Imaging dose from cone beam computed tomography in radiation therapy

Imaging dose in radiation therapy has traditionally been ignored due to its low magnitude and frequency in comparison to therapeutic dose used to treat patients. The advent of modern, volumetric, imaging modalities, often as an integral part of linear accelerators, has facilitated the implementation of image-guided radiation therapy (IGRT), which is often accomplished by daily imaging of patients. Daily imaging results in additional dose delivered to patient that warrants new attention be given to imaging dose. This review summarizes the imaging dose delivered to patients as the result of cone beam computed tomography (CBCT) imaging performed in radiation therapy using current methods and equipment. This review also summarizes methods to calculate the imaging dose, including the use of Monte Carlo (MC) and treatment planning systems (TPS). Peripheral dose from CBCT imaging, dose reduction methods, the use of effective dose in describing imaging dose, and the measurement of CT dose index (CTDI) in CBCT systems are also reviewed.     

Evaluation of patient organ doses from kilovoltage cone-beam CT imaging in radiation therapy

BackgroundCurrently, CBCT system is an indispensable component of radiation therapy units. Because of that, it is important in treatment planning and diagnosis. CBCT is also an crucial tool for patient positioning and verification in image-guided radiation therapy (IGRT). Therefore, it is critical to investigate the patient organ doses arising from CBCT imaging. The purpose of this study is to evaluate patient organ doses and effective dose to patients from three different protocols of Elekta Synergy XVI system for kV CBCT imaging examinations in image guided radiation therapy.Materials and methodsOrgan dose measurements were done with thermoluminescent dosimeters in Alderson RA NDO male phantom for head & neck (H&N), chest and pelvis protocols of the Elekta Synergy XVI kV CBCT system. From the measured organ dose, effective dose to patients were calculated according to the International Commission on Radiological Protection 103 report recommendations.ResultsFor H&N, chest and pelvis scans, the organ doses were in the range of 0.03–3.43 mGy, 6.04–22.94 mGy and 2.5–25.28 mGy, respectively. The calculated effective doses were 0.25 mSv, 5.56 mSv and 4.72 mSv, respectively.ConclusionThe obtained results were consistent with the most published studies in the literature. Although the doses to patient organs from the kV CBCT system were relatively low when compared with the prescribed treatment dose, the amount of delivered dose should be monitored and recorded carefully in order to avoid secondary cancer risk, especially in pediatric examinations.     

Feasibility of Tomotherapy-Based Image-Guided Radiotherapy to Reduce Aspiration Risk in Patients with Non-Laryngeal and Non-Pharyngeal Head and Neck Cancer

Purpose

The study aims to assess the feasibility of Tomotherapy-based image-guided radiotherapy (IGRT) to reduce the aspiration risk in patients with non-laryngeal and non-hypopharyngeal cancer. A retrospective review of 48 patients undergoing radiation for non-laryngeal and non-hypopharyngeal head and neck cancers was conducted. All patients had a modified barium swallow (MBS) prior to treatment, which was repeated one month following radiotherapy. Mean middle and inferior pharyngeal dose was recorded and correlated with the MBS results to determine aspiration risk.

Results

Mean pharyngeal dose was 23.2 Gy for the whole group. Two patients (4.2%) developed trace aspiration following radiotherapy which resolved with swallowing therapy. At a median follow-up of 19 months (1–48 months), all patients were able to resume normal oral feeding without aspiration.

Conclusion and Clinical Relevance

IGRT may reduce the aspiration risk by decreasing the mean pharyngeal dose in the presence of large cervical lymph nodes. Further prospective studies with IGRT should be performed in patients with non-laryngeal and non-hypopharyngeal head and neck cancers to verify this hypothesis.     

