Clinical use of electronic portal imaging

Impact of electronic portal image quality on Elekta AQUA® collimator isocenter

PURPOSE

The collimator radiation isocenter position determined in AQUA^® v3.0 [Elekta AB, Stockholm, Sweden] software test "MLC Leaf and Jaw Position" was independently validated using an in-house MATLAB [Natick, MA: The MathWorks Inc.] script.

METHODS

The AQUA test determines radiation isocenter using the mean field center of nine 4 cm × 4 cm electronic portal imager (EPID) exposures at equidistant collimator angles. Impact of EPID image quality on AQUA reported isocenter for thirteen Elekta linear accelerators with Agility MLC heads were evaluated.

RESULTS

Of the thirteen, three had visually and quantitatively identifiable artifacts. For the ten good EPID's there was a systematic 0.25 mm offset of the MATLAB calculated mean field center relative to AQUA in the X-axis and Y-axis. This corresponds to one image pixel and was found to be due to differences in software co-ordinate convention. After subtracting this offset there was no significant difference in AQUA and MATLAB calculated isocenter.

CONCLUSIONS

For the three machines with poor image quality there was a demonstrated variation in AQUA calculated field center and therefore radiation isocenter relative to MATLAB. Restricting the region of interest (ROI) in AQUA software to only the irradiated section of the EPID brought AQUA and MATLAB result for these three machines into agreement.

Assessing setup errors and shifting margins for planning target volume in head, neck, and breast cancer

Accurately calculating setup errors is crucial in ensuring quality assurance for patients undergoing radiation therapy treatment. This cross-sectional study aimed to determine the systematic, random, and planning target volume (PTV) margin errors for patients with head and neck cancer (n=48) and breast cancer (n=50). The treatment setup was performed using electronic portal imaging (EPIDs) and irradiated using Elekta linac. The errors were calculated using the van Herk formula. The systematic error for the head and neck was 0.89, 0.43, and 1.49 mm on the x, y, and z-axis, respectively, and 0.39, 0.74, 0.38 for the breast cases. The random error was 0.82, 0.68, 0.94 mm for the head and neck and 0.66, 0.72, 0.79 mm for the breast. The PTV margin shifting error for the head and neck were 2.79, 1.55, and 4.38 mm, while it was 1.43, 2.35, and 1.50 mm for the breast. The setup errors varied according to the tumor location. The study highlights the potential benefits of using EPIDs for reducing uncertainties in setup verification procedures.

In-vivo dose measurements with MOSFET dosimeters during MV portal imaging

Background

The purpose of this study was to investigate the feasibility of MOSFET dosimeter in measuring eye dose during 2D MV portal imaging for setup verification in radiotherapy.

Materials and methods

The in-vivo dose measurements were performed by placing the dosimeters over the eyes of 30 brain patients during the acquisition of portal images in linear accelerator by delivering 1 MU with the field sizes of 10 × 10 cm² and 15 × 15 cm².

Results

The mean doses received by the left and right eyes of 10 out of 30 patients when both eyes were completely inside the anterior portal field were found to be 2.56 ± 0.2 cGy and 2.75 ± 0.2, respectively. Similarly, for next 10 patients out of the same 30 patients the mean doses to left and right eyes when both eyes were completely out of the anterior portal fields were found to be 0.13 ± 0.02 cGy and 0.17 ± 0.02 cGy, respectively. The mean doses to ipsilateral and contralateral eye for the last 10 patients when one eye was inside the anterior portal field were found to be 3.28 ± 0.2 cGy and 0.36 ± 0.1 cGy, respectively.

Conclusion

The promising results obtained during 2D MV portal imaging using MOSFET have shown that this dosimeter is well suitable for assessing low doses during imaging thereby enabling to optimize the imaging procedure using the dosimetric data obtained. In addition, the documentation of the dose received by the patient during imaging procedure is possible with the help of an in-built software in conjunction with the MOSFET reader module.

Paediatric image-guided radiation therapy: determining and evaluating appropriate kilovoltage planar exposure factors for the Varian on-board imager

INTRODUCTION

Kilovoltage (kV) orthogonal imaging is commonly used for image-guided radiation therapy (IGRT) in paediatrics. Paediatrics have an increased sensitivity to radiation. Exposure factors need to be optimised so that imaging dose is kept as low as reasonably achievable (ALARA).

METHODS

A table of low-dose IGRT radiographic exposure factors for paediatric IGRT was determined through a phantom study. Four anatomical sites, head and neck, thorax, abdomen and pelvis, were investigated. The table was evaluated against standard manufacturer pre-sets. Dose was evaluated in terms of system-reported entrance surface air kerma (ESAK). Qualified participants volunteered to perform offline image matching in a further phantom study, recording misalignments detected and providing subjective assessments of image quality using an electronic survey tool. A statistical comparison of matching accuracy was conducted.

RESULTS

Twelve radiation therapists or radiation oncologists completed the image matching task and survey. The low-dose exposure table reduced imaging dose by 20-94% compared to manufacturer pre-sets. No significant difference was observed in the accuracy of image matching (head and neck P = 0.82, thorax P = 0.15, abdomen P = 0.33, pelvis P = 0.59). Participant image exposure preference was largely equivocal.

CONCLUSIONS

Optimising radiographic exposures in paediatric IGRT is feasible, logical and therefore reasonably achievable. Implementation of the low-dose exposure table presented in this study should be considered by paediatric radiotherapy departments wishing to image gently without compromising the potential to detect set up errors. Further study using a contrast detail phantom and contrast to noise image analysis software is recommended.

Use of artificial neural network for pretreatment verification of intensity modulation radiation therapy fields

OBJECTIVE

The accuracy of dose delivery for intensity modulated radiotherapy (IMRT) treatments should be determined by an accurate quality assurance procedure. In this work, we used artificial neural networks (ANNs) as an application for the pre-treatment dose verification of IMRT fields based two-dimensional-fluence maps acquired by an electronic portal imaging device (EPID).

METHODS

The ANN must be trained and validated before use for the pretreatment dose verification. Hence, 60 EPID fluence maps of the anteroposterior prostate and nasopharynx IMRT fields were used as an input for the ANN (feed forward type), and a dose map of those fluence maps that were acquired by two-dimensional Array Seven29TM as an output for the ANN.

RESULTS

After the training and validation of the neural network, the analysis of 20 IMRT anteroposterior fields showed excellent agreement between the ANN output and the dose map predicted by the treatment planning system. The average overall global and local γ field pass rate was greater than 90% for the prostate and nasopharynx fields, with the 2 mm/3% criteria.

CONCLUSION

The results indicated that the ANN can be used as a fast and powerful tool for pretreatment dose verification, based on an EPID fluence map.

ADVANCES IN KNOWLEDGE

In this study, ANN is proposed for EPID based dose validation of IMRT fields. The proposed method has good accuracy and high speed in response to problems. Neural network show to be low price and precise method for IMRT fields verification.

Contrast Enhancement for Portal Imaging in Nanoparticle-Enhanced Radiotherapy: A Monte Carlo Phantom Evaluation Using Flattening-Filter-Free Photon Beams

Our team evaluated contrast enhancement for portal imaging using Monte Carlo simulation in nanoparticle-enhanced radiotherapy. Dependencies of percentage contrast enhancement on flattening-filter (FF) and flattening-filter-free (FFF) photon beams were determined by varying the nanoparticle material (gold, platinum, iodine, silver, iron oxide), nanoparticle concentration (3–40 mg/mL) and photon beam energy (6 and 10 MV). Phase-space files and energy spectra of the 6 MV FF, 6 MV FFF, 10 MV FF and 10 MV FFF photon beams were generated based on a Varian TrueBeam linear accelerator. We found that gold and platinum nanoparticles (NP) produced the highest contrast enhancement for portal imaging, compared to other NP with lower atomic numbers. The maximum percentage contrast enhancements for the gold and platinum NP were 18.9% and 18.5% with a concentration equal to 40 mg/mL. The contrast enhancement was also found to increase with the nanoparticle concentration. The maximum rate of increase of contrast enhancement for the gold NP was equal to 0.29%/mg/mL. Using the 6 MV photon beams, the maximum contrast enhancements for the gold NP were 79% (FF) and 78% (FFF) higher than those using the 10 MV beams. For the FFF beams, the maximum contrast enhancements for the gold NP were 53.6% (6 MV) and 53.8% (10 MV) higher than those using the FF beams. It is concluded that contrast enhancement for portal imaging can be increased when a higher atomic number of NP, higher nanoparticle concentration, lower photon beam energy and no flattening filter of photon beam are used in nanoparticle-enhanced radiotherapy.

