|Year : 2018 | Volume
| Issue : 2 | Page : 300-307
In vivo dose estimations through transit signal measured with thimble chamber positioned along the central axis at electronic portal imaging device level in medical linear accelerator in carcinoma of the middle-third esophagus patients undergoing three-dimensional conformal radiotherapy
Putha Suman Kumar1, S Banerjee1, ES Arun Kumar1, Challapalli Srinivas1, BM Vadhiraja2, PU Saxena1, Ramamoorthy Ravichandran3, Dinesh Pai Kasturi1
1 Department of Radiotherapy and Oncology, Kasturba Medical College Hospital (An Associated Teaching Hospital of Manipal University), Mangalore, India
2 Department of Radiation Oncology, Manipal Hospital, Bengaluru, India
3 Department of Medical Radiation Physics, School of Allied Health Sciences, Manipal University, Manipal, Karnataka, India
|Date of Web Publication||8-Mar-2018|
Dr. Challapalli Srinivas
Department of Radiotherapy and Oncology, Kasturba Medical College Hospital (An associated teaching hospital of Manipal University), Attavar, Mangalore - 575 001, Karnataka
Source of Support: None, Conflict of Interest: None
Objective: This study presents a method to estimate midplane dose (Diso, transit) in vivo from transit signal (St) measured with thimble ionization chamber in cancer of the middle-third esophagus patients treated with three-dimensional radiotherapy (RT). This detector is positioned at the level of electronic portal imaging device in the gantry of a medical linear accelerator.
Materials and Methods: Efficacy of inhomogeneity corrections of three dose calculation algorithms available in XiO treatment planning system (TPS) for planned dose (for open fields) (Diso, TPS) was studied with three heterogeneous phantoms. Diso, transit represents measured signal at transit point (St) far away correlating to dose at isocenter. A locally fabricated thorax phantom was used to measure the in vivo midplane dose (Diso, mid) which was also estimated through St. Thirteen patients with carcinoma of the middle-third esophagus treated with three-dimensional conformal RT were studied. St was recorded (three times, with a gap of 5–6 fractions during the treatment) to estimate Diso, transit, which was compared with the doses calculated by TPS.
Results: The dose predictions by superposition algorithm were superior compared to the other algorithms. Percentage deviation of Diso, transit, Diso, mid with Diso, TPS combined all fields was 2.7 and –2.6%, respectively, with the thorax phantom. The mean percentage deviation with standard deviation of estimated Diso, transit with Diso, TPS observed in patients was within standard deviation –0.73% ±2.09% (n = 39).
Conclusions: Midplane dose estimates in vivo using this method provide accurate determination of delivered dose in the middle-third esophagus RT treatments. This method could be useful in similar clinical circumstances for dose confirmation and documentation.
Keywords: Conformal radiotherapy, esophagus cancer, in vivo dose, transit signal
|How to cite this article:|
Kumar PS, Banerjee S, Arun Kumar E S, Srinivas C, Vadhiraja B M, Saxena P U, Ravichandran R, Kasturi DP. In vivo dose estimations through transit signal measured with thimble chamber positioned along the central axis at electronic portal imaging device level in medical linear accelerator in carcinoma of the middle-third esophagus patients undergoing three-dimensional conformal radiotherapy. J Can Res Ther 2018;14:300-7
|How to cite this URL:|
Kumar PS, Banerjee S, Arun Kumar E S, Srinivas C, Vadhiraja B M, Saxena P U, Ravichandran R, Kasturi DP. In vivo dose estimations through transit signal measured with thimble chamber positioned along the central axis at electronic portal imaging device level in medical linear accelerator in carcinoma of the middle-third esophagus patients undergoing three-dimensional conformal radiotherapy. J Can Res Ther [serial online] 2018 [cited 2019 Dec 12];14:300-7. Available from: http://www.cancerjournal.net/text.asp?2018/14/2/300/214517
| > Introduction|| |
Esophageal cancer is a highly virulent neoplasm with high morbidity and mortality. It is the second most common malignancy among Indian males and fifth in females. Keeping into account the complications associated with surgery, along with the benefit of radiotherapy (RT) combined with chemotherapy clearly established, the challenge currently lies in the accurate and safe delivery of RT. Improved understandings of patterns of esophageal cancer relapse and tumor spread and of organ motion in the upper thorax and abdomen have allowed for the implementation of three-dimensional conformal radiotherapy (3DCRT), respiratory-gated RT, image-guided RT, and intensity-modulated RT. As a minimum, successful implementation of 3DCRT requires a detailed understanding of esophageal anatomy, radiobiological principles, an individualized assessment of organ motion, precise patient immobilization techniques, and adequate physics and dosimetry expertise.