Prospective image planning in radiation therapy for optimization of image quality and reduction of patient dose

IntroductionCT simulation data in image-guided radiation therapy (IGRT) provides patient-specific subject contrast. This information can be exploited to establish, a priori, a suitable imaging goal and to select patient-specific imaging acquisition parameters that optimize the similarity between reference and daily set-up images and reduce imaging dose. This study aims to describe and clinically validate a computerized algorithm designed to provide such optimization.Material and methodsAn image planning system (IPS) was developed to assist in planar kV imaging technique selection for radiation therapy. The system's patient-specific image quality and dose reduction capabilities were validated herein. Anthropomorphic phantom and clinical data were acquired. Mutual information (MI) was used to compare simulated and measured images in both phantom and clinical tests. Variations in contrast resolution resulting from imaging panel underexposure, saturation and a contrast plateau were investigated. For evaluation of patient-specific imaging dose reduction, the IPS was used to modify acquisition settings for six patients.ResultsPhantom data confirmed the IPS's predictive capability regarding image contrast. Measured and simulated images showed similar progressions from under-exposure, image quality peak, and loss of contrast due to detector saturation. Clinical data demonstrated that contrast resolution and imaging dose could be prospectively improved without loss of image contrast. The algorithm reduced imaging dose by an average of 47%, and a maximum of 80%.ConclusionsLoss of image contrast resulting from under-exposure or over-exposure, as well as a contrast plateau can be predicted by use of a prospective image planning algorithm. Image acquisition parameters can be predicted that reduce patient dose without loss of useful contrast.     

Cardiac toxicity of lung cancer radiotherapy

Radical radiotherapy of lung cancer with dose escalation has been associated with increased tumor control. However, these attempts to continually improve local control through dose escalation, have met mixed results culminating in the findings of the RTOG trial 0617, where the heart dose was associated with a worse overall survival, indicating a significant contribution to radiation-induced cardiac morbidity. It is, therefore, very likely that poorly understood cardiac toxicity may have offset any potential improvement in overall survival derived from dose escalation and may be an obstacle that limits disease control and survival of patients. The manifestations of cardiac toxicity are relatively common after high dose radiotherapy of advanced lung cancers and are independently associated with both heart dose and baseline cardiac risk. Toxicity following the treatment may occur earlier than previously thought and, therefore, heart doses should be minimized. In patients with lung cancer, who not only receive substantial heart dose, but are also older with more comorbidities, all cardiac events have the potential to be clinically significant and life-threatening.Sophisticated radiation treatment planning techniques, charged particle therapy, and modern imaging methods in radiotherapy planning, may lead to reduction of the heart dose, which could potentially improve the clinical outcomes in patients with lung cancer. Efforts should be made to minimize heart radiation exposure whenever possible even at doses lower than those generally recommended. Heart doses should be limited as much as possible.A heart dosimetry as a whole is important for patient outcomes, rather than emphasizing just one parameter.     

A support vector machine tool for adaptive tomotherapy treatments: Prediction of head and neck patients criticalities

PurposeAdaptive radiation therapy (ART) is an advanced field of radiation oncology. Image-guided radiation therapy (IGRT) methods can support daily setup and assess anatomical variations during therapy, which could prevent incorrect dose distribution and unexpected toxicities. A re-planning to correct these anatomical variations should be done daily/weekly, but to be applicable to a large number of patients, still require time consumption and resources. Using unsupervised machine learning on retrospective data, we have developed a predictive network, to identify patients that would benefit of a re-planning.Methods1200 MVCT of 40 head and neck (H&N) cases were re-contoured, automatically, using deformable hybrid registration and structures mapping. Deformable algorithm and MATLAB^® homemade machine learning process, developed, allow prediction of criticalities for Tomotherapy treatments.ResultsUsing retrospective analysis of H&N treatments, we have investigated and predicted tumor shrinkage and organ at risk (OAR) deformations. Support vector machine (SVM) and cluster analysis have identified cases or treatment sessions with potential criticalities, based on dose and volume discrepancies between fractions. During 1st weeks of treatment, 84% of patients shown an output comparable to average standard radiation treatment behavior. Starting from the 4th week, significant morpho-dosimetric changes affect 77% of patients, suggesting need for re-planning. The comparison of treatment delivered and ART simulation was carried out with receiver operating characteristic (ROC) curves, showing monotonous increase of ROC area.ConclusionsWarping methods, supported by daily image analysis and predictive tools, can improve personalization and monitoring of each treatment, thereby minimizing anatomic and dosimetric divergences from initial constraints.     