On the direct acquisition of beam's-eye-view images in MRI for integration with external beam radiotherapy

The recent interest in the integration of external beam radiotherapy with a magnetic resonance (MR) imaging unit offers the potential for real-time adaptive tumour tracking during radiation treatment. The tracking of large tumours which follow a rapid trajectory may best be served by the generation of a projection image from the perspective of the beam source, or 'beam's eye view' (BEV). This type of image projection represents the path of the radiation beam, thus enabling rapid compensations for target translations, rotations and deformations, as well time-dependent critical structure avoidance. MR units have been traditionally incapable of this type of imaging except through lengthy 3D acquisitions and ray tracing procedures. This work investigates some changes to the traditional MR scanner architecture that would permit the direct acquisition of a BEV image suitable for integration with external beam radiotherapy. Based on the theory presented in this work, a phantom was imaged with nonlinear encoding-gradient field patterns to demonstrate the technique. The phantom was constructed with agarose gel tubes spaced two cm apart at their base and oriented to converge towards an imaginary beam source 100 cm away. A corresponding virtual phantom was also created and subjected to the same encoding technique as in the physical demonstration, allowing the method to be tested without hardware limitations. The experimentally acquired and simulated images indicate the feasibility of the technique, showing a substantial amount of blur reduction in a diverging phantom compared to the conventional imaging geometry, particularly with the nonlinear gradients ideally implemented. The theory is developed to demonstrate that the method can be adapted in a number of different configurations to accommodate all proposed integration schemes for MR units and radiotherapy sources. Depending on the configuration, the implementation of this technique will require between two and four additional nonlinear encoding coils.

Normal tissue doses from MV image-guided radiation therapy (IGRT) using orthogonal MV and MV-CBCT

Purpose

The aim of this study was to measure and compare the mega‐voltage imaging dose from the Halcyon medical linear accelerator (Varian Medical Systems) with measured imaging doses with the dose calculated by Eclipse treatment planning system.

Methods

An anthropomorphic thorax phantom was imaged using all imaging techniques available with the Halcyon linac — MV cone‐beam computed tomography (MV‐CBCT) and orthogonal anterior‐posterior/lateral pairs (MV‐MV), both with high‐quality and low‐dose modes. In total, 54 imaging technique, isocenter position, and field size combinations were evaluated. The imaging doses delivered to 11 points in the phantom (in‐target and extra‐target) were measured using an ion chamber, and compared with the imaging doses calculated using Eclipse.

Results

For high‐quality MV‐MV mode, the mean extra‐target doses delivered to the heart, left lung, right lung and spine were 1.18, 1.64, 0.80, and 1.11 cGy per fraction, respectively. The corresponding mean in‐target doses were 3.36, 3.72, 2.61, and 2.69 cGy per fraction, respectively. For MV‐MV technique, the extra‐target imaging dose had greater variation and dependency on imaging field size than did the in‐target dose. Compared to MV‐MV technique, the imaging dose from MV‐CBCT was less sensitive to the location of the organ relative to the treatment field. For high‐quality MV‐CBCT mode, the mean imaging doses to the heart, left lung, right lung, and spine were 8.45, 7.16, 7.19, and 6.51 cGy per fraction, respectively. For both MV‐MV and MV‐CBCT techniques, the low‐dose mode resulted in an imaging dose about half of that in high‐quality mode.

Conclusion

The in‐target doses due to MV imaging using the Halcyon ranged from 0.59 to 9.75 cGy, depending on the choice of imaging technique. Extra‐target doses from MV‐MV technique ranged from 0 to 2.54 cGy. The MV imaging dose was accurately calculated by Eclipse, with maximum differences less than 0.5% of a typical treatment dose (assuming a 60 Gy prescription). Therefore, the cumulative imaging and treatment plan dose distribution can be expected to accurately reflect the actual dose.

Characterization of the a-Si EPID in the unity MR-linac for dosimetric applications

Electronic portal imaging devices (EPIDs) are frequently used in external beam radiation therapy for dose verification purposes. The aim of this study was to investigate the dose-response characteristics of the EPID in the Unity MR-linac (Elekta AB, Stockholm, Sweden) relevant for dosimetric applications under clinical conditions. EPID images and ionization chamber (IC) measurements were used to study the effects of the magnetic field, the scatter generated in the MR housing reaching the EPID, and inhomogeneous attenuation from the MR housing. Dose linearity and dose rate dependencies were also determined. The magnetic field strength at EPID level did not exceed 10 mT, and dose linearity and dose rate dependencies proved to be comparable to that on a conventional linac. Profiles of fields, delivered with and without the magnetic field, were indistinguishable. The EPID center had an offset of 5.6 cm in the longitudinal direction, compared to the beam central axis, meaning that large fields in this direction will partially fall outside the detector area and not be suitable for verification. Beam attenuation by the MRI scanner and the table is gantry angle dependent, presenting a minimum attenuation of 67% relative to the 90° measurement. Repeatability, observed over two months, was within 0.5% (1 SD). In order to use the EPID for dosimetric applications in the MR-linac, challenges related to the EPID position, scatter from the MR housing, and the inhomogeneous, gantry angle-dependent attenuation of the beam will need to be solved.

EPID-based dosimetry to verify IMRT planar dose distribution for the aS1200 EPID and FFF beams

We proposed to perform a basic dosimetry commissioning on a new imager system, the Varian aS1200 electronic portal imaging device (EPID) and TrueBeam 2.0 linear accelerator for flattened (FF) and flattening filter‐free (FFF) beams, then to develop an image‐based quality assurance (QA) model for verification of the system delivery accuracy for intensity‐modulated radiation therapy (IMRT) treatments. For dosimetry testing, linearity of dose response with MU, imager lag, and effectiveness of backscatter shielding were investigated. Then, an image‐based model was developed to convert images to planar dose onto a virtual water phantom. The model parameters were identified using energy fluence of the Acuros treatment planning system (TPS) and, reference dose profiles and output factors measured at depths of 5, 10, 15, and 20 cm in water phantom for square fields. To validate the model, its calculated dose was compared to measured dose from MapCHECK 2 diode arrays for 36 IMRT fields at 10 cm depth delivered with 6X, 6XFFF, 10X, and 10XFFF energies. An in‐house gamma function was used to compare planar doses pixel‐by‐pixel. Finally, the method was applied to the same IMRT fields to verify their pretreatment delivery dose compared with Eclipse TPS dose. For the EPID commissioning, dose linearity was within 0.4% above 5 MU and ∼1% above 2 MU, measured lag was smaller than the previous EPIDs, and profile symmetry was improved. The model was validated with mean gamma pass rates (standard deviation) of 99.0% (0.4%), 99.5% (0.6%), 99.3% (0.4%), and 98.0% (0.8%) at 3%/3 mm for respectively 6X, 6XFFF, 10X, and 10XFFF beams. Using the same comparison criteria, the beam deliveries were verified with mean pass rates of 100% (0.0%), 99.6% (0.3%), 99.9% (0.1%), and 98.7% (1.4%). Improvements were observed in dosimetric response of the aS1200 imager compared to previous EPID models, and the model was successfully developed for the new system and delivery energies of 6 and 10 MV, FF, and FFF modes.

