Contrôle qualité d’un système d’imagerie cone beam mégavoltage

[Ballistic quality assurance]

This review describes the ballistic quality assurance for stereotactic intracranial irradiation treatments delivered with Gamma Knife® either dedicated or adapted medical linear accelerators. Specific and periodic controls should be performed in order to check the mechanical stability for both irradiation and collimation systems. If this step remains under the responsibility of the medical physicist, it should be done in agreement with the manufacturer's technical support. At this time, there are no recent published guidelines. With technological developments, both frequency and accuracy should be assessed in each institution according to the treatment mode: single versus hypofractionnated dose, circular collimator versus micro-multileaf collimators. In addition, "end-to-end" techniques are mandatory to find the origin of potential discrepancies and to estimate the global ballistic accuracy of the delivered treatment. Indeed, they include frames, non-invasive immobilization devices, localizers, multimodal imaging for delineation and in-room positioning imaging systems. The final precision that could be reasonably achieved is more or less 1mm.

[Quality assurance of enhanced dynamic wedge using the aS500-II, EPID]

PURPOSE

The work presented herein rests on the study of the Varian EPID aS500-II and the Image Acquisition system IAS3. We assessed the dosimetric performance of this EPID for measurements and quality assurance of enhanced dynamic wedge profiles and wedge factors.

MATERIALS AND METHODS

We evaluated the dosimeter properties using the integrated asynchronous mode of acquisition in treatments with enhanced dynamic wedges (EDW). We studied the performance, stability and the reproducibility in measurements of the transmission factors and profiles of the fields with dynamic wedges. EPID profiles were compared to the "Profiler Sun Nuclear" diode array and PTW ion chamber. Analytical functions were developed in order to correct EDW profiles. The dependence of EPID measurements on wedge direction, beam dimensions and source to EPID distance was assessed.

RESULTS

The backscatter produced by the "exact arm" was evaluated; EPID profiles depended on the EDW direction and on the detector source distance. Wedge factors were determined using this detector and compared to the ion chamber response, differences were all within 1 %. Two empirical correction functions were developed to produce EPID wedge profiles that correspond to diode for all wedge angles and energies depending on the wedge direction.

CONCLUSION

The EPID is highly suited to regular measurement of EDW due to the reproducibility of the EPID-measured wedge factors and profiles.

[Quality control of a kV cone beam computed tomography imaging system]

PURPOSE

This work presents the introduction of a quality assurance program for the On-Board Imager (OBI, Varian) kVCBCT system, together with the results of 1 year monthly testing.

MATERIALS AND METHODS

Firstly the geometric precision and stability of the equipment and of the associated software were evaluated using the Marker phantom. The coincidence of the accelerator isocenter and the imager isocenter was verified as well as the accuracy of the registration of kVCBCT with reference CT images. Then, the kVCBCT image quality was evaluated using the Catphan 504 phantom and ArtiScan software (Aquilab) for both full-fan (FF) and half-fan (HF) imaging modes.

RESULTS

The kVCBCT isocenter and image registration with correction of the table position were found to be within a tolerance of 2.0mm. Concerning the kVCBCT image quality, image noise and uniformity, the Hounsfield units (HU) stability and linearity, geometric distorsion and high contrast resolution were all found to be within the manufacturer's recommendations for both FF and HF modes. However, the low contrast resolution for the HF mode did not meet the manufacturer's specifications.

CONCLUSION

The quality assurance tests introduced have defined the initial system characteristics and their evolution during a period of 1 year, demonstrating the stability of the OBI.