kV Cone-Beam CT-Based IGRT

Assessment of the therapeutic accuracy of cone beam computed tomography-guided nasopharyngeal carcinoma radiotherapy

The aim of the present study was to determine the ability of cone beam computed tomography (CBCT) to improve the accuracy of nasopharyngeal carcinoma (NPC) radiotherapy by analyzing the setup and inter-fraction errors at different levels and directions of the target volumes. A total of 113 patients with NPC who were undergoing intensity-modulated radiotherapy were recruited for the present study. Each patient had at least three CBCT exams prior to the start of radiation therapy. Three anatomic bony landmarks, including the upper neck, lower neck and head, were used to represent the different levels of assessment. The positioning errors were registered in three planes throughout the course of radiotherapy: The right-left (RL), superior-inferior (SI) and anterior-posterior (AP) directions. The planning CT images were matched with the CBCT images to determine the naso-pharynx shifts. A receiver operating characteristic curve was plotted to establish the specificity and sensitivity of CBCT. The planning target volume margin (MPTV) for the head was 0.9 mm, 1.4 mm for the upper neck and 2.0 mm for the lower neck. MPTVs of 1.5, 0.6 and 2.2 mm in the RL, SI and AP directions, respectively, were detected. In addition, there was evidence of setup errors in the three planes (RL, SI and AP) with the greatest error observed in the AP direction. Furthermore, the setup uncertainties in the neck region were greater than those of the head. In conclusion, CBCT could greatly improve the accuracy of radiotherapy by minimizing the setup errors and MPTV.

Comparison of treatment plan quality among MRI-based IMRT with a linac, MRI-based IMRT with tri-Co-60 sources, and VMAT for spine SABR

PURPOSE

This study compares the plan quality of magnetic-resonance image (MRI)-based intensity modulated radiation therapy (IMRT) using a linac (MR-linac-IMRT), MRI-based IMRT using tri-Co-60 sources (MR-Co-60-IMRT), and volumetric modulated arc therapy (VMAT) for spine stereotactic ablative radiotherapy (SABR).

METHODS

Twenty patients with thoracic spine metastasis were retrospectively selected for this study. For each patient, the MR-linac-IMRT, MR-Co-60-IMRT, and VMAT plans were generated using an identical CT image set and structures, except for the spinal cord and spinal cord planning organ-at-risk volume (PRV). Those two structures were contoured based on CT image sets for VMAT planning while those were contoured based on MR image sets for MR-linac-IMRT and MR-Co-60-IMRT planning. The initial prescription doses were 18 Gy in a single fraction for every plan in this study. If the tolerance level of the spinal cord was not met, the prescription doses were reduced to meet the tolerance level of the spinal cord. Dose-volumetric parameters of each plan were analyzed.

RESULTS

The average spinal cord volumes contoured based on the CT and MR images were 3.8±1.6 cm3 and 1.1±1.0 cm3, respectively (p<0.001). For four patients, the prescription doses of VMAT plans were reduced to 16 Gy to satisfy the spinal cord tolerance level. For thirteen patients, the prescription doses of MR-Co-60-IMRT plans were reduced to be less than 16 Gy to meet the spinal cord tolerance level. However, for every MR-linac-IMRT plan, the initial prescription doses of 18 Gy could be delivered to the target volume while satisfying the spinal cord tolerance. The average values of D10%, V10Gy, and V14Gy of the spinal cord PRV consistently indicated that the doses to the spinal cord PRV in the MR-linac-IMRT plans were the lowest among three types of plans in this study (all with p≤0.003).

CONCLUSION

MR-linac-IMRT appears promising for spine SABR.

A supervised network for fast image-guided radiotherapy (IGRT) registration

3D/3D image registration in IGRT, which aligns planning Computed Tomography (CT) image set with on board Cone Beam CT (CBCT) image set in a short time with high accuracy, is still a challenge due to its high computational cost and complex anatomical structure of medical image. In order to overcome these difficulties, a new method is proposed which contains a coarse registration and a fine registration. For the coarse registration, a supervised regression convolutional neural networks (CNNs) is used to optimize the spatial variation by minimizing the loss when combine the CT images with the CBCT images. For the fine registration, intensity-based image registration is used to calculate the accurate spatial difference of the input image pairs. A coarse registration can get a rough result with a wide capture range in less than 0.5 s. Sequentially a fine registration can get accurate results in a reasonable short time. RSD-111 T chest phantom was used to test our new method. The set-up error was calculated in less than 10s in time scale, and was reduced to sub-millimeter level in spatial scale. The average residual errors in translation and rotation are within ±0.5 mm and ± 0.2°.