PACS number(s): 87.53.Oq, 87.53.Xd

Four dimensional digital tomosynthesis using on-board imager for the verification of respiratory motion

Purpose

To evaluate respiratory motion of a patient by generating four-dimensional digital tomosynthesis (4D DTS), extracting respiratory signal from patients' on-board projection data, and ensuring the feasibility of 4D DTS as a localization tool for the targets which have respiratory movement.

Methods and Materials

Four patients with lung and liver cancer were included to verify the feasibility of 4D-DTS with an on-board imager. CBCT acquisition (650–670 projections) was used to reconstruct 4D DTS images and the breath signal of the patients was generated by extracting the motion of diaphragm during data acquisition. Based on the extracted signal, the projection data was divided into four phases: peak-exhale phase, mid-inhale phase, peak-inhale phase, and mid-exhale phase. The binned projection data was then used to generate 4D DTS, where the total scan angle was assigned as ±22.5° from rotation center, centered on 0° and 180° for coronal “half-fan” 4D DTS, and 90° and 270° for sagittal “half-fan” 4D DTS. The result was then compared with 4D CBCT which we have also generated with the same phase distribution.

Results

The motion of the diaphragm was evident from the 4D DTS results for peak-exhale, mid-inhale, peak-inhale and mid-exhale phase assignment which was absent in 3D DTS. Compared to the result of 4D CBCT, the view aliasing effect due to arbitrary angle reconstruction was less severe. In addition, the severity of metal artifacts, the image distortion due to presence of metal, was less than that of the 4D CBCT results.

Conclusion

We have implemented on-board 4D DTS on patients data to visualize the movement of anatomy due to respiratory motion. The results indicate that 4D-DTS could be a promising alternative to 4D CBCT for acquiring the respiratory motion of internal organs just prior to radiotherapy treatment.

Image guidance in radiation therapy: techniques and applications

In modern day radiotherapy, the emphasis on reduction on volume exposed to high radiotherapy doses, improving treatment precision as well as reducing radiation-related normal tissue toxicity has increased, and thus there is greater importance given to accurate position verification and correction before delivering radiotherapy. At present, several techniques that accomplish these goals impeccably have been developed, though all of them have their limitations. There is no single method available that eliminates treatment-related uncertainties without considerably adding to the cost. However, delivering “high precision radiotherapy” without periodic image guidance would do more harm than treating large volumes to compensate for setup errors. In the present review, we discuss the concept of image guidance in radiotherapy, the current techniques available, and their expected benefits and pitfalls.

Assessing the Efficiency and Consistency of Daily Image-guided Radiation Therapy in a Modern Radiotherapy Centre

BACKGROUND

Patients at Radiation Oncology Queensland Toowoomba are treated using the assistance of daily image-guided radiation therapy (IGRT). Each patient's daily setup is exposed to a number of variables. This study investigates the effect that these variables have on the total time taken to analyse field placement and the total time taken for treatment, as well accessing setup error across a variety of treatment types.

METHODS

This is a retrospective study of 80 patients across a variety of treatment sites where daily IGRT was undertaken using kilovoltage and megavoltage orthogonal images. Variables investigated include the treatment type, the imaging modality used, and the setup error of each session. Statistical analysis was then performed on the data.

RESULTS

Patients being treated in the thoracic region had the greatest random setup error. The mean matching times were also longer for chest patients (197 seconds), whereas there were minimal differences in times regarding modality. Treatment times were longest for head and neck variables (399-405 seconds).

CONCLUSIONS

Pretreatment daily IGRT is beneficial to all patients and can be performed efficiently. Pelvic variables were the strongest performer, with fiducial markers providing the most consistent and rapid match times. Chest variables were the worst performer specifically regarding random setup error and match times indicating future work required on chest stabilization.

Evaluation of imaging performance of megavoltage cone-beam CT over an extended period

A linear accelerator vendor and the AAPM TG-142 report propose that quality assurance testing for image-guided devices such megavoltage cone-beam CT (MV-CBCT) be conducted on a monthly basis. In clinical settings, however, unpredictable errors such as image artifacts can occur even when quality assurance results performed at this frequency are within tolerance limits. Here, we evaluated the imaging performance of MV-CBCT on a weekly basis for ∼ 1 year using a Siemens ONCOR machine with a 6-MV X-ray and an image-quality phantom. Image acquisition was undertaken using 15 monitor units. Geometric distortion was evaluated with beads evenly distributed in the phantom, and the results were compared with the expected position in three dimensions. Image-quality characteristics of the system were measured and assessed qualitatively and quantitatively, including image noise and uniformity, low-contrast resolution, high-contrast resolution and spatial resolution. All evaluations were performed 100 times each. For geometric distortion, deviation between the measured and expected values was within the tolerance limit of 2 mm. However, a subtle systematic error was found which meant that the phantom was rotated slightly in a clockwise manner, possibly due to geometry calibration of the MV-CBCT system. Regarding image noise and uniformity, two incidents over tolerance occurred in 100 measurements. This phenomenon disappeared after dose calibration of beam output for MV-CBCT. In contrast, all results for low-contrast resolution, high-contrast resolution and spatial resolution were within their respective tolerances.

Image quality improvements of electronic portal imaging devices by multi-level gain calibration and temperature correction

Amorphous silicon (aSi:H) flat panel detectors are prevalent in radiotherapy for megavoltage imaging tasks. Any clinical and dosimetrical application requires a well-defined dose response of the system to achieve meaningful results. Due to radiation damages, panels deteriorate and the linearity of pixel response to dose as well as the stability with regard to changing operating temperatures get worse with time. Using a single level gain correction can lead to an error of about 23% when irradiating a flood field image with 100 MU min(-1) on an old detector. A multi-level gain (MLG) correction is introduced, emending the nonlinearities and subpanel-related artifacts caused by insufficient radiation hardness of amplifiers in the read-out electronics. With rising temperature, offset values typically increase (up to 300 gray values) while the response at higher dose values per frame remain constant for a majority of pixels. To account for temperature-related image artifacts, two additional temperature correction methods have been developed. MLG in combination with temperature corrections can re-establish the aSi:H image quality to the performance required by reliable medical verification tools. Furthermore, the life span and recalibration intervals of these costly devices can be prolonged decisively.

Interobserver variability of radiation therapists aligning to fiducial markers for prostate radiation therapy

INTRODUCTION

As the use of fiducial markers (FMs) for the localisation of the prostate during external beam radiation therapy (EBRT) has become part of routine practice, radiation therapists (RTs) have become increasingly responsible for online image interpretation. The aim of this investigation was to quantify the limits of agreement (LoA) between RTs when localising to FMs with orthogonal kilovoltage (kV) imaging.

METHODS

Six patients receiving prostate EBRT utilising FMs were included in this study. Treatment localisation was performed using kV imaging prior to each fraction. Online stereoscopic assessment of FMs, performed by the treating RTs, was compared with the offline assessment by three RTs. Observer agreement was determined by pairwise Bland-Altman analysis.

RESULTS

Stereoscopic analysis of 225 image pairs was performed online at the time of treatment, and offline by three RT observers. Eighteen pairwise Bland-Altman analyses were completed to assess the level of agreement between observers. Localisation by RTs was found to be within clinically acceptable 95% LoAs.

CONCLUSIONS

Small differences between RTs, in both the online and offline setting, were found to be within clinically acceptable limits. RTs were able to make consistent and reliable judgements when matching FMs on planar kV imaging.

