Setup in a Clinical Workflow and Results of In Vivo Dosimetry Procedure in an Overload Radiation Therapy Department

Abstract

High conformal techniques, as intensity modulated radiation therapy (IMRT) and volumetric modulated arc therapy (VMAT) are widely used in overloaded radiotherapy departments. In vivo dosimetric screening appears to be essential in this environment to avoid important dosimetric errors. The work reports the results of the assessment of an in vivo dosimetry (IVD) procedure in a surcharged clinical workflow, analyses the causes that could be responsible of dosimetric disagreements between the delivered and planned treatments and evaluates the added workload needed for the whole procedure. An IVD method based on the results of an electronic portal imaging device (EPID) was used. A total of 3484 IVD tests were performed for 150 patients treated for head and neck, abdomen, pelvis, breast and thorax tumors with IMRT and VMAT techniques. Two types of tests were considered: the ratio R between the reconstructed and planned isocenter doses and a transit dosimetry based on the γ-analysis of the electronic portal images. The tests were considered in tolerance if R is within 5%, γ% >90% (i.e. the percentage of points with γ <1 is greater than 90%) and if γ mean ≤ 0.4. Therefore, within the EPID irradiated area, a maximum of 10% of the points in disagreement were accepted, while the weight of the discrepancy was supplied by the distribution of the γ values, characterized by a mean value lower than 0.4. If even only one of these indices was outside the tolerance level, the reason was investigated and corrected prior to the next fraction and the IVD test was repeated. The time needed for each step of the IVD procedure was registered. IMRT and VMAT treatments for pelvis, abdomen and head and neck presented 10% of the tests out of the tolerance level, while IMRT treatments for the breast and thorax regions accounted for up to 25%. Of the off-tolerance tests, 90% were due to inadequate standard quality controls that can lead to errors in the patient setup. The remaining 10% of the tests out of the tolerance level, were mainly ascribable to patient morphological changes as different filling of bladder and rectum for pelvis treatments, tumor shrinkage for head and neck, but also loss of weight. For breast and thorax treatments, the patient positioning was the main cause of the out of tolerance of the γ analysis. Once corrected the cause of the errors, the percentage of patients ending the treatment with each mean index within the tolerance level tends towards 100% for every region treated and technique used. An average time of 42 minutes/day was sufficient to monitor a daily workload of 60 patients in treatment. IVD performed with an electronic portal imaging device is feasible in an overloaded department. Its use enables the timely realignment of the treatment quality indices within the tolerance level in order to achieve 100% of patient’s final treatment compliant with the one prescribed.