A literature review of electronic portal imaging for radiotherapy dosimetry

The American Association of Physicists in Medicine Task Group 119 instructed institutions to use a low‐dose threshold of 10% or a region of interest determined by the jaw setting when they collected gamma analysis quality assurance (QA) data for the planar dose distribution. However, there are no clinical data to quantitatively demonstrate the impact of the low‐dose threshold on the gamma index. Therefore, we performed a gamma analysis with various low‐dose thresholds in the range of 0% to 15% according to both global and local normalization and different acceptance criteria (3%/3 mm, 2%/2 mm, and 1%/1 mm). A total of 30 treatment plans — 10 head and neck, 10 brain, and 10 prostate cancer cases — were randomly selected from the Varian Eclipse treatment planning system (TPS). For the gamma analysis, a calculated portal image was acquired through a portal dose calculation algorithm in the Eclipse TPS, and a measured portal image was obtained using an electronic portal‐imaging device. Then, the gamma analysis was performed using the Portal Dosimetry software (Varian Medical Systems, Palo Alto, CA). The gamma passing rate (%GP) for the global normalization decreased as the low‐dose threshold increased, and all low‐dose thresholds led to %GP values above 95% for both the 3%/3 mm and 2%/2 mm criteria. However, for the local normalization, %GP for a low‐dose threshold of 10% was 7.47%, 10.23%, and 6.71% greater than the low‐dose threshold of 0% for head and neck, brain, and prostate for the 3%/3 mm criteria, respectively. The results indicate that applying the low‐dose threshold to global normalization does not have a critical impact on patient‐specific QA results. However, in the local normalization, the low‐dose threshold level should be carefully selected because the excluded low‐dose points could cause the average %GP to increase rapidly.PACS number: 87.55.Qr

Section: Methodsmentioning

confidence: 99%

Gamma analysis dependence on specified low‐dose thresholds for VMAT QA

Song

Kim

Park

et al. 2015

“…The intensity of the transit portal image (acquired through the patient during treatment) can be related to the dose absorbed by the patient. Amorphous‐silicon (a‐Si) EPIDs, in particular, have desirable dosimetric properties including linearity with dose, nondependency with dose rate, and good reproducibility 7 , 8 , 9 , 10 , 11 . Additional strengths of the EPID as a dosimeter are: it is readily available, easy to operate, and can produce two‐dimensional dose maps.…”

Section: Introductionmentioning

confidence: 99%

“…Cine‐mode EPID imaging has found application in the realm of gantry motion verification for dynamic RT techniques 23 , 24 and as a pretreatment dose verification tool, 8 , 25 but real‐time EPID IVD is not current clinical practice. For further applications of portal imaging IVD the reader is referred to comprehensive review papers available in literature 2 , 11 …”

Section: Introductionmentioning

confidence: 99%

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

Peca

Brown

2014

The electronic portal imaging device (EPID) has the potential to be used for in vivo dosimetry during radiation therapy as an additional dose delivery check. In this study we have extended a method developed by A. Piermattei and colleagues in 2006 that made use of EPID transit images (acquired during treatment) to calculate dose in the isocenter point. The extension allows calculation of two‐dimensional dose maps of the entire radiation field at the depth of isocenter. We quantified the variability of the ratio of EPID signal to dose in the isocenter plane in Solid Water phantoms of various thicknesses and with various field sizes, and designed a field edge dose calculation correction. To validate the method, we designed three realistic conventional radiation therapy treatment plans on a thorax and head anthropomorphic phantom (whole brain, brain primary, lung tumor). Using CT data, EPID transit images, EPID signal‐to‐dose correlation, and our edge correction, we calculated dose in the isocenter plane and compared it with the treatment planning system's prediction. Gamma evaluation (3%, 3 mm) showed good agreement (Pγ<1 ≥ 96.5%) for all fields of the whole brain and brain primary plans. In the presence of lung, however, our algorithm overestimated dose by 7%–9%. This 2D EPID‐based in vivo dosimetry method can be used for posttreatment dose verification, thereby improving the safety and quality of patient treatments. With future work, it may be extended to measure dose in real time and to prevent harmful delivery errors.PACS numbers: 87.55.km, 87.55.Qr, 87.55.T‐

“…( 2 ) When compared to the traditional EPIDs such as fluoroscopic screen/camera‐based and liquid‐filled matrix ionization chambers, the new generation of EPIDs, equipped with amorphous silicon (aSi) flat panels, supply more stable transit signals and are suitable as transit detectors. Recently, the authors have developed an in‐vivo dosimetry method based on correlation functions defined by the ratios between the transit signal,

s_{normalt}

, measured by aSi EPIDs, and the solid water phantom midplane doses,

D_{normalm}

, measured by an ion chamber positioned along the central axis.…”

Section: Introductionmentioning

confidence: 99%

Correlation functions for Elekta aSi EPIDs used as transit dosimeter for open fields

Cilla

Fidanzio

Greco

et al. 2010

In‐vivo dosimetry techniques are currently being applied only by a few Centers because they require time‐consuming implementation measurements, and workload for detector positioning and data analysis. The transit in‐vivo dosimetry performed by the electronic portal imaging device (EPID) avoids the problem of solid‐state detector positioning on the patient. Moreover, the dosimetric characterization of the recent Elekta aSi EPIDs in terms of signal stability and linearity make these detectors useful for the transit in‐vivo dosimetry with 6, 10 and 15 MV photon beams. However, the implementation of the EPID transit dosimetry requires several measurements. Recently, the present authors have developed an in‐vivo dosimetry method for 3D CRT based on correlation functions defined by the ratios between the transit signal, normalsnormalt(w,L), by the EPID and the phantom midplane dose, normalDnormalm(w,L), at the source to axis distance (SAD) as a function of the phantom thickness, w, and the square field dimensions, L. When the phantom midplane was positioned at distance, d, from the SAD, the ratios normalsnormalt(w,L)/snormalt'(d,w,L) were used to take into account the variation of the scattered photon contributions on the EPID as a function of d and L.The aim of this paper is the implementation of a procedure that uses generalized correlation functions obtained by nine Elekta Precise linac beams. The procedure can be used by other Elekta Precise linacs equipped with the same aSi EPIDs, assuming the stabilities of the beam output factors and the EPID signals. The procedure here reported avoids measurements in solid water equivalent phantoms needed to implement the in‐vivo dosimetry method in the radiotherapy department. A tolerance level ranging between ±5% and ±6% (depending on the type of tumor) was estimated for the comparison between the reconstructed isocenter dose, Diso, and the computed dose, Diso,TPS, by the treatment planning system (TPS).PACS number: 87.55.Qr; 87.56.Fc