Feasibility of Tomotherapy-Based Image-Guided Radiotherapy for Locally Advanced Oropharyngeal Cancer

Purpose

The study aims to assess the feasibility of tomotherapy-based image-guided (IGRT) radiotherapy for locally advanced oropharyngeal cancer. A retrospective review of 33 patients undergoing concurrent chemoradiation for locally advanced oropharyngeal cancers was conducted. Radiotherapy planning, treatment toxicity and loco-regional control were assessed.

Results

At a median follow-up of 32 months (6–47 months), no patient developed loco-regional recurrence. Two patients (6%) developed distant metastases. Grade 3–4 acute toxicity was respectively 72% and 25% for mucositis and gastrointestinal toxicity. Two patients (6%) had long-term dependence on tube feedings. Dose-volume histogram demonstrated excellent target volume coverage and low radiation dose to the organs at risk for complications.

Conclusions and clinical relevance

IGRT provides excellent loco-regional control but acute toxicity remains significant and needs to be addressed in future prospective trials. The feasibility of Tomotherapy to decrease radiation dose to the normal tissues merits further investigations.     

Dosimetric assessment of the exposure of radiotherapy patients due to cone-beam CT procedures

Cone-beam computed tomography (CBCT) is widely used for pre-treatment verification and patient setup in image-guided radiation therapy (IGRT). CBCT imaging is employed daily and several times per patient, resulting in potentially high cumulative imaging doses to healthy tissues that surround exposed target organs. Computed tomography dose index (CTDI) is the parameter used by CBCT equipment as indication of the radiation output to patients. This study aimed to increase the knowledge on the relation between CBCT organ doses and weighted CTDI (CTDI_W) for a thorax scanning protocol. A CBCT system was modelled using the Monte Carlo (MC) radiation transport program MCNPX2.7.0. Simulation results were validated against half-value layer (HVL), axial beam profile, patient skin dose (PSD) and CTDI measurements. For organ dose calculations, a male voxel phantom (“Golem”) was implemented with the CBCT scanner computational model. After a successful MC model validation with measurements, a systematic comparison was performed between organ doses (and their distribution) and CTDI dosimetry concepts [CTDI_W and cumulative dose quantities f₁₀₀(150) and \({\text{CTD}}{{\text{I}}_\infty }\)]. The results obtained show that CBCT organ doses vary between 1.2 ± 0.1 mGy and 3.3 ± 0.2 mGy for organs located within the primary beam. It was also verified that CTDI_W allows prediction of absorbed doses to tissues at distances of about 5 cm from the isocentre of the CBCT system, whereas f₁₀₀(150) allows prediction of organ doses at distances of about 10 cm from the isocentre, independently from its location. This study demonstrates that these dosimetric concepts are suitable methods that easily allow a good approximation of the additional CBCT imaging doses during a typical lung cancer IGRT treatment.

     

PET guidance in prostate cancer radiotherapy: Quantitative imaging to predict response and guide treatment

Positron emission tomography (PET) allows a monitoring and recording of the spatial and temporal distribution of molecular/cellular processes for diagnostic and therapeutic applications.The aim of this review is to describe the current applications and to explore the role of PET in prostate cancer management, mainly in the radiation therapy (RT) scenario.The state-of-the art of PET for prostate cancer will be presented together with the impact of new specific PET tracers and technological developments aiming at obtaining better imaging quality, increased tumor detectability and more accurate volume delineation.An increased number of studies have been focusing on PET quantification methods as predictive biomarkers capable of guiding individualized treatment and improving patient outcome; the sophisticated advanced intensity modulated and imaged guided radiation therapy techniques (IMRT/IGRT) are capable of boosting more radioresistant tumor (sub)volumes.The use of advanced feature analyses of PET images is an approach that holds great promise with regard to several oncological diseases, but needs further validation in managing prostate diseases.     

