A Feasibility Study for in vivo Dosimetry Procedure in Routine Clinical Practice

Abstract

Purpose:The aim of the in vivo dosimetry, during the fractionated radiation therapy, is the verification of the correct dose delivery to patient. Nowadays, in vivo dosimetry procedures for photon beams are based on the use of the electronic portal imaging device and dedicated software to elaborate electronic portal imaging device images.Methods:In total, 8474 in vivo dosimetry tests were carried out for 386 patients treated with 3-dimensional conformal radiotherapy, intensity-modulated radiotherapy, and volumetric modulated arc therapy techniques, using the SOFTDISO. SOFTDISO is a dedicated software that uses electronic portal imaging device images in order to (1) calculate the R index, that is, the ratio between daily reconstructed dose and the planned one at isocenter and (2) perform a γ-like analysis between the signals, S, of a reference electronic portal imaging device image and that obtained in a daily fraction. It supplies 2 indexes, the percentage γ% of points with γ < 1 and the mean γ value, γmean. In γ-like analysis, the pass criteria for the signals agreement ΔS% and distance to agreement Δd have been selected based on the clinical experience and technology used. The adopted tolerance levels for the 3 indexes were fixed in 0.95 ≤ R ≤ 1.05, γ% ≥ 90%, and γmean ≤ 0.5.Results:The results of R ratio, γ-like, and a visual inspection of these data reported on a monitor screen permitted to individuate 2 classes of errors (1) class 1 that included errors due to inadequate standard quality controls and (2) class 2, due to patient morphological changes. Depending on the technique and anatomical site, a maximum of 18% of tests had at least 1 index out of tolerance; once removed the causes of class-1 errors, almost all patients (except patients with 4 lung and 2 breast cancer treated with 3-dimensional conformal radiotherapy) presented mean indexes values (trueR¯, trueγ¯%, and γ¯mean ) within tolerance at the end of treatment course. Class-2 errors were found in some patients.Conclusions:The in vivo dosimetry procedure with SOFTDISO resulted easily implementable, able to individuate errors with a limited workload.

Evaluation of interfraction setup variations for postmastectomy radiation therapy using EPID‐based in vivo dosimetry

“…The percentage of patients, P%, with the mean value of the indices within the tolerance levels at the end of the course of radiotherapy was greater than 96.0%, regardless of the patient group to which they belonged or the technique used, while T% ranged from 92.0% to 95.8%. This feature of EPID‐based IVD — which enables the timely realignment of the treatment quality indices within the tolerance level — has already been shown in previous studies . This indicates that the effect of the corrections was evident and, as expected, every patient treated using the IMRT and VMAT delivery technique ended the treatment with the indices within the tolerance levels.…”

“…Setup errors are a major source of deviations during EPID‐based IVD verification programs . If the setup of a patient is irreproducible, large variations in the dose reconstruction will be observed especially when high‐conformal techniques are utilized.…”

“…We adopted as pass criteria 5% and 5 mm. The current choice of pass‐fail criteria aligns with previous literature data and is based on our experience with 2D in vivo dose verification of IMRT‐VMAT using gamma evaluation since the start of routine clinical implementation in 2016 . Δ d = 5 mm is also the maximum displacement value acceptable in clinical practice by the radiation oncologists, while the Δ S was defined by considering the presence of dose gradients (interface lung‐PTV) and mobility of the irradiated organs (breath).…”

“…For these reasons it has been recommended by several international organizations and has become mandatory in some western countries . Moreover, it is widely used in Europe to evaluate the interfractional variations in dose delivery and patient setup for many treatment regions; many studies have validated the technique and presented the results obtained in the clinical practice . Mijnheer et al, analyzed with EPID‐based IVD more than 15 000 plans on different treatment sites, and found that more than 30% exceeded the alert criteria, attributing most of the errors to deviations from the routine clinical procedure and to anatomical changes.…”

Combined Use of a Transmission Detector and an EPID-Based In Vivo Dose Monitoring System in External Beam Whole Breast Irradiation: A Study with an Anthropomorphic Female Phantom

“…The strength of the combined use of the two systems in detecting all different kinds of errors, even when very small, has been evidenced in this work. Separate use of IQM or SoftDiso devices has been widely discussed for different irradiation modalities [43][44][45][46][47][48][49], but to our knowledge, no study exists of their combined use, in particular for breast irradiation. A clinical study of the SoftDiso-based quality assurance procedure for post-mastectomy IMRT and VMAT radiation therapy is reported in reference [50], where specific thresholds for R-value, γ PR and γ mean indexes were proposed, based on the clinical experience and validated with a phantom chest.…”