[High-quality reconstruction of four-dimensional cone beam CT from motion registration prior image]

Four-dimensional cone beam CT (4D-CBCT) imaging can provide accurate location information of real-time breathing for imaging-guided radiotherapy. How to improve the accuracy of 4D-CBCT reconstruction image is a hot topic in current studies. PICCS algorithm performs remarkably in all 4D-CBCT reconstruction algorithms based on CS theory. The improved PICCS algorithm proposed in this paper improves the prior image on the basis of the traditional PICCS algorithm. According to the location information of each phase, the corresponding prior image is constructed, which completely eliminates the motion blur of the reconstructed image caused by the mismatch of the projection data. Meanwhile, the data fidelity model of the proposed method is consistent with the traditional PICCS algorithm. The experimental results showed that the reconstructed image using the proposed method had a clearer organization boundary compared with that of images reconstructed using the traditional PICCS algorithm. This proposed method significantly reduced the motion artifact and improved the image resolution.

A Noninvasive Method to Reduce Radiotherapy Positioning Error Caused by Respiration for Patients With Abdominal or Pelvic Cancers

Purpose:

To develop an infrared optical method of reducing surface-based registration error caused by respiration to improve radiotherapy setup accuracy for patients with abdominal or pelvic tumors.

Materials and Methods:

Fifteen patients with abdominal or pelvic tumors who received radiation therapy were prospectively included in our study. All patients were immobilized with vacuum cushion and underwent cone-beam computed tomography to validate positioning error before treatment. For each patient, after his or her setup based on markers fixed on immobilization device, initial positioning errors in patient left-right, anterior-posterior, and superior-inferior directions were validated by cone-beam computed tomography. Then, our method calculated mismatch between patient and immobilization device based on surface registration by interpolating between expiratory- and inspiratory-phase surface to find the specific phase to best match the surface in planning computed tomography scans. After adjusting the position of treatment couch by the shift proposed by our method, a second cone-beam computed tomography was performed to determine the final positioning error. A comparison between initial and final setup error will be made to validate the effectiveness of our method.

Results:

Final positioning error confirmed by cone-beam computed tomography is 1.59 (1.82), 1.61 (1.84), and 1.31 (1.38) mm, reducing initial setup error by 24.52%, 51.04%, and 53.63% in patient left-right, anterior-posterior, and superior-inferior directions, respectively. Wilcoxon test showed that our method significantly reduced the 3-dimensional distance of positioning error (P < .001).

Conclusion:

Our method can significantly improve the setup precision for patients with abdominal or pelvic tumors in a noninvasive way by reducing the surface-based registration error caused by respiration.

A prospective study of weekly intensity modulated radiation therapy plan adaptation for head and neck cancer: improved target coverage and organ at risk sparing

This prospective study of weekly CT scanning and plan adaption during H&N IMRT reports on the frequency of plan adaptations based on dosimetric differences between original and re-optimised IMRT plans. The volumetric and geometric change occurring in target volumes and salivary glands is also described. Ten H&N cancer patients underwent weekly planning CT imaging and re-optimisation of the IMRT plan if PTV or OAR coverage was unacceptable. Comparisons of PTV and parotid gland dosimetry between the original and adaptive plans were made. Parotid and submandibular gland volume changes and shift were calculated. Eight of ten patients required one or more plan adaptations, with 41% of adaptations occurring by fraction ten. Salivary glands reduced in volume, with a medial shift of the lateral border of the parotid gland and a superior shift of the submandibular gland. Change in PTV coverage did not correlate with weight loss or nutritional score. Inadequate PTV coverage, requiring plan adaptation, occurs early in the course of IMRT. A weekly Adaptive RT (ART) protocol results in significant improvement of PTV coverage. Implementation of a clinical ART protocol should include imaging and dose calculation within the first ten fractions.

Cone-beam CT-based inter-fraction localization errors for tumors in the pelvic region

PURPOSE

To evaluate inter-fraction tumor localization errors (TE) in the RapidArc® treatment of pelvic cancers based on CBCT. Appropriate CTV-to PTV margins in a non-IGRT scenario have been proposed.

