Gel dosimetry in tridimensional quality control radiotherapy treatments with Intensity Modulated and SMART Technique (Simultaneous Modulate Accelerated Radiation Therapy)

Intensity Modulated Radiation Therapy (IMRT) treatments in the radiotherapy practice made possible the simultaneously treatment of multiple targets at an accelerated treatment regimen, this scheme is known as Simultaneous Modulated Accelerated Radiation Therapy (SMART). These treatments require a strict quality control (QC) that should be done, ideally, in three dimensions. A potential tool for tridimensional dosimetry (3D) is the use of polymeric gel with Nuclear Magnetic Resonance Images (NMRI). In this paper, the 3D dosimetry with MAGIC-f gel was applied in 10 QC radiotherapy planning using the SMART technique. Initially, the dose distributions of assessed plans were remeasured in simulator geometry based on a CT scan. All plans used were previously approved in punctual and planar conventional CQ, which validated the dose distribution planning system (TPS) for comparison with the gel dosimetry. The simulators were irradiated with established schedules and dose distributions were obtained by relaxometry in NMRI. The comparison of dose distributions calculated by TPS and measured in the gel was made by gamma analysis (3% / 3mm) and by comparing the dose-volume histograms (DVHs) calculated and measured for two volumes created in the equivalent regions to target volumes of treatment. In 5 analyzed cases, the distributions of dose measurements agree with the distribution expected by TPS, in these cases the protocol already established for the gel dosimetry has been fulfilled. The average approval in the gamma analysis was greater than 94% and at the DVHs the differences between the measured and expected curves were very insignificant. For the ve remaining planning, changes in dosimetry protocol were introduced and it was found that the obtained dose distributions were different from those calculated by the TPS. For a phantom, the temperature during irradiation was lower than the ambient temperature and in it, the percentage points approved by the analysis range was 58%. In the other four phantoms, the acquisition of NMRI was done three months after irradiation and an average approval of 57% was obtained. For these phantoms, the differences between DVHs are considerable. The gel temperature during irradiation and acquisition of NMRI by long periods after irradiation are responsible for significant deviations from the results of the 3D dosimetry. These results highlight the potential use of the polymer gel dosimetry in IMRT planning 3D CQ with SMART technique, providing complete 3D information of the absorbed dose in each target volume planned, if carried out following the pre-established protocols. (author)