Retrospective comparison of rectal toxicity between carbon-ion radiotherapy and intensity-modulated radiation therapy based on treatment plan,normal tissue complication probability model,and clinical outcomes in prostate cancer

This retrospective study assessed the treatment planning data and clinical outcomes for 152 prostate cancer patients: 76 consecutive patients treated by carbon-ion radiation therapy and 76 consequtive patients treated by moderate hypo-fractionated intensity-modulated photon radiation therapy. These two modalities were compared using linear quadratic model equivalent doses in 2 Gy per fraction for rectal or rectal wall dose–volume histogram, 3.6 Gy per fraction-converted rectal dose–volume histogram, normal tissue complication probability model, and actual clinical outcomes. Carbon-ion radiation therapy was predicted to have a lower probability of rectal adverse events than intensity-modulated photon radiation therapy based on dose–volume histograms and normal tissue complication probability model. There was no difference in the clinical outcome of rectal adverse events between the two modalities compared in this study.     

Feasibility of intensity-modulated and image-guided radiotherapy for functional organ preservation in locally advanced laryngeal cancer

Purpose

The study aims to assess the feasibility of intensity-modulated and image-guided radiotherapy (IMRT, and IGRT, respectively) for functional preservation in locally advanced laryngeal cancer. A retrospective review of 27 patients undergoing concurrent chemoradiation for locally advanced laryngeal cancers (8 IMRT, 19 IGRT) was undertaken. In addition to regular clinical examinations, all patients had PET imaging at 4 months and 10 months after radiotherapy, then yearly. Loco-regional control, speech quality and feeding-tube dependency were assessed during follow-up visits.

Results

At a median follow-up of 20 months (range 6–57 months), four out of 27 patients (14.8%) developed local recurrence and underwent salvage total laryngectomy. One patient developed distant metastases following salvage surgery. Among the 23 patients who conserved their larynx with no sign of recurrence at last follow-up, 22 (95%) reported normal or near normal voice quality, allowing them to communicate adequately. Four patients (14.8%) had long-term tube feeding-dependency because of severe dysphagia (2 patients) and chronic aspiration (2 patients, with ensuing death from aspiration pneumonia in one patient).

Conclusions and Clinical Relevance

Functional laryngeal preservation is feasible with IMRT and IGRT for locally advanced laryngeal cancer. However, dysphagia and aspiration remain serious complications, due most likely to high radiation dose delivery to the pharyngeal musculatures.     

Polymer gel dosimeters for pretreatment radiotherapy verification using the three-dimensional gamma evaluation and pass rate maps

Polymer gel dosimeters (PGDs) have been widely studied for use in the pretreatment verification of clinical radiation therapy. However, the readability of PGDs in three-dimensional (3D) dosimetry remain unclear. In this study, the pretreatment verifications of clinical radiation therapy were performed using an N-isopropyl-acrylamide (NIPAM) PGD, and the results were used to evaluate the performance of the NIPAM PGD on 3D dose measurement. A gel phantom was used to measure the dose distribution of a clinical case of intensity-modulated radiation therapy. Magnetic resonance imaging scans were performed for dose readouts. The measured dose volumes were compared with the planned dose volume. The relative volume histograms showed that relative volumes with a negative percent dose difference decreased as time elapsed. Furthermore, the histograms revealed few changes after 24 h postirradiation. For the 3%/3 mm and 2%/2 mm criteria, the pass rates of the 12- and 24-h dose volumes were higher than 95%, respectively. This study thus concludes that the pass rate map can be used to evaluate the dose-temporal readability of PGDs and that the NIPAM PGD can be used for clinical pretreatment verifications.     