Reliable detection of fluence anomalies in EPID-based IMRT pretreatment quality assurance using pixel intensity deviations

PURPOSE

This work uses repeat images of intensity modulated radiation therapy (IMRT) fields to quantify fluence anomalies (i.e., delivery errors) that can be reliably detected in electronic portal images used for IMRT pretreatment quality assurance.

METHODS

Repeat images of 11 clinical IMRT fields are acquired on a Varian Trilogy linear accelerator at energies of 6 MV and 18 MV. Acquired images are corrected for output variations and registered to minimize the impact of linear accelerator and electronic portal imaging device (EPID) positioning deviations. Detection studies are performed in which rectangular anomalies of various sizes are inserted into the images. The performance of detection strategies based on pixel intensity deviations (PIDs) and gamma indices is evaluated using receiver operating characteristic analysis.

RESULTS

Residual differences between registered images are due to interfraction positional deviations of jaws and multileaf collimator leaves, plus imager noise. Positional deviations produce large intensity differences that degrade anomaly detection. Gradient effects are suppressed in PIDs using gradient scaling. Background noise is suppressed using median filtering. In the majority of images, PID-based detection strategies can reliably detect fluence anomalies of ≥5% in ∼1 mm(2) areas and ≥2% in ∼20 mm(2) areas.

CONCLUSIONS

The ability to detect small dose differences (≤2%) depends strongly on the level of background noise. This in turn depends on the accuracy of image registration, the quality of the reference image, and field properties. The longer term aim of this work is to develop accurate and reliable methods of detecting IMRT delivery errors and variations. The ability to resolve small anomalies will allow the accuracy of advanced treatment techniques, such as image guided, adaptive, and arc therapies, to be quantified.

Image-guided radiotherapy: has it influenced patient outcomes?

Cancer control and toxicity outcomes are the mainstay of evidence-based medicine in radiation oncology. However, radiotherapy is an intricate therapy involving numerous processes that need to be executed appropriately in order for the therapy to be delivered successfully. The use of image-guided radiation therapy (IGRT), referring to imaging occurring in the radiation therapy room with per-patient adjustments, can increase the agreement between the planned and the actual dose delivered. However, the absence of direct evidence regarding the clinical benefit of IGRT has been a criticism. Here, we dissect the role of IGRT in the radiotherapy (RT) process and emphasize its role in improving the quality of the intervention. The literature is reviewed to collect evidence that supports that higher-quality dose delivery enabled by IGRT results in higher clinical control rates, reduced toxicity, and new treatment options for patients that previously were without viable options.

Dose to craniofacial region through portal imaging of pediatric brain tumors

The purpose of this study was to determine dose to the planning target volume (PTV) and organs at risk (OARs) from portal imaging (PI) of the craniofacial region in pediatric brain tumor patients treated with intensity‐modulated radiation therapy (IMRT). Twenty pediatric brain tumor patients were retrospectively studied. Each received portal imaging of treatment fields and orthogonal setup fields in the craniofacial region. The number of PI and monitor units used for PI were documented for each patient. Dose distributions and dose‐volume histograms were generated to quantify the maximum, minimum, and mean dose to the PTV, and the mean dose to OARs through PI acquisition. The doses resulting from PI are reported as percentage of prescribed dose. The average maximum, minimum, and mean doses to PTV from PI were 2.9±0.7%, 2.2±1.0%, and 2.5±0.7%, respectively. The mean dose to the OARs from PI were brainstem 2.8±1.1%, optic nerves/chiasm 2.6±0.9%, cochlea 2.6±0.9%, hypothalamus/pituitary 2.4±0.6%, temporal lobes 2.3±0.6%, thyroid 1.6±0.8%, and eyes 2.6±0.9%. The mean number of portal images and the mean number of PI monitor units per patient were 58.8 and 173.3, respectively. The dose from PI while treating pediatric brain tumors using IMRT is significant (2%–3% of the prescribed dose). This may result in exceeding the tolerance limit of many critical structures and lead to unwanted late complications and secondary malignancies. Dose contributions from PI should be considered in the final documented dose. Attempts must be made in PI practices to lower the imaging dose when feasible.

PACS numbers: 87.55ne, 87.55Qr

Correction of systematic set-up error in breast and head and neck irradiation through a no-action level (NAL) protocol

PURPOSE

To quantify systematic and random patient set-up errors in breast and head and neck conventional irradiation and to evaluate a no-action level (NAL) protocol for systematic set-up error off-line correction in head and neck cancer and breast cancer patients.

MATERIAL AND METHODS

Verification electronic portal images of orthogonal set-up fields were obtained daily for the initial four consecutive fractions for 20 patients treated for breast cancer and for 20 head and neck cancer patients. The calculated systematic error was used to shift the isocentre accordingly on the fifth treatment day. From then until the end of the treatment course, pair orthogonal portal images of set-up fields were obtained weekly. To assess the impact of the protocol, pre- and post-correction systematic errors were compared and PTV margins were estimated before and after correction using published margin recipes.

RESULTS

Population systematic set-up error decreased in the breast cancer patient group after the implementation of NAL protocol from 4.0 to 1.7 mm on the x-axis, from 4.7 to 2.1 mm on the y-axis and from 2.8 to 0.9 mm on the z axis. The percentage of patients with individual systematic set-up error reduction was 80%, 90% and 80% on the x-, y and z-axes respectively. Population systematic set-up error decreased also in the head and neck cancer patient group from 2.3 to 1.1 mm on the x-axis, from 1.6 to 1.4 mm on the y-axis and from 1.7 to 0.7 mm on the z-axis. The percentage of patients with individual systematic set-up error reduction was 70%, 65% and 85% on the x-, y- and z-axes respectively. Margin reduction achievable with NAL protocol implementation on the x-, y- and z-axes was 6.3, 7.2 and 4.8 mm for breast cancer patients and 3.3, 0.6 and 2.8 mm for head and neck cancer patients.

CONCLUSION

NAL off-line protocol is useful for systematic set-up error correction and PTV margin reduction in conventional breast and head and neck irradiation.

Set-up errors analyses in IMRT treatments for nasopharyngeal carcinoma to evaluate time trends, PTV and PRV margins

INTRODUCTION

the aims of this study were to analyze the systematic and random interfractional set-up errors during Intensity Modulated Radiation Therapy (IMRT) in 20 consecutive nasopharyngeal carcinoma (NPC) patients by means of Electronic Portal Images Device (EPID), to define appropriate Planning Target Volume (PTV) and Planning Risk Volume (PRV) margins, as well as to investigate set-up displacement trend as a function of time during fractionated RT course.

MATERIAL AND METHODS

before EPID clinical implementation, an anthropomorphic phantom was shifted intentionally 5 mm to all directions and the EPIs were compared with the digitally reconstructed radiographs (DRRs) to test the system's capability to recognize displacements observed in clinical studies. Then, 578 clinical images were analyzed with a mean of 29 images for each patient.

RESULTS

phantom data showed that the system was able to correct shifts with an accuracy of 1 mm. As regards clinical data, the estimated population systematic errors were 1.3 mm for left-right (L-R), 1 mm for superior-inferior (S-I) and 1.1 mm for anterior-posterior (A-P) directions, respectively. Population random errors were 1.3 mm, 1.5 mm and 1.3 mm for L-R, S-I and A-P directions, respectively. PTV margin was at least 3.4, 3 and 3.2 mm for L-R, S-I and A-P direction, respectively. PRV margins for brainstem and spinal cord were 2.3, 2 and 2.1 mm and 3.8, 3.5 and 3.2 mm for L-R, A-P and S-I directions, respectively. Set-up error displacements showed no significant changes as the therapy progressed (p>0.05), although displacements >3 mm were found more frequently when severe weight loss or tumor nodal shrinkage occurred.

DISCUSSION

these results enable us to choose margins that guarantee with sufficient accuracy the coverage of PTVs and organs at risk sparing. Collected data confirmed the need for a strict check of patient position reproducibility in case of anatomical changes.