The International Commission on Radiation Units and Measurements has recommended that radiation dose must be delivered to be within ±5% of the prescribed dose to the target. Uncertainty in dose delivery includes patient- and machine-specific parameters, e.g., outline of patient structure along with associated inhomogeneity, positioning error, variations in patient geometry from time of planning to the time of treatment, organ motion (internal and external), accuracy of computerized treatment planning system (TPS) dose calculation algorithm, and random variations in linear accelerator (linac) output. The use of inhomogeneity corrections for tissue density variations has become standard practice in most radiation therapy departments that have direct access to computerized tomography (CT) scanning for radiation therapy planning. Therefore, irradiation has to be planned under these challenging conditions with accurate dose calculation done in TPS, in which the functionality and quality depend on the type of algorithm used in planning process. The accuracy of dose calculations has continuously improved by moving from the simple scatter in homogeneity corrections over pencil beam algorithms to point Kernel-based convolution/superposition (SP) methods. Many studies relating to the guidelines and protocols recommend implementation of suitable quality assurance (QA) for the TPS have been performed in the department. Hence, there is a definite need to check the inhomogeneity corrections for dose calculations by the TPS.
Increasing complexity in treatment techniques warrants the verification of planned dose delivery to target as an important part of QA, a point emphasized by several national and international organizations.,,,In vivo dosimetry has been recommended by the International Atomic Energy Agency (IAEA) in sites with uniform body contour (e.g., pelvis) and for techniques that do not entail high-dose gradients. It will enable the identification of potential errors in dose calculation, data transfer, dose delivery, patient setup, and changes in patient anatomy. There are various methods listed in the literature for the documentation of in vivo dose with different detectors and protocols.,
Since the introduction of rotation therapy with deep X-rays and telecobalt beam, measurement of transmission of radiation flux at a point distant from the patient has been used for in vivo dosimetry. Piermattei et al., in an elegant method, positioned a small air ionization chamber (IC) at the typical location of the electronic portal imaging device (EPID) in relation to the beam source along the beam central axis to obtain the in vivo midplane dose as extrapolated from the IC signal obtained. Francois et al. used similar principles wherein they calibrated the EPID against an IC and used it to measure the transit dose by back projection of the portal dose. In a recent study by Putha et al., the midplane dose was estimated through transit signal (St) measured with IC positioned at EPID level, in group of patients with pelvic malignancies undergoing 3DCRT. These methods have the potential to identify alterations in tissues thickness and probable setup errors.
The present work illustrates (a) validation of dosimetric performance of TPS with inhomogeneous phantoms using different dose calculation algorithms, (b) estimation of in vivo midplane dose through St with a locally fabricated thorax phantom, (c) estimation of in vivo midplane dose through St which were measured with an IC positioned along central axis at EPID level of linac in a group of carcinoma of middle-third esophagus patients undergoing 3DCRT (by accounting water equivalent path length principle) and their comparison with the TPS calculated values.
| > Materials and Methods|| |
A medical linac (Model Elekta Compact), with 6 MV photons, motorized wedge and 40 pairs Multi Leaf Collimator (MLCi2) having leaf thickness of 1 cm at 100 cm isocenter available at our center was used. This is routinely operated at 350 MU/min which is calibrated for the output of 1 cGy/MU at isocenter followed as per the IAEA protocol Technical Report Series (TRS)-398.
Dosimetric performance of treatment planning system with and without homogenous media
Computerized TPS (CMS XiO®, Elekta Ltd, UK, version 4.80.02) which has Clarkson, convolution, SP, and fast superposition (FSP) algorithms was commissioned with beam data measurements of the linac and was used.
