Interfractional trend analysis of dose differences based on 2D transit portal dosimetry

Abstract

Dose delivery of a radiotherapy treatment can be influenced by a number of factors. It has been demonstrated that the electronic portal imaging device (EPID) is valuable for transit portal dosimetry verification. Patient related dose differences can emerge at any time during treatment and can be categorized in two types: (1) systematic-appearing repeatedly, (2) random-appearing sporadically during treatment. The aim of this study is to investigate how systematic and random information appears in 2D transit dose distributions measured in the EPID plane over the entire course of a treatment and how this information can be used to examine interfractional trends, building toward a methodology to support adaptive radiotherapy. To create a trend overview of the interfractional changes in transit dose, the predicted portal dose for the different beams is compared to a measured portal dose using a γ evaluation. For each beam of the delivered fraction, information is extracted from the γ images to differentiate systematic from random dose delivery errors. From the systematic differences of a fraction for a projected anatomical structures, several metrics are extracted like percentage pixels with |γ| > 1. We demonstrate for four example cases the trends and dose difference causes which can be detected with this method. Two sample prostate cases show the occurrence of a random and systematic difference and identify the organ that causes the difference. In a lung cancer case a trend is shown of a rapidly diminishing atelectasis (lung fluid) during the course of treatment, which was detected with this trend analysis method. The final example is a breast cancer case where we show the influence of set-up differences on the 2D transit dose. A method is presented based on 2D portal transit dosimetry to record dose changes throughout the course of treatment, and to allow trend analysis of dose discrepancies. We show in example cases that this method can identify the causes of dose delivery differences and that treatment adaptation can be triggered as a result. It provides an important element toward informed decision-making for adaptive radiotherapy.

Two‐dimensional in vivo dose verification using portal imaging and correlation ratios

“…Nonetheless, there are many arguments in favor of in vivo dosimetry (IVD), that is, a method to measure the dose deposited in the patient during treatment, as an auxiliary optimization and safety procedure. IVD can identify errors in dose calculation, data transfer, patient setup, and dose delivery, and may be used as a trigger for adaptive radiotherapy in cases of changing patient anatomy 1 , 2 , 3 . More importantly, most RT errors which have led to serious patient injury or death 4 , 5 , 6 could have been avoided or reduced with IVD.…”

First Experience With Real-Time EPID-Based Delivery Verification During IMRT and VMAT Sessions

“…These will be designed to detect clinically relevant errors and not small errors owing to transient anatomical changes [26][27][28]. The system is not able to verify fields where collisions between EPID and the couch or the patient are possible, such as non-coplanar beams.…”

Establishing action threshold for change in patient anatomy using EPID gamma analysis and PTV coverage for head and neck radiotherapy treatment

“…Since the advent of the Electronic Portal Imaging Device (EPID) in radiotherapy, interest in this detector system supports its potential for in vivo dosimetry . Over the last decade, EPID‐based in vivo dosimetry has been used in many centers with methods ranging from a single point measurement to the reconstruction of a dose distribution within a 2D plane or a 3D volume . These systems are usually designed to detect errors occurring during data transfer or in the treatment plan, or to evaluate plan feasibility, rather than monitoring a patient's morphological changes over time.…”

“…These systems are usually designed to detect errors occurring during data transfer or in the treatment plan, or to evaluate plan feasibility, rather than monitoring a patient's morphological changes over time. Some groups have nevertheless found that valuable information such as trends in anatomical changes, can be derived for 2D transit portal dosimetry …”