Characterization of a real-time surface image-guided stereotactic positioning system

PURPOSE

The AlignRT3C system is an image-guided stereotactic positioning system (IGSPS) that provides real-time target localization. This study involves the first use of this system with three camera pods. The authors have evaluated its localization accuracy and tracking ability using a cone-beam computed tomography (CBCT) system and an optical tracking system in a clinical setting.

METHODS

A modified Rando head-and-neck phantom and five patients receiving intracranial stereotactic radiotherapy (SRT) were used to evaluate the calibration, registration, and position-tracking accuracies of the AlignRT3C system and to study surface reconstruction uncertainties, including the effects due to interfractional and intrafractional motion, skin tone, room light level, camera temperature, and image registration region of interest selection. System accuracy was validated through comparison with the Elekta kV CBCT system (XVI) and the Varian frameless SonArray (FSA) optical tracking system. Surface-image data sets were acquired with the AlignRT3C daily for the evaluation of pretreatment and interfractional and intrafractional motion for each patient. Results for two different reference image sets, planning CT surface contours (CTS) and previously recorded AlignRT3C optical surface images (ARTS), are reported.

RESULTS

The system origin displacements for the AlignRT3C and XVI systems agreed to within 1.3 mm and 0.7 degrees. Similar results were seen for AlignRT3C vs FSA. For the phantom displacements having couch angles of 0 degrees, those that utilized ART_S references resulted in a mean difference of 0.9 mm/0.4 degrees with respect to XVI and 0.3 mm/0.2 degrees with respect to FSA. For phantom displacements of more than +/- 10 mm and +/- 3 degrees, the maximum discrepancies between AlignRT and the XVI and FSA systems were 3.0 and 0.4 mm, respectively. For couch angles up to +/- 90 degrees, the mean (max.) difference between the AlignRT3C and FSA was 1.2 (2.3) mm/0.7 degrees (1.2 degrees). For all tests, the mean registration errors obtained using the CT_S references were approximately 1.3 mm/1.0 degrees larger than those obtained using the ART_S references. For the patient study, the mean differences in the pretreatment displacements were 0.3 mm/0.2 degrees between the AlignRT3C and XVI systems and 1.3 mm/1 degrees between the FSA and XVI systems. For noncoplanar treatments, interfractional motion displacements obtained using the ART_S and CT_S references resulted in 90th percentile differences within 2.1 mm/0.8 degrees and 3.3 mm/0.3 degrees, respectively, compared to the FSA system. Intrafractional displacements that were tracked for a maximum of 14 min were within 1 mm/1 degrees of those obtained with the FSA system. Uncertainties introduced by the bite-tray were as high as 3 mm/2 degrees for one patient. The combination of gantry, aSi detector panel, and x-ray tube blockage effects during the CBCT acquisition resulted in a registration error of approximately 3 mm. No skin-tone or surface deformation effects were seen with the limited patient sample.

CONCLUSIONS

AlignRT3C can be used as a nonionizing IGSPS with accuracy comparable to current image/marker-based systems. IGSPS and CBCT can be combined for high-precision positioning without the need for patient-attached localization devices.

Megavoltage versus kilovoltage image guidance for efficiency and accuracy in head and neck IMRT

Accurate patient positioning is vitally important in intensity modulated radiation therapy (IMRT) for head and neck (H&N) cancer. The introduction of kilovoltage (kV) on-board imaging (OBI) at our centre was anticipated to improve the accuracy and efficiency of H&N IMRT patient position verification over traditional megavoltage (MV) electronic portal imaging (EPI). This study compares these imaging systems with a phantom accuracy study and retrospective analysis of imaging workload in H&N IMRT at our centre. Six therapists performed online evaluation of phantom images, and residual positional errors for each system were recorded. The largest residual error was 1 mm for OBI and 3 mm for EPI. The estimated improvement in residual error in OBI over EPI was 0.57 mm (95% confidence interval 0.33–0.81 mm), suggesting treatment staff would be better able to detect set-up deviations with OBI. Electronic treatment records of 20 H&N IMRT patients (10 verified daily with MV EPI and 10 with kV OBI) were analysed. Mean imaging session duration was 9.51 min for EPI and 9.76 min for OBI. Analysis found no evidence of an effect on duration due to the imaging system used for this subset of patients (p = 0.664).

Clinical experience with image-guided radiotherapy in an accelerated partial breast intensity-modulated radiotherapy protocol

PURPOSE

To explore the feasibility of fiducial markers for the use of image-guided radiotherapy (IGRT) in an accelerated partial breast intensity modulated radiotherapy protocol.

METHODS AND MATERIALS

Nineteen patients consented to an institutional review board approved protocol of accelerated partial breast intensity-modulated radiotherapy with fiducial marker placement and treatment with IGRT. Patients (1 patient with bilateral breast cancer; 20 total breasts) underwent ultrasound guided implantation of three 1.2- x 3-mm gold markers placed around the surgical cavity. For each patient, table shifts (inferior/superior, right/left lateral, and anterior/posterior) and minimum, maximum, mean error with standard deviation were recorded for each of the 10 BID treatments. The dose contribution of daily orthogonal films was also examined.

RESULTS

All IGRT patients underwent successful marker placement. In all, 200 IGRT treatment sessions were performed. The average vector displacement was 4 mm (range, 2-7 mm). The average superior/inferior shift was 2 mm (range, 0-5 mm), the average lateral shift was 2 mm (range, 1-4 mm), and the average anterior/posterior shift was 3 mm (range, 1 5 mm).

CONCLUSIONS

This study shows that the use of IGRT can be successfully used in an accelerated partial breast intensity-modulated radiotherapy protocol. The authors believe that this technique has increased daily treatment accuracy and permitted reduction in the margin added to the clinical target volume to form the planning target volume.

Clinical evaluation of positioning verification using digital tomosynthesis and bony anatomy and soft tissues for prostate image-guided radiotherapy

PURPOSE

To evaluate on-board digital tomosynthesis (DTS) for patient positioning vs. two-dimensional (2D) radiography and three-dimensional cone beam (CBCT).

METHODS AND MATERIALS

A total of 92 image sessions from 9 prostate cancer patients were analyzed. An on-board image set was registered to a corresponding reference image set. Four pairs of image sets were used: digitally reconstructed radiographs vs. on-board orthogonal paired radiographs for the 2D method, coronal-reference DTS vs. on-board coronal DTS for the coronal-DTS method, sagittal-reference DTS vs. on-board sagittal DTS for the sagittal-DTS method, and planning CT vs. CBCT for the CBCT method. The registration results were compared.

RESULTS

The systematic errors in all methods were <1 mm/1 degrees . When registering the bony anatomy, the mean vector difference was 0.21 +/- 0.11 cm between 2D and CBCT, 0.11 +/- 0.08 cm between CBCT and coronal DTS, and 0.14 +/- 0.07 cm between CBCT and sagittal DTS. The correlation between CBCT to DTS was stronger (coefficient = 0.92-0.95) than the correlation between 2D and CBCT or DTS (coefficient = 0.81-0.83). When registering the soft tissue, the mean vector difference was 0.18 +/- 0.11 cm between CBCT and coronal DTS and 0.29 +/- 0.17 cm between CBCT and sagittal DTS. The correlation coefficient of CBCT to sagittal DTS and to coronal DTS was 0.84 and 0.92, respectively.

CONCLUSION

DTS could provide equivalent results to CBCT when the bony anatomy is used as landmarks for prostate image-guided radiotherapy. For soft tissue-based positioning verification, coronal DTS produced equivalent results to CBCT, but sagittal DTS alone was insufficient. DTS could allow for comparable soft tissue-based target localization with faster scanning time and a lower imaging dose compared with CBCT.