To evaluate the dosimetric performance of the TPS with homogenous media, simple test cases were created as per the guidelines of TRS-340 and absolute dose measurements were done for the each test case with the MU calculation given by the TPS with water phantom. A deviation of 1.1% was observed between the measured doses with the corresponding calculated ones. To assess the dosimetric performance of TPS with inhomogeneous media, three phantoms [A, B, and C as shown in [Figure 1] were made with the combination of sheets having water equivalent (solid water of density ρ≈ 1.045 g/cm3 having dimensions of 30 cm × 30 cm × 1 cm for each slab), bone tissue equivalent (Teflon sheet of density ρ≈ 1.6 g/cm3 having dimensions of 30 cm × 30 cm × 2 cm), and lung tissue equivalent (cork sheet of density ρ≈ 0.28 g/cm3 having dimensions of 30 cm × 30 cm × 1 cm for each sheet). Calibrated Farmer-type IC model FC65 (Iba Dosimetry GmbH, Schwarzenbruck, Germany) was used for dosimetric measurements which was kept in an adapter plate at a depth of 12 cm in all combinations.
|Figure 1: Inhomogeneous phantoms of A, B, and C for dose verification at a physical depth of 12.0 cm|
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The arrangement of slabs with IC adapter plate to generate three inhomogeneous phantoms for planned dose verification is shown in [Table 1]. CT (M/s Wipro GE “High Speed”) images of 5 mm slice thickness were acquired with 120 kVp for all three phantoms having IC in adapter plate. Scanned serial CT images were exported to FocalSim Contouring Station (M/s Elekta Ltd., Crawly, UK) via Digital Imaging and Communications in Medicine (DICOM) network. Contouring of target volume (active volume of IC) was done in all transverse slices. Dose calculations were performed using convolution, SP, and FSP algorithms for central axis open anterior fields (5, 10, 15, and 20 cm2) for 200 cGy delivery, placing isocenter at center of the target using the CT calibration curve generated for 120 kVp, which is used in phantoms image acquisition. [Figure 2] shows treatment planning windows of inhomogeneous phantoms A, B, and C with IC in position.
|Table 1: Inhomogeneous phantoms A, B, and C (combination of water, lung and bone equivalent slabs with IC adapter plate)|
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|Figure 2: Treatment planning windows corresponding to geometries (A, B, and C) shown in Figure.1 along with ionization chamber in position|
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Thorax phantom (design, fabrication, and dosimetric aspects)
To validate the in vivo dose estimations in carcinoma of the middle-third esophagus patients, a thorax phantom was designed and locally fabricated. The objective in preparation of this phantom was based on ease of utility, handling, and capability to perform dosimetric studies in clinical RT test cases in the thorax region. A water phantom of dimensions 30 cm × 30 cm × 30 cm having prefixed IC slot for FC65 at midplane depth with leveling screws was considered to design the thorax phantom. Two acrylic sheets of dimensions 30 cm × 30 cm × 1 cm were taken to make the slots for lung (Cork), spine (Teflon rod of density ρ≈ 1.60 g/cm3 having 12 cm in length and 4 cm in diameter) equivalents and also provision to place miniature IC (CC13) at different locations (numbered 1–5 and 7) as shown in [Figure 3] for in vivo dosimetric measurements. [Figure 3] shows the axial and lateral views of the thorax phantom with dimensions (along with the acrylic sheets sandwiched with cork and spine inserts) along with chamber slots. Slot no. 6 was used as surrogate for middle-third esophagus where FC 65 was inserted for measurements in this study. [Figure 4] shows the water phantom with leveling screws with FC65 chamber slot at midplane, acrylic sheets with lung (Cork), bone (Teflon) along with miniature IC (CC13) slots, and thorax phantom with acrylic sheets final assembly.