IGRT and motion management during lung SBRT delivery

Patient motion can cause misalignment of the tumour and toxicities to the healthy lung tissue during lung stereotactic body radiation therapy (SBRT). Any deviations from the reference setup can miss the target and have acute toxic effects on the patient with consequences onto its quality of life and survival outcomes. Correction for motion, either immediately prior to treatment or intra-treatment, can be realized with image-guided radiation therapy (IGRT) and motion management devices. The use of these techniques has demonstrated the feasibility of integrating complex technology with clinical linear accelerator to provide a higher standard of care for the patients and increase their quality of life.     

An end-to-end postal audit test to examine the coincidence between the imaging isocenter and treatment beam isocenter of the IGRT linac system for Japan Clinical Oncology Group (JCOG) clinical trials

PurposeThe aim of this study was to develop an end-to-end postal audit test to examine the coincidence between the imaging isocenter and treatment beam isocenter of the image guided radiotherapy (IGRT) linac system for Japan Clinical Oncology Group (JCOG) trials, as a part of IGRT credentialing of institutions participating in JCOG trials.MethodsWe developed an end-to-end postal audit test to verify radiation positional errors associated with IGRT techniques. This test is intended for simulating a clinical IGRT flow and uses a static cubic phantom measuring 15 × 15 × 15 cm³ and weighing approximately 3.4 kg. The phantom has four gold fiducial markers and a spherical dummy target for setup, with known shift values from the phantom center. Two pairs of Gafchromic RTQA2 films were inserted 5 mm from the phantom’s anterior-posterior and right-left surfaces. Radiation positional errors at the isocenter were determined by analyzing the center of the radiation field on the films and the known shift values of the dummy target. The test was performed on 47 IGRT devices at 35 institutions.ResultsRadiation positional errors were within acceptance levels (1 mm/1°) for 42 IGRT devices (89.4%) in the first check. Median time to complete IGRT credentialing was 11.5 days. This audit method was applicable for any radiotherapy machine with an IGRT device.ConclusionsA postal audit test to verify radiation positional errors for JCOG trials was successfully developed. In the postal audit, all but one institution passed this credentialing item within two trials.     

A realistic utilization of nanotechnology in molecular imaging and targeted radiotherapy of solid tumors

Precise dose delivery to malignant tissue in radiotherapy is of paramount importance for treatment efficacy while minimizing morbidity of surrounding normal tissues. Current conventional imaging techniques, such as magnetic resonance imaging (MRI) and computerized tomography (CT), are used to define the three-dimensional shape and volume of the tumor for radiation therapy. In many cases, these radiographic imaging (RI) techniques are ambiguous or provide limited information with regard to tumor margins and histopathology. Molecular imaging (MI) modalities, such as positron emission tomography (PET) and single photon-emission computed-tomography (SPECT) that can characterize tumor tissue, are rapidly becoming routine in radiation therapy. However, their inherent low spatial resolution impedes tumor delineation for the purposes of radiation treatment planning. This review will focus on applications of nanotechnology to synergize imaging modalities in order to accurately highlight, as well as subsequently target, tumor cells. Furthermore, using such nano-agents for imaging, simultaneous coupling of novel therapeutics including radiosensitizers can be delivered specifically to the tumor to maximize tumor cell killing while sparing normal tissue.     

An iterative approach to plan combination in radiotherapy

Radiation therapy concerns the delivery of a proper dose of radiation to a tumor volume without causing irreparable damage to surrounding healthy tissue and critical organs. The problem of plan combination in radiation therapy treatment planning (RTTP) proposed, formulated and studied here, addresses a situation when for a specific clinical case, a set of several treatment plans is proposed, but each one of them violates the prescribed dose in at least one significant region of the volume that has to be treated. We represent treatment plans as vectors in the Euclidean space, and define their equivalence, acceptability and realizability. A simple linear algebraic model for combining them is then used in order to derive, from the given set of approximate plans, a combined treatment plan, which will be both acceptable, and technically realizable. In the event that such a combined plan does not exist, the alternatives for relaxing the treatment requirements can be systematically considered.     