METHODS

Data of 928 patients with prostate, gynecological, and rectum/anal canal cancers were retrospectively analyzed to determine systematic and random localization errors. Two protocols were used: daily online IGRT (d-IGRT) and weekly IGRT. The latter consisted in acquiring a CBCT for the first 3 fractions and subsequently once a week. TE for patients who underwent d-IGRT protocol were calculated using either all CBCTs or the first 3.

RESULTS

The systematic (and random) TE in the AP, LL, and SI direction were: for prostate bed 2.7(3.2), 2.3(2.8) and 1.9(2.2) mm; for prostate 4.2(3.1), 2.9(2.8) and 2.3(2.2) mm; for gynecological 3.0(3.6), 2.4(2.7) and 2.3(2.5) mm; for rectum 2.8(2.8), 2.4(2.8) and 2.3(2.5) mm; for anal canal 3.1(3.3), 2.1(2.5) and 2.2(2.7) mm. CTV-to-PTV margins determined from all CBCTs were 14 mm in the AP, 10 mm in the LL and 9-9.5 mm in the SI directions for the prostate and the gynecological groups and 9.5-10.5 mm in AP, 9 mm in LL and 8-10 mm in the SI direction for the prostate bed and the rectum/anal canal groups. If assessed on the basis of the first 3 CBCTs, the calculated CTV-to-PTV margins were slightly larger.

CONCLUSIONS

without IGRT, large CTV-to-PTV margins up to 15 mm are required to account for inter-fraction tumor localization errors. Daily IGRT should be used for all hypo-fractionated treatments to reduce margins and avoid increased toxicity to critical organs.

A Noninvasive Body Setup Method for Radiotherapy by Using a Multimodal Image Fusion Technique

Purpose:

To minimize the mismatch error between patient surface and immobilization system for tumor location by a noninvasive patient setup method.

Materials and Methods:

The method, based on a point set registration, proposes a shift for patient positioning by integrating information of the computed tomography scans and that of optical surface landmarks. An evaluation of the method included 3 areas: (1) a validation on a phantom by estimating 100 known mismatch errors between patient surface and immobilization system. (2) Five patients with pelvic tumors were considered. The tumor location errors of the method were measured using the difference between the proposal shift of cone-beam computed tomography and that of our method. (3) The collected setup data from the evaluation of patients were compared with the published performance data of other 2 similar systems.

Results:

The phantom verification results showed that the method was capable of estimating mismatch error between patient surface and immobilization system in a precision of <0.22 mm. For the pelvic tumor, the method had an average tumor location error of 1.303, 2.602, and 1.684 mm in left–right, anterior–posterior, and superior–inferior directions, respectively. The performance comparison with other 2 similar systems suggested that the method had a better positioning accuracy for pelvic tumor location.

Conclusion:

By effectively decreasing an interfraction uncertainty source (mismatch error between patient surface and immobilization system) in radiotherapy, the method can improve patient positioning precision for pelvic tumor.

Investigation of the radiation dose from cone-beam CT for image-guided radiotherapy: A comparison of methodologies

Four methodologies were evaluated for quantifying kilovoltage cone‐beam computed tomography (CBCT) dose: the Cone‐Beam Dose Index (CBDI), IAEA Report 5 recommended methodology (IAEA), the AAPM Task Group 111 methodology (TG111), and the current dose metric; the Computed Tomography Dose Index (CTDI) on two commercial Varian cone‐beam CT imaging systems; the Clinac iX On‐Board Imager (OBI); and the TrueBeam X‐ray Imaging system (XI). The TG111 methodology measured the highest overall dose (21.199 ± 0.035 mGy OBI and 22.420 ± 0.002 XI for pelvis imaging) due to the full scatter of the TG111 phantom and was within 5% of CTDI measurements taken using a full scatter TG111 phantom and 30‐cm film strips. CBDI measured the second highest overall dose, within 10% of the TG111, with IAEA measuring the third highest dose. For head CBCT protocols, CBDI measured the highest dose, followed by IAEA. The CTDI method measured lowest across all scan modes highlighting its limitations for CBCT dosimetry. The XI imaging system delivered lower doses for head and thorax scan modes and similar doses to the OBI system for pelvis scan modes due to additional beam hardening filtration in the XI system. The TG111 method measured the highest dose in the center of a CBCT scan during image guidance procedures; however, CBDI provided a good approximation to TG111 with existing CTDI equipment and may be more applicable clinically.