|Figure 3: Thorax phantom frontal view and lateral view with dimensions along with the acrylic sheets sandwiched with cork sheets, spine insert and provision for miniature ionization chamber (CC13) slots (1–5 and 7) for in vivo measurements|
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|Figure 4: (a) Water phantom with FC65 chamber slot at midplane having leveling screws. (b) Acrylic sheets with lung (Cork), bone (Teflon) along with miniature ionization chamber (CC13) slots. (c) Thorax phantom with acrylic sheets final assembly|
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Transit signal measurements
The method of estimation of in vivo midplane dose (Diso, transit) through St measured (in nano-Coulombs) with an IC kept at EPID level developed by Putha et al., which was implemented in a group of patients with pelvic malignancies, treated with 3DCRT, was applied to generate the data for this study. In their method, St was obtained for 100 MU delivery with a water phantom (dimensions of 40 cm × 40 cm × 40 cm) positioned on treatment couch in isocenter condition for various water thicknesses (ranging from 6 cm to 30 cm with a step size of 2 cm) and for square fields (from 5 cm × 5 cm to 25 cm × 25 cm at intervals of 5 cm). All readings were corrected for temperature, pressure, phantom (base), and couch transmission. A calibration table was generated by taking the ratio of St versus Diso, TPS values which were acquired from the CMS XiO TPS by calculating the midplane doses from the virtual water phantoms (in nC/cGy) for corresponding water thickness and field size combinations. These data were fed to MATLAB (M/S The MathWorks Inc., Version R 2015a) software to generate a code for calculation of in vivo midplane dose (Diso, transit) through St.
The above-fabricated thorax phantom was scanned under CT machine with FC65 chamber in the slot no. 6, which simulates the target position as in the case of carcinoma of middle-third esophagus patient. Scanned serial CT images were exported to contouring station via DICOM network. Target volume, i.e., the active volume of IC was contoured in all transverse slices and the resultant images transferred to CMS XiO® (M/s Elekta Ltd., Crawly, UK) version 4.80.02 TPS for beam placement and dose calculations. A set of four beams with gantry angles 0°, 90°, 180°, and 270° with a field size of 12 cm × 6 cm positioned the isocenter at the center of the target. SP algorithm was chosen (based on the results found from inhomogeneous phantom study) for dose calculations. A treatment plan was generated with a dose prescription of 200 cGy and normalized to the 100% isodose line that is encompassed the target. [Figure 5] shows the treatment planning window with the 95% isodose coverage around the target in transverse, coronal, sagittal planes, anterior field beam's eye view, 3D view, along with dose volume histogram of thorax phantom in Xio TPS.
|Figure 5: Isodose (95%) coverage around the target (ionization chamber) in slot no. 6 the in transverse, coronal, sagittal planes, anterior field beam's eye view, three-dimensional coverage along with dose volume histogram of thorax phantom in Xio treatment planning system|
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The St and midplane dose (Diso, mid) were taken with Dose 2 electrometer (M/s Iba, Germany) simultaneously with two FC65 ICs for all four beam angles with thorax phantom positioned in isocenter condition under linac during treatment planning execution. [Figure 6] shows the execution of treatment plan of anterior field with ICs in target (slot no. 6 of thorax phantom) and transit position. Measured in vivo midplane dose (Diso, mid) was compared with the estimated midplane dose from St measurements (Diso, transit) using the method developed by Putha et al. To check the uniformity of both ICs, the absolute dose was measured for 100 MUs, following the IAEA protocol TRS-398 at a reference depth of 10 cm and field size 10 cm × 10 cm in a water phantom and a deviation of 0.2% was found.
|Figure 6: Simultaneous measurement of in vivo midplane dose and transit signal with two FC65 ionization chambers with thorax phantom positioned in isocenter condition under linear accelerator|
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Estimation of in vivo midplane dose (Diso, transit) through transit signal in the middle-third esophagus patients
A total of 13 patients diagnosed with carcinoma of middle-third esophagus planned with 3DCRT were included in this study. Patients were immobilized in supine position using “Vacloc” device (M/s Klarity Medical, USA). Following CT simulation the scanned images were exported to FocalSim Workstation (M/s Elekta Ltd, Crawly, UK) where the clinical target volume (CTV) was created encompassing the gross tumor volume with a 1.5 cm radial margin and a 5 cm superior and inferior margin along with the appropriate nodal basins; surrounding organs at risk were also delineated. Based on the data from the investigating institution, a margin of 0.5 cm was added to the CTV to create the planning target volume (PTV).