Dosimetry in photodynamic therapy: oxygen and the critical importance of capillary density.

Recently published results of tumor response to various photoradiation protocols in photodynamic therapy appear to contradict accepted definitions of photodynamic dose. In this report, the failure of standard dosimetry models to predict therapeutic outcome is interpreted on the basis of PDT-induced oxygen consumption in tumors with relatively low capillary densities. Calculated estimates of oxygen consumption in photodynamic therapy are combined with the Krogh cylinder model of oxygen diffusion. It is shown that, for tissue volumes in which the intercapillary spacing is less than a specific critical distance, oxygen may be considered constant and unaffected by the therapy. Under these conditions, the 1O2 delivered to a given volume of tissue is spatially uniform and proportional to the number of photons absorbed by the sensitizer. When the intercapillary spacing exceeds the critical distance, the dose of 1O2 varies with radial distance from the capillary wall. In this situation, dose may no longer be considered simply in terms of the product of the photon fluence and the sensitizer absorption coefficient. Since fractionation will increase the 1O2 dose only to cells relatively remote from the capillary wall, the analysis further suggests that fractionating the radiation dose should result in an improved therapeutic ratio for photodynamic therapy.     

Prostate cancer radiation therapy: A physician’s perspective

Prostate cancer is the second most common cancer in men and a major cause of cancer deaths worldwide. Ionizing radiation has played a substantial role in the curative treatment of this disease. The historical evolution of radiotherapy techniques through 3D-conformal radiotherapy (3D-CRT), intensity-modulated radiotherapy (IMRT), and image-guided radiotherapy (IGRT) has allowed more accurate and precise treatments toward significant improvements in the therapeutic ratio. The addition of androgen deprivation therapy has significantly improved overall survival becoming the standard therapy for intermediate- and high-risk disease. Many randomized controlled trials have shown improved local control with dose escalation, and hypofractionated RT has been consolidated with proven efficacy and safe clinical results. However, several questions remain open in the radiotherapeutic management of prostate cancer patients and hopefully ongoing studies will shed light on these uncertainties. More individualized approaches are essential through better prognostic and novel predictive biomarkers of prostate radiotherapy response. Clinicians should critically interpret the evolving technologies in prostate cancer radiotherapy with important optimism but balancing the costs and the actual magnitude of clinical benefit. This article provides an overview of the basic aspects of radiotherapy treatment in localized prostate cancer from a physician’s perspective.     

Displacements of fiducial markers in patients with prostate cancer treated with image guided radiotherapy: A single-institution descriptive study

Aim

To describe daily displacements when using fiducial markers as surrogates for the target volume in patients with prostate cancer treated with IGRT.

Background

The higher grade of conformity achieved with the use of modern radiation technologies in prostate cancer can increase the risk of geographical miss; therefore, an associated protocol of IGRT is recommended.

Materials and methods

A single-institution, retrospective, consecutive study was designed. 128 prostate cancer patients treated with daily on-line IGRT based on 2D kV orthogonal images were included. Daily displacement of the fiducial markers was considered as the difference between the position of the patient when using skin tattoos and the position after being relocated using fiducial markers. Measures of central tendency and dispersion were used to describe fiducial displacements.

Results

The implant itself took a mean time of 15 min. We did not detect any complications derived from the implant. 4296 sets of orthogonal images were identified, 128 sets of images corresponding to treatment initiation were excluded; 91 (2.1%) sets of images were excluded from the analysis after having identified that these images contained extreme outlier values. If IGRT had not been performed 25%, 10% or 5% of the treatments would have had displacements superior to 4, 7 or 9 mm respectively in any axis.

Conclusions

Image guidance is required when using highly conformal techniques; otherwise, at least 10% of daily treatments could have significant displacements. IGRT based on fiducial markers, with 2D kV orthogonal images is a convenient and fast method for performing image guidance.