Evaluation of targeting errors in ultrasound-assisted radiotherapy

A method for validating the start-to-end accuracy of a 3-D ultrasound (US)-based patient positioning system for radiotherapy is described. A radiosensitive polymer gel is used to record the actual dose delivered to a rigid phantom after being positioned using 3-D US guidance. Comparison of the delivered dose with the treatment plan allows accuracy of the entire radiotherapy treatment process, from simulation to 3-D US guidance, and finally delivery of radiation, to be evaluated. The 3-D US patient positioning system has a number of features for achieving high accuracy and reducing operator dependence. These include using tracked 3-D US scans of the target anatomy acquired using a dedicated 3-D ultrasound probe during both the simulation and treatment sessions, automatic 3-D US-to-US registration and use of infrared LED (IRED) markers of the optical position-sensing system for registering simulation computed tomography to US data. The mean target localization accuracy of this system was 2.5 mm for four target locations inside the phantom, compared with 1.6 mm obtained using the conventional patient positioning method of laser alignment. Because the phantom is rigid, this represents the best possible set-up accuracy of the system. Thus, these results suggest that 3-D US-based target localization is practically feasible and potentially capable of increasing the accuracy of patient positioning for radiotherapy in sites where day-to-day organ shifts are greater than 1 mm in magnitude.

Comparison of various online IGRT strategies: The benefits of online treatment plan re-optimization

PURPOSE

To compare the dosimetric differences of various online IGRT strategies and to predict potential benefits of online re-optimization techniques in prostate cancer radiation treatments.

MATERIALS AND METHODS

Nine prostate patients were recruited in this study. Each patient has one treatment planning CT images and 10-treatment day CT images. Five different online IGRT strategies were evaluated which include 3D conformal with bone alignment, 3D conformal re-planning via aperture changes, intensity modulated radiation treatment (IMRT) with bone alignment, IMRT with target alignment and IMRT daily re-optimization. Treatment planning and virtual treatment delivery were performed. The delivered doses were obtained using in-house deformable dose mapping software. The results were analyzed using equivalent uniform dose (EUD).

RESULTS

With the same margin, rectum and bladder doses in IMRT plans were about 10% and 5% less than those in CRT plans, respectively. Rectum and bladder doses were reduced as much as 20% if motion margin is reduced by 1cm. IMRT is more sensitive to organ motion. Large discrepancies of bladder and rectum doses were observed compared to the actual delivered dose with treatment plan predication. The therapeutic ratio can be improved by 14% and 25% for rectum and bladder, respectively, if IMRT online re-planning is employed compared to the IMRT bone alignment approach. The improvement of target alignment approach is similar with 11% and 21% dose reduction to rectum and bladder, respectively. However, underdosing in seminal vesicles was observed on certain patients.

CONCLUSIONS

Online treatment plan re-optimization may significantly improve therapeutic ratio in prostate cancer treatments mostly due to the reduction of PTV margin. However, for low risk patient with only prostate involved, online target alignment IMRT treatment would achieve similar results as online re-planning. For all IGRT approaches, the delivered organ-at-risk doses may be significantly different from treatment planning prediction.

A literature review of electronic portal imaging for radiotherapy dosimetry

Electronic portal imaging devices (EPIDs) have been the preferred tools for verification of patient positioning for radiotherapy in recent decades. Since EPID images contain dose information, many groups have investigated their use for radiotherapy dose measurement. With the introduction of the amorphous-silicon EPIDs, the interest in EPID dosimetry has been accelerated because of the favourable characteristics such as fast image acquisition, high resolution, digital format, and potential for in vivo measurements and 3D dose verification. As a result, the number of publications dealing with EPID dosimetry has increased considerably over the past approximately 15 years. The purpose of this paper was to review the information provided in these publications. Information available in the literature included dosimetric characteristics and calibration procedures of various types of EPIDs, strategies to use EPIDs for dose verification, clinical approaches to EPID dosimetry, ranging from point dose to full 3D dose distribution verification, and current clinical experience. Quality control of a linear accelerator, pre-treatment dose verification and in vivo dosimetry using EPIDs are now routinely used in a growing number of clinics. The use of EPIDs for dosimetry purposes has matured and is now a reliable and accurate dose verification method that can be used in a large number of situations. Methods to integrate 3D in vivo dosimetry and image-guided radiotherapy (IGRT) procedures, such as the use of kV or MV cone-beam CT, are under development. It has been shown that EPID dosimetry can play an integral role in the total chain of verification procedures that are implemented in a radiotherapy department. It provides a safety net for simple to advanced treatments, as well as a full account of the dose delivered. Despite these favourable characteristics and the vast range of publications on the subject, there is still a lack of commercially available solutions for EPID dosimetry. As strategies evolve and commercial products become available, EPID dosimetry has the potential to become an accurate and efficient means of large-scale patient-specific IMRT dose verification for any radiotherapy department.

ESTERR-PRO: a setup verification software system using electronic portal imaging

The purpose of the paper is to present and evaluate the performance of a new software-based registration system for patient setup verification, during radiotherapy, using electronic portal images. The estimation of setup errors, using the proposed system, can be accomplished by means of two alternate registration methods. (a) The portal image of the current fraction of the treatment is registered directly with the reference image (digitally reconstructed radiograph (DRR) or simulator image) using a modified manual technique. (b) The portal image of the current fraction of the treatment is registered with the portal image of the first fraction of the treatment (reference portal image) by applying a nearly automated technique based on self-organizing maps, whereas the reference portal has already been registered with a DRR or a simulator image. The proposed system was tested on phantom data and on data from six patients. The root mean square error (RMSE) of the setup estimates was 0.8 ± 0.3 (mean value ± standard deviation) for the phantom data and 0.3 ± 0.3 for the patient data, respectively, by applying the two methodologies. Furthermore, statistical analysis by means of the Wilcoxon nonparametric signed test showed that the results that were obtained by the two methods did not differ significantly (P value > 0.05).

Benefits of Six Degrees of Freedom for Optically Driven Patient Set-up Correction in SBRT

To quantify the advantages of a 6 degrees of freedom (dof) versus the conventional 3- or 4-dof correction modality for stereotactic body radiation therapy (SBRT) treatments. Eighty-five patients were fitted with 5–7 infra-red passive markers for optical localization. Data, acquired during the treatment, were analyzed retrospectively to simulate and evaluate the best approach for correcting patient misalignments. After the implementation of each correction, the new position of the target (tumor's center of mass) was estimated by means of a dedicated stereotactic algorithm. The Euclidean distance between the corrected and the planned location of target point was calculated and compared to the initial mismatching. Initial and after correction median±quartile displacements affecting external control points were 3.74±2.55 mm (initial), 2.45±0.91 mm (3-dof), 2.37±0.95 mm (4-dof), and 2.03±1.47 mm (6-dof). The benefit of a six-parameter adjustment was particularly evident when evaluating the results relative to the target position before and after the re-alignment. In this context, the Euclidean distance between the planned and the current target point turned to 0.82±1.12mm (median±quartile values) after the roto-translation versus the initial displacement of 2.98±2.32mm. No statistical improvements were found after 3- and 4-dof correction (2.73±1.22 mm and 2.60±1.31 mm, respectively). Angular errors were 0.09±0.93° (mean±std). Pitch rotation in abdomen site showed the most relevant deviation, being − 0.46±1.27° with a peak value of 5.46°. Translational misalignments were −0.68±2.60 mm (mean±std) with the maximum value of 12 mm along the cranio-caudal direction. We conclude that positioning system platforms featuring 6-dof are preferred for high precision radiation therapy. Data are in line with previous results relative to other sites and represent a relevant record in the framework of SBRT.