For patients treated with curative intent, the first phase of treatment entailing 45 Gy in 25 fractions, reduced by 2 cm during the second phase wherein 5.4 Gy was delivered in 3 fractions was planned to the PTV. For patients treated with palliative intent, 30 Gy in 10 fractions was planned to the PTV. Contoured image data set was transferred to TPS for beams placement. Four-field (0°, 90°, 180° and 270°) technique with all beams positioned at the center and conformed to PTV by MLC with a margin of 5 mm was used for all patients. Planned dose was normalized to 100% to the center of PTV. Field-in-field (subfield) and/or wedge technique was used, as and when required, to reduce the hot spots around the target region. Beam weightage was distributed as 67% to the anteroposterior and 33% to the left and right lateral beams to achieve the lung and spinal doses within the current Quantec guidelines and also to get 95% of prescribed dose coverage around PTV. SP algorithm was chosen for dose calculations. A 3DCRT plan was generated with a planned dose per fraction which was evaluated and approved by the radiation oncologist. The plan was exported to record and verification system (MOSAIQ®) for scheduling and execution.
Positional verification before execution of treatment was carried out on the digitally reconstructed radiograph, i.e., digitally reconstructed radiography images (both anterior and lateral portal images) that had been previously exported to EPID (iViewC – camera based). Based on institutional data, a 3-mm margin of translational shift (X, Y, and Z axes) was deemed to be acceptable; couch corrections were applied as and where indicated. The scheduled 3DCRT plan was subsequently executed with linac for all patients. St readings were collected only for conformal fields, but not for wedged and subfields, during the treatment with IC positioned in transmission geometry. Measurements were repeated three times (with an interval of 5–6 fractions) during the RT.In vivo midplane dose was estimated (Diso, transit) using the method developed by Putha et al. from the acquired St which was compared with the TPS calculated value (Diso, TPS).
Schematic representation of position of IC with build up cap vis-à-vis linac treatment head, representative isocentric transverse slice of a middle-third esophagus patient used as model at all four gantry angles (0°, 90°, 180°, and 270°) under transit study condition is shown in [Figure 7].
|Figure 7: Schematic representation of position of ionization chamber with buildup cap vis-à-vis linear accelerator treatment head, representative isocenter transverse slice of a middle third esophagus patient used as model at all four gantry angles (0°, 90°, 180°, and 270°) under transit study condition|
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| > Results|| |
In [Table 2], the measured doses by our methodology are compared with the calculated doses in different field combinations by different algorithms. The percentage deviation of measured doses is tabulated for three inhomogeneous phantoms (A, B, and C), for which TPS has calculated doses by different algorithms. It could be seen that the deviation in doses for phantom A is −4.8 ± 1.3, 1.5 ± 0.2, and −1.1 ± 1.0 for convolution, SP, and FSP algorithms respectively. For phantom B, the respective values are −5.0 ± 0.6, −0.4 ± 0.5, and −0.8 ± 2.0; and for phantom C, these values are 2.2 ± 0.9, 0.6 ± 0.8, and 1.0 ± 1.1, correspondingly.
|Table 2: Percentage deviation of measured doses versus calculated ones with three algorithms (convolution, superposition, and fast superposition) available in XiO treatment planning system for different square field sizes in inhomogeneous phantoms (A, B, and C)|
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The in vivo midplane doses (Diso, transit, Diso, mid) and the calculated one (Diso, TPS) obtained with thorax phantom for four gantry angles are shown in [Table 3]. The mean with standard deviations of Diso, transit and Diso, mid versus Diso, TPS for individual fields is within 2.7 ± 2.4 and −2.65 ± 2.0, respectively. However, the percentage deviation of Diso, transit, Diso, mid with Diso, TPS combined all fields was 2.7 and −2.6%, respectively. [Table 4] shows the results of estimated Diso, transit(39 measurements) of 13 patients taken for three times each during the treatments along with Diso, TPS values. The mean percentage deviation of Diso, transit with Diso, TPS in individual patients during the treatments ranges from −5.37% to 3.26%. The overall estimated accuracy of Diso, transit with Diso, TPS observed from this group was −0.73% ± 2.09 (n = 39). [Figure 8] represents the scatter diagram of percentage deviation of estimated Diso, transit with Diso, TPS in a group of 13 patients (taken 3 times during the 3DCRT).
