OSL nanodot response at high dose gradient regions: Validation with phantom and in vivo tests

Abstract

Introduction: In vivo dosimetry (IVD) is the tool that has been shown to be suitable for verification and improvement on the accuracy of dose delivery and to prevent accidents during radiotherapy treatment. OSLD nanoDot complies with the characteristics to perform intracavitary IVD: being small-sized, they show little or no dependence on variables such as temperature, dose rate, field size, among others. The goal of this work is to define confidence and action levels for intracavitary IVD in intensity modulated radiotherapy (IMRT) at high dose gradient regions (HDGR). Method: OSLD nanoDot were prepared following the TG-191 AAPM international guideline. Commissioning test: OSLD response at HDGR in the penumbra region of an open field with a dedicated acrylic phantom was evaluated and compared with an ionization chamber. Geometrical uncertainties were evaluated as i) dosimeter location at planar digital images, ii) internal anatomical movements, iii) construction and placement of compensator filters and iv) mechanical indicators of the linear accelerator such as gantry, telemeter, laser, among others. Dosimetric impact was evaluated by relative difference considering calculated dose at distant points ±5 mm on anteroposterior direction at HDGR at rectum as organ at risk (OAR) in prostate cancer treatments. In vivo dosimetry test: a small ergonomic silicone device contained two dosimeters and they were placed on 23 patients. Patients gave their consent to participate in this statistical study. Geometrical coordinates of the dosimeters location were acquired by anteroposterior and lateral planar digital images. Coordinates were imported to the treatment planning system Modular Integrated Radiotherapy System (MIRS) v.5.1 (Nuclemed S.A., Argentina) and dose was determined at these points. Results: OSLD response followed with good agreement ionization chamber response over the entire penumbra region and the maximum distance to agreement (DTA) is 1.2 mm. Total geometrical uncertainty was ±5 mm. On the basis of intracavitary IVD, confidence (CL) and action levels (AL) were defined for HDGR. CL was ±19.2% and AL was ±37.1%. Conclusion: Geometrical uncertainty analysis associated to IMRT and intracavitary IVD has excellent correspondence to action level defined. With the correct characterization of the OSLD nanoDot, this detector becomes a good option for its implementation in intracavitary IVD at HDGR.