Anatomical imaging for radiotherapy

The goal of radiation therapy is to achieve maximal therapeutic benefit expressed in terms of a high probability of local control of disease with minimal side effects. Physically this often equates to the delivery of a high dose of radiation to the tumour or target region whilst maintaining an acceptably low dose to other tissues, particularly those adjacent to the target. Techniques such as intensity modulated radiotherapy (IMRT), stereotactic radiosurgery and computer planned brachytherapy provide the means to calculate the radiation dose delivery to achieve the desired dose distribution. Imaging is an essential tool in all state of the art planning and delivery techniques: (i) to enable planning of the desired treatment, (ii) to verify the treatment is delivered as planned and (iii) to follow-up treatment outcome to monitor that the treatment has had the desired effect. Clinical imaging techniques can be loosely classified into anatomic methods which measure the basic physical characteristics of tissue such as their density and biological imaging techniques which measure functional characteristics such as metabolism. In this review we consider anatomical imaging techniques. Biological imaging is considered in another article. Anatomical imaging is generally used for goals (i) and (ii) above. Computed tomography (CT) has been the mainstay of anatomical treatment planning for many years, enabling some delineation of soft tissue as well as radiation attenuation estimation for dose prediction. Magnetic resonance imaging is fast becoming widespread alongside CT, enabling superior soft-tissue visualization. Traditionally scanning for treatment planning has relied on the use of a single snapshot scan. Recent years have seen the development of techniques such as 4D CT and adaptive radiotherapy (ART). In 4D CT raw data are encoded with phase information and reconstructed to yield a set of scans detailing motion through the breathing, or cardiac, cycle. In ART a set of scans is taken on different days. Both allow planning to account for variability intrinsic to the patient. Treatment verification has been carried out using a variety of technologies including: MV portal imaging, kV portal/fluoroscopy, MVCT, conebeam kVCT, ultrasound and optical surface imaging. The various methods have their pros and cons. The four x-ray methods involve an extra radiation dose to normal tissue. The portal methods may not generally be used to visualize soft tissue, consequently they are often used in conjunction with implanted fiducial markers. The two CT-based methods allow measurement of inter-fraction variation only. Ultrasound allows soft-tissue measurement with zero dose but requires skilled interpretation, and there is evidence of systematic differences between ultrasound and other data sources, perhaps due to the effects of the probe pressure. Optical imaging also involves zero dose but requires good correlation between the target and the external measurement and thus is often used in conjunction with an x-ray method. The use of anatomical imaging in radiotherapy allows treatment uncertainties to be determined. These include errors between the mean position at treatment and that at planning (the systematic error) and the day-to-day variation in treatment set-up (the random error). Positional variations may also be categorized in terms of inter- and intra-fraction errors. Various empirical treatment margin formulae and intervention approaches exist to determine the optimum strategies for treatment in the presence of these known errors. Other methods exist to try to minimize error margins drastically including the currently available breath-hold techniques and the tracking methods which are largely in development. This paper will review anatomical imaging techniques in radiotherapy and how they are used to boost the therapeutic benefit of the treatment.

Radiation therapy treatment verification imaging in Australia and New Zealand

An original questionnaire was used to investigate the available types of reference and treatment image verification equipment and specific practices related to image analysis. A section on treatment site-specific imaging was included. The questionnaire was distributed to all radiation oncology facilities in Australia and New Zealand. A response rate of 87% (40/46) was achieved. Most facilities (90%) in Australia and New Zealand reported the availability of electronic portal imaging devices. Use of computer software to assist with image interpretation was indicated by 92% of centres. Frequency of image acquisition and tolerance levels used for radical treatment sites were variable, but palliative treatment site protocols were more consistent between treatment facilities. In conclusion, departments should strive to use evidence-based protocols and guidelines to ensure acceptable accuracy in treatment delivery.

Interactive image registration tool for positioning verification in head and neck radiotherapy

This paper presents the development of a computer-aided image management tool, named CAIMT, used for patient setup error measurement in radiation treatment planning. The CAIMT incorporates user interaction to facilitate contour detection from both portal film and simulation film. The detected contours were then used as features for image registration through application of the generalized Hough transform (GHT). Positioning error was measured when the optimal registration was achieved. The CAIMT has been applied to register the image pairs of a rando(TM) phantom and five real subjects, in order to evaluate its reliability and stability. The promising results suggest that the CAIMT can assist radiotherapists to align the simulation film and the portal film precisely and steadily and therefore adjust patient positioning to the optimum in a more efficient way.

Assessment of three-dimensional set-up errors in conventional head and neck radiotherapy using electronic portal imaging device

Background

Set-up errors are an inherent part of radiation treatment process. Coverage of target volume is a direct function of set-up margins, which should be optimized to prevent inadvertent irradiation of adjacent normal tissues. The aim of this study was to evaluate three-dimensional (3D) set-up errors and propose optimum margins for target volume coverage in head and neck radiotherapy.

Methods

The dataset consisted of 93 pairs of orthogonal simulator and corresponding portal images on which 558 point positions were measured to calculate translational displacement in 25 patients undergoing conventional head and neck radiotherapy with antero-lateral wedge pair technique. Mean displacements, population systematic (Σ) and random (σ) errors and 3D vector of displacement was calculated. Set-up margins were calculated using published margin recipes.

Results

The mean displacement in antero-posterior (AP), medio-lateral (ML) and supero-inferior (SI) direction was -0.25 mm (-6.50 to +7.70 mm), -0.48 mm (-5.50 to +7.80 mm) and +0.45 mm (-7.30 to +7.40 mm) respectively. Ninety three percent of the displacements were within 5 mm in all three cardinal directions. Population systematic (Σ) and random errors (σ) were 0.96, 0.98 and 1.20 mm and 1.94, 1.97 and 2.48 mm in AP, ML and SI direction respectively. The mean 3D vector of displacement was 3.84 cm. Using van Herk's formula, the clinical target volume to planning target volume margins were 3.76, 3.83 and 4.74 mm in AP, ML and SI direction respectively.

Conclusion

The present study report compares well with published set-up error data relevant to head and neck radiotherapy practice. The set-up margins were <5 mm in all directions. Caution is warranted against adopting generic margin recipes as different margin generating recipes lead to a different probability of target volume coverage.

The management of imaging dose during image-guided radiotherapy: report of the AAPM Task Group 75

Radiographic image guidance has emerged as the new paradigm for patient positioning, target localization, and external beam alignment in radiotherapy. Although widely varied in modality and method, all radiographic guidance techniques have one thing in common--they can give a significant radiation dose to the patient. As with all medical uses of ionizing radiation, the general view is that this exposure should be carefully managed. The philosophy for dose management adopted by the diagnostic imaging community is summarized by the acronym ALARA, i.e., as low as reasonably achievable. But unlike the general situation with diagnostic imaging and image-guided surgery, image-guided radiotherapy (IGRT) adds the imaging dose to an already high level of therapeutic radiation. There is furthermore an interplay between increased imaging and improved therapeutic dose conformity that suggests the possibility of optimizing rather than simply minimizing the imaging dose. For this reason, the management of imaging dose during radiotherapy is a different problem than its management during routine diagnostic or image-guided surgical procedures. The imaging dose received as part of a radiotherapy treatment has long been regarded as negligible and thus has been quantified in a fairly loose manner. On the other hand, radiation oncologists examine the therapy dose distribution in minute detail. The introduction of more intensive imaging procedures for IGRT now obligates the clinician to evaluate therapeutic and imaging doses in a more balanced manner. This task group is charged with addressing the issue of radiation dose delivered via image guidance techniques during radiotherapy. The group has developed this charge into three objectives: (1) Compile an overview of image-guidance techniques and their associated radiation dose levels, to provide the clinician using a particular set of image guidance techniques with enough data to estimate the total diagnostic dose for a specific treatment scenario, (2) identify ways to reduce the total imaging dose without sacrificing essential imaging information, and (3) recommend optimization strategies to trade off imaging dose with improvements in therapeutic dose delivery. The end goal is to enable the design of image guidance regimens that are as effective and efficient as possible.

Optimization of image quality and dose for Varian aS500 electronic portal imaging devices (EPIDs)

This study was carried out to investigate whether the electronic portal imaging (EPI) acquisition process could be optimized, and as a result tolerance and action levels be set for the PIPSPro QC-3V phantom image quality assessment. The aim of the optimization process was to reduce the dose delivered to the patient while maintaining a clinically acceptable image quality. This is of interest when images are acquired in addition to the planned patient treatment, rather than images being acquired using the treatment field during a patient's treatment. A series of phantoms were used to assess image quality for different acquisition settings relative to the baseline values obtained following acceptance testing. Eight Varian aS500 EPID systems on four matched Varian 600C/D linacs and four matched Varian 2100C/D linacs were compared for consistency of performance and images were acquired at the four main orthogonal gantry angles. Images were acquired using a 6 MV beam operating at 100 MU min(-1) and the low-dose acquisition mode. Doses used in the comparison were measured using a Farmer ionization chamber placed at d(max) in solid water. The results demonstrated that the number of reset frames did not have any influence on the image contrast, but the number of frame averages did. The expected increase in noise with corresponding decrease in contrast was also observed when reducing the number of frame averages. The optimal settings for the low-dose acquisition mode with respect to image quality and dose were found to be one reset frame and three frame averages. All patients at the Northern Ireland Cancer Centre are now imaged using one reset frame and three frame averages in the 6 MV 100 MU min(-1) low-dose acquisition mode. Routine EPID QC contrast tolerance (+/-10) and action (+/-20) levels using the PIPSPro phantom based around expected values of 190 (Varian 600C/D) and 225 (Varian 2100C/D) have been introduced. The dose at dmax from electronic portal imaging has been reduced by approximately 28%, and while the image quality has been reduced, the images produced are still clinically acceptable.

Practical issues in the implementation of image-guided radiotherapy for the treatment of prostate cancer within a UK department

AIMS

To study the feasibility of using implanted gold seeds in combination with a commercial software system for daily localisation of the prostate gland during conformal radiotherapy, and to assess the effect this may have on departmental workload.

MATERIALS AND METHODS

Six patients had three gold radio-opaque seeds implanted into the prostate gland before starting a course of radiotherapy. The seeds were identified on daily portal images and an automated online system provided immediate vector analysis of discrepancies between the planned and actual daily position of the intraprostatic seeds. In total, 138 interfractional displacements were analysed. The workload impact for the department was assessed using the basic treatment equivalence model, by comparing measurements of daily treatment session durations with a control group of patients receiving standard conformal radiotherapy, matched for treatment complexity.

RESULTS

No acute complications of seed insertion were observed. A number of developmental issues required solutions to be identified before clinical implementation was possible. The standard deviations of the set-up and organ motion systematic errors in the left-right, superior-inferior and anterior-posterior directions were 2.4, 3.0 and 2.5 mm, respectively. The standard deviations of the set-up and organ motion random errors calculated were 2.5, 2.9 and 3.7 mm. The mean treatment session duration with this daily prostate localisation system was increased by 3 min compared with matched controls using standard imaging practice. If all radical prostate patients in our department were to receive image-guided radiotherapy in this way, this would increase machine workload time by 2.2 h/day.

CONCLUSIONS

The implementation of this image-guided system is feasible. No additional linear accelerator modification is required and standard imaging devices can be used. It would be a useful addition to any department's image-guided radiotherapy developmental strategy.

A technique for verification of isocenter position in tangential field breast irradiation

Treatment verification and reproducibility of the breast treatment portals play a very important role in breast radiotherapy. We propose a simple technique to verify the planned isocenter position during treatment using an electronic portal imaging device. Ten patients were recruited in this study and (CT) computed tomography-based planning was performed with a conventional tangential field technique. For verification purposes, in addition to the standard medial (F1) and lateral (F2) tangential fields, a field (F3) perpendicular to the medial field was used for verification of the treatment portals. Lead markers were placed along the central axis of the 2 defined fields (F1 and F3) and the separation between the markers was measured on the portal images and verified with the marker separation on the digitally reconstructed radiographs (DRRs). Any deviation will identify the shift in the planned isocenter position during treatment. The average deviation observed between the markers measured from the DRR and portal image was 1.6 and 2.1 mm, with a standard deviation of 0.4 and 0.9 mm for fields F1 and F3, respectively. The maximum deviation observed was 3.0 mm for field F3. This technique will be very useful in patient setup for tangential breast radiotherapy.

Developments in electronic portal imaging systems

Verification of geometric accuracy at the time of treatment delivery has always been a necessary part of the radiotherapy process. Since the introduction of conformal and intensity-modulated radiotherapy, the consequences of patient positioning errors are more serious. Portal imaging has played a large part in fulfilling the need for improved geometric accuracy. This review examines how portal imaging has progressed through the development and evolution of electronic portal imaging devices (EPIDs). Changes in technology, including the current commercial systems, and how image quality has changed are presented. The clinical usage of EPIDs and the technological innovations being devised for further improvements in image quality and systems are considered.

Physics and imaging for targeting of oligometastases

Oligometastases refer to metastases that are limited in number and location and are amenable to regional treatment. The majority of these metastases appear in the brain, lung, liver, and bone. Although the focus of interest in the past within radiation oncology has been on the treatment of intracranial metastases, there has been growing interest in extracranial sites such as the liver and lung. This is largely because of the rapid development of targeting techniques for oligometastases such as intensity-modulated and image-guided radiation therapy, which has made it possible to deliver single or a few fractions of high-dose radiation treatments, highly conformal to the target. The clinical decision to use radiation to treat oligometastases is based on both radiobiological and physics considerations. The radiobiological considerations involve improvement of treatment schema for time, dose, and volume. Areas of interests are hypofractionation, tumor and normal tissue tolerance, and hypoxia. The physics considerations for oligometastases treatment are focused mainly on ensuring treatment accuracy and precision. This article discusses the physics and imaging aspects involved in each step of the radiation treatment process for oligometastases, including target definition, treatment simulation, treatment planning, pretreatment target localization, radiation delivery, treatment verification, and treatment evaluation.

Breast movement during normal and deep breathing, respiratory training and set up errors: implications for external beam partial breast irradiation

This study was designed to evaluate interfraction and intrafraction breast movement and to study the effect of respiratory training on respiratory indices. Five patients were immobilized in supine position in a vacuum bag and three-dimensional set up errors, respiratory movement of the breast during normal and deep breathing, tidal volume and breath hold time were recorded. All patients underwent respiratory training and all the respiratory indices were re-evaluated at the end of training. Cumulative maximum movement error (CMME) was calculated by adding directional maximum set up error and maximum post training movement during normal breathing. The mean set up deviation was 1.3 mm (SD +/- 0.5 mm), 1.3 mm (SD +/- 0.3 mm) and 4.4 mm (SD +/- 2.6 mm) in the mediolateral, superoinferior and anteroposterior dimensions. Pre-training mean of the maximum marker movement during normal breathing was 1.07 mm, 1.94 mm and 1.86 mm in the mediolateral, superoinferior and anteroposterior dimensions. During deep breathing these values were 2 mm, 5.5 mm and 4.8 mm. While respiratory training had negligible effect on breast movement during normal breathing, it resulted in a modest reduction during deep breathing (p = 0.2). The mean CMME recorded for these patients was 3.4 mm, 4.5 mm and 7.1 mm in the mediolateral, superoinferior and anteroposterior dimension. Respiratory training also resulted in an increase in breath hold time from a mean of 31 s to 44 s (p = 0.04) and tidal volume from a mean of 560 cm(3) to 1160 cm(3) (p = 0.04). With patients immobilized in the vacuum bag the CMMEs are relatively less. Individualized directional margins may aid in reduction of planning target volume (PTV).