Magnetic resonance imaging in precision radiation therapy for lung cancer

Radiotherapy remains the cornerstone of curative treatment for inoperable locally advanced lung cancer, given concomitantly with platinum-based chemotherapy. With poor overall survival, research efforts continue to explore whether integration of advanced radiation techniques will assist safe treatment intensification with the potential for improving outcomes. One advance is the integration of magnetic resonance imaging (MRI) in the treatment pathway, providing anatomical and functional information with excellent soft tissue contrast without exposure of the patient to radiation. MRI may complement or improve the diagnostic staging accuracy of F-18 fluorodeoxyglucose position emission tomography and computerized tomography imaging, particularly in assessing local tumour invasion and is also effective for identification of nodal and distant metastatic disease. Incorporating anatomical MRI sequences into lung radiotherapy treatment planning is a novel application and may improve target volume and organs at risk delineation reproducibility. Furthermore, functional MRI may facilitate dose painting for heterogeneous target volumes and prediction of normal tissue toxicity to guide adaptive strategies. MRI sequences are rapidly developing and although the issue of intra-thoracic motion has historically hindered the quality of MRI due to the effect of motion, progress is being made in this field. Four-dimensional MRI has the potential to complement or supersede 4D CT and 4D F-18-FDG PET, by providing superior spatial resolution. A number of MR-guided radiotherapy delivery units are now available, combining a radiotherapy delivery machine (linear accelerator or cobalt-60 unit) with MRI at varying magnetic field strengths. This novel hybrid technology is evolving with many technical challenges to overcome. It is anticipated that the clinical benefits of MR-guided radiotherapy will be derived from the ability to adapt treatment on the fly for each fraction and in real-time, using 'beam-on' imaging. The lung tumour site group of the Atlantic MR-Linac consortium is working to generate a challenging MR-guided adaptive workflow for multi-institution treatment intensification trials in this patient group.

A Projection Quality-Driven Tube Current Modulation Method in Cone-Beam CT for IGRT: Proof of Concept

Purpose:

To develop a projection quality-driven tube current modulation method in cone-beam computed tomography for image-guided radiotherapy based on the prior attenuation information obtained by the planning computed tomography and then evaluate its effect on a reduction in the imaging dose.

Materials and Methods:

The QCKV-1 phantom with different thicknesses (0-400 mm) of solid water upon it was used to simulate different attenuation (μ). Projections were acquired with a series of tube current–exposure time product (mAs) settings, and a 2-dimensional contrast to noise ratio was analyzed for each projection to create a lookup table of mAs versus 2-dimensional contrast to noise ratio, μ. Before a patient underwent computed tomography, the maximum attenuation μmaxθ within the 95% range of each projection angle (θ) was estimated according to the planning computed tomography images. Then, a desired 2-dimensional contrast to noise ratio value was selected, and the mAs setting at θ was calculated with the lookup table of mAs versus 2-dimensional contrast to noise ratio,μmaxθ. Three-dimensional cone-beam computed tomography images were reconstructed using the projections acquired with the selected mAs. The imaging dose was evaluated with a polymethyl methacrylate dosimetry phantom in terms of volume computed tomography dose index. Image quality was analyzed using a Catphan 503 phantom with an oval body annulus and a pelvis phantom.

Results:

For the Catphan 503 phantom, the cone-beam computed tomography image obtained by the projection quality-driven tube current modulation method had a similar quality to that of conventional cone-beam computed tomography . However, the proposed method could reduce the imaging dose by 16% to 33% to achieve an equivalent contrast to noise ratio value. For the pelvis phantom, the structural similarity index was 0.992 with a dose reduction of 39.7% for the projection quality-driven tube current modulation method.

Conclusions:

The proposed method could reduce the additional dose to the patient while not degrading the image quality for cone-beam computed tomography. The projection quality-driven tube current modulation method could be especially beneficial to patients who undergo cone-beam computed tomography frequently during a treatment course.

Phantom-based evaluation of dose exposure of ultrafast combined kV-MV-CBCT towards clinical implementation for IGRT of lung cancer

PURPOSE

Combined ultrafast 90°+90° kV-MV-CBCT within single breath-hold of 15s has high clinical potential for accelerating imaging for lung cancer patients treated with deep inspiration breath-hold (DIBH). For clinical feasibility of kV-MV-CBCT, dose exposure has to be small compared to prescribed dose. In this study, kV-MV dose output is evaluated and compared to clinically-established kV-CBCT.