|Table 3: The in vivo midplane doses (Diso,TPS, Diso,mid, and Diso,transit) and the percentage deviation (Diso,transit vs. Diso,TPS and Diso,transit vs. Diso,mid) obtained with thorax phantom for different gantry angles|
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|Table 4: The calculated Diso,TPS with the estimated Diso,transit values (obtained in three fractions) along with the percentage deviation in a group of 13 patients|
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|Figure 8: Percentage deviation of estimated Diso, transitwith Diso, TPSin a group of 13 patients (taken 3 times during the three-dimensional conformal radiotherapy)|
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| > Discussion|| |
From the results, it is observed that the convolution algorithm was less accurate than the SP and FSP calculation in predicting the actual dose under inhomogeneity conditions. Most calculated results were within 1.5% of the corresponding measurements for both SP and FSP algorithms in this study. These results are in accordance with the studies done by Asnaashari et al. and Muralidhar et al.
Transit dosimetry in different geometries has been attempted by various researchers.,,, However, implementation of most of the methods explained therein may not be feasible in a department having limited infrastructure facilities.
A multicenter dose audit using similar transit measurement has shown an efficacy of dose estimation within an action level accuracy of 4%–5%, a measured deviation within 2% for brain treatments and 4% for thorax/pelvic treatments.
Francois et al. presented a simple method for verification of dose delivered on 38 patients treated with conformal therapy for various localizations using photon beams. During the treatment session, a transit dose is measured with calibrated IC and the dose in the patient is estimated from back projection algorithm. Central axis doses estimated by their formalism were compared with measured dose which was found within the accepted tolerance of classical in vivo dosimetry (SD of 3.5%).
Goldenberg et al. did a study to determine lung correction factors under actual treatment conditions in eight patients with proven squamous cell carcinoma of middle-third of the esophagus who were treated with cobalt 60 teletherapy unit by rotational therapy. They compared the estimated doses obtained through transit dose measurements by IC, with those obtained by direct measurements (temperature corrected) using intraluminal dosimeter in the same patient, the results of which were found to be within 3%.
In a recent work by Putha et al., estimations of in vivo midplane dose (in pelvic malignancies) were carried out through St measured by farmer type ion chamber with build up cap mounted along the beam central axis kept at the EPID level.
In the current study, an attempt has been made to verify the accuracy of computerized TPS dose calculation algorithm in heterogeneous media since the patient treatment sites involved in this study are surrounded by viscera of variable electron densities. The present work highlights a simple and elegant method of fabricating a thorax phantom with the above attributes which can be used for validating the accuracy of TPS in such a setting and also for delivery of radiation dose to a target lodged amid tissues of variable electron densities. Results from a small population of patients confirmed accuracy levels similar to those published in literature.,,,
| > Conclusion|| |
In vivo midplane dosimetry is potentially useful in settings where the region of treatment is adjacent to large tissue inhomogeneity owing to variable electron density of the organs concerned. The methodology implemented in this study carries with it a further advantage in view of its simplicity and easy implementability a clinic as the detector is placed at a location distant from the patient for all gantry angles during transit measurements, thereby negating potential pitfalls in position, reproducibility, or calibration. Repeated measurements on separate treatment sessions allow the opportunity to develop new strategies for validation of data by appropriate statistics. This method estimates the true path length of the beam in a real patient to check the validity of delivered dose, thus complementing the dose calculation and display from the treatment planning algorithm. The method mentioned in the paper is applicable only when patient having isocenter at the middle of water equivalent thickness for anteroposterior-posteroanterior and lateral fields.
It must be stressed upon, however, that this method should not be construed as a substitute for rigorous pretreatment QA but rather as an adjuvant mode of tracking dose delivery and detecting such errors as may be to the detriment of the patient. Furthermore, this study also attempts to address a potential difficulty in setups where sophisticated EPID-based methods of dosimetry through transmission measurements may not be available. Future work in this direction may assist in guiding adaptive RT by evaluating interfractional variations in dose delivery secondary to edema, weight loss, or similar pathophysiological alterations or even variability in machine output parameters which can be subsequently addressed and corrected.
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7], [Figure 8]
[Table 1], [Table 2], [Table 3], [Table 4]