patients with high-risk prostate cancer had surgery first followed by intensity-modulated radiotherapy and androgen deprivation. Four treatment planning were simulated for each patient, based on two CT scans performed before and after surgery. The target volumes were: prostate-seminal vesicles-lymph nodes, prostate-lymph nodes, prostate-seminal vesicles and prostate only. The total dose was 46Gy in the seminal vesicles and lymph nodes, and 80Gy in the prostate. RESULTS: Compared to prostate target volume only, the addition of seminal vesicles and lymph nodes multiplied by a factor of 1.6 and 6.5 the target volume, respectively. Decreasing the target volume from prostate-seminal vesicles-lymph nodes to prostate-seminal vesicles, to prostate only decreased the rectal wall mean dose from 49Gy to 42Gy, to 36Gy, and the risk of late rectal bleeding from 4.4% to 3.2%, to 2.4% (P<0.05), respectively. The bladder wall mean dose decreased from 51Gy to 40Gy, to 35Gy (P<0,05), respectively. Not irradiating the lymph nodes decreased the absolute risk of diarrhea by 11%. CONCLUSION: Lymph node and seminal vesicle dissection before prostate cancer intensity-modulated radiotherapy allows decreasing moderately the risk of digestive toxicity.

[Which IMRT? From "step and shoot" to VMAT: physicist point of view]. / Quelle RCMI? Du step and shoot au VMAT: point de vue du physicien.

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.