METHODS

Accurate dose calibration was performed for kV and MV energy; beam quality was determined. For direct comparison of MV and kV dose output, relative biological effectiveness (RBE) was considered. CT dose index (CTDI) was determined and measurements in various representative locations of an inhomogeneous thorax phantom were performed to simulate the patient situation.

RESULTS

A measured dose of 20.5mGE (Gray-equivalent) in the target region was comparable to kV-CBCT (31.2mGy for widely-used, and 9.1mGy for latest available preset), whereas kV-MV spared healthy tissue and reduced dose to 6.6mGE (30%) due to asymmetric dose distribution. The measured weighted CTDI of 12mGE for kV-MV lay in between both clinical presets.

CONCLUSIONS

Dosimetric properties were in agreement with established imaging techniques, whereas exposure to healthy tissue was reduced. By reducing the imaging time to a single breath-hold of 15s, ultrafast combined kV-MV CBCT shortens patient time at the treatment couch and thus improves patient comfort. It is therefore usable for imaging of hypofractionated lung DIBH patients.

Pelvic bone anatomy vs implanted gold seed marker registration for image-guided intensity modulated radiotherapy for prostate carcinoma: Comparative analysis of inter-fraction motion and toxicities

OBJECTIVES

We compared the prostate motion variability and toxicities between patients treated with gold marker registration based IG-IMRT (IG-IMRT-M) and bony landmark registration based IG-IMRT (IG-IMRT-B).

METHODS

T1c-T3b (node negative), intermediate and high risk (non-metastatic) adenocarcinoma of prostate, age ≥18years, Karnofsky Performance Status of ≥70 were included in this retrospective study. The prostate motion variability, acute and late radiation toxicities between the two treatment arms (IG-IMRT-M versus IG-IMRT-B) were compared.

RESULTS

Total of 35 patients (17 for IG-IMRT-M and 18 for IG-IMRT-B) were treated with a median radiotherapy dose of 76 Gray. The prostate variability observed with and without markers in millimeter was 4.1±2.3 vs 3.7±2.1 [Antero-Posterior (A-P); p=0.001], 2.3±1.5 vs 2.1±1.2 [Superior-Inferior (S-I); p=0.095] and 1.1±1.7 vs 0.4±1.4 [Left-Right (L-R); p=0.003]. There was higher acute toxicity in IG-IMRT-B arm compared to IG-IMRT-M arm in terms of grade ≥2 diarrhea [50% vs 11% OR=7.5 (1.3-42.7); p=0.02] and grade ≥2 proctitis [38% vs 5.8%, OR=10.1 (1.09-94.1); p=0.04]. At a median follow up of 36months, the late genitourinary toxicities grade ≥2 [27% vs 0%; p=0.04] were higher in the IG-IMRT-B arm compared to IG-IMRT-M arm.

CONCLUSIONS

IG-IMRT-M detects higher prostate motion variability as compared to IG-IMRT-B, inferring a significant prostate motion inside fixed pelvic bony cavity. The addition of marker based image guidance results in higher precision of prostate localization and lesser acute and late toxicities.

Daily CT guidance improves target coverage during definitive radiation therapy for gastric MALT lymphoma

PURPOSE

Radiation therapy (RT) for gastric mucosa-associated lymphoid tissue (MALT) lymphoma is challenging because of variation in the stomach's position, size, and shape. We investigated the interfractional changes in stomach location, consequent dosimetric effects, and impact of daily computed tomography image guidance RT (CT-IGRT).

METHODS AND MATERIALS

Twelve patients treated for gastric MALT lymphoma with intensity modulated radiation therapy, using a breath-hold technique and restriction of oral intake, were studied retrospectively. The planning target volume (PTV) comprised a 0.5 to 1.0 cm expansion of the stomach. The prescription dose was 30 Gy in 15 to 20 fractions. CT-IGRT was performed daily using CT-on-Rails. Dosimetry was calculated on 229 daily CT images after bony versus CT-based soft tissue alignment, and doses delivered to the target and adjacent structures were compared with the treatment plan. Target coverage was expressed as the percent of the clinical target volume (CTV) and PTV receiving ≥95% of the prescribed dose (V95%).

RESULTS

The average change in stomach volume was -12.4% (range, -47.6% to 38.6%). The average shift required for target coverage was 1.0 cm (maximum, 2.2 cm). With CT-based alignment to the stomach, the average V95% was 98.5% for CTV and 94.9% for PTV; with bony alignment, these values were 94.5% and 90.4%, respectively (P < .01 for CTV and PTV). With bony alignment, the PTV V95% was ≤90% in 4 patients (33%) over the course of treatment and was as low as 72.5% for 1 fraction. The kidney position varied with respect to the stomach and bony anatomy. Consequently, the dose to the left kidney was higher based on daily CT scans than on planning scans. Dose to other organs at risk did not vary significantly.

CONCLUSIONS

Substantial interfractional variation in stomach volume was observed, despite treatment with breath-hold and restriction of oral intake. Daily CT-IGRT improved target coverage, enabling excellent coverage despite the use of small PTV margins.

Interobserver variability of patient positioning using four different CT datasets for image registration in lung stereotactic body radiotherapy

PURPOSE

To assess the impact of different reference CT datasets on manual image registration with free-breathing three-dimensional (3D) cone beam CTs (FB-CBCT) for patient positioning by several observers.

METHODS

For 48 patients with lung lesions, manual image registration with FB-CBCTs was performed by four observers. A slow planning CT (PCT), average intensity projection (AIP), maximum intensity projection (MIP), and midventilation CT (MidV) were used as reference images. Couch shift differences between the four reference CT datasets for each observer as well as shift differences between the observers for the same reference CT dataset were determined. Statistical analyses were performed and correlations between the registration differences and the 3D tumor motion and the CBCT score were calculated.

RESULTS

The mean 3D shift difference between different reference CT datasets was the smallest for AIPvsMIP (range 1.1-2.2 mm) and the largest for MidVvsPCT (2.8-3.5 mm) with differences >10 mm. The 3D shifts showed partially significant correlations to 3D tumor motion and CBCT score. The interobserver comparison for the same reference CTs resulted in the smallest ∆3D mean differences and mean ∆3D standard deviation for ∆AIP (1.5 ± 0.7 mm, 0.7 ± 0.4 mm). The maximal 3D shift difference between observers was 10.4 mm (∆MidV). Both 3D tumor motion and mean CBCT score correlated with the shift differences (R_s = 0.336-0.740).

CONCLUSION

The applied reference CT dataset impacts image registration and causes interobserver variabilities. The 3D tumor motion and CBCT quality affect shift differences. The smallest differences were found for AIP which might be the most appropriate CT dataset for image registration with FB-CBCT.

Deep inspiration breath-hold radiotherapy for lung cancer: impact on image quality and registration uncertainty in cone beam CT image guidance

OBJECTIVE

We investigated the impact of deep inspiration breath-hold (DIBH) and tumour baseline shifts on image quality and registration uncertainty in image-guided DIBH radiotherapy (RT) for locally advanced lung cancer.

METHODS

Patients treated with daily cone beam CT (CBCT)-guided free-breathing (FB) RT had an additional CBCT in DIBH at three fractions. These CBCT scans were offline rigidly registered (on tumour) to FB and DIBH CT scans acquired at planning. All registrations were repeated to evaluate the intraobserver uncertainty. CBCT scans were scored on degree of streak artefacts and visualization of tumour and anatomical structures. We examined the impact of tumour baseline shift between consecutive DIBHs on CBCT image quality.

RESULTS

CBCT scans from 15 patients were analysed. Intraobserver image registration uncertainty was approximately 2 mm in both FB and DIBH, except for the craniocaudal direction in FB, where it was >3 mm. On the 31st fraction, the intraobserver uncertainty increased compared with the second fraction. This increase was more pronounced in FB. Image quality scores improved in DIBH compared with FB for all parameters in all patients. Simulated tumour baseline shifts ≤2 mm did not affect the CBCT image quality considerably.

CONCLUSION

DIBH CBCT improved image quality and reduced registration uncertainty in the craniocaudal direction in image-guided RT of locally advanced lung cancer. Baseline shifts ≤2 mm in DIBH during CBCT acquisition did not affect image quality. Advances in knowledge: DIBH RT has dosimetric advantages over FB; this work demonstrates an additional benefit of DIBH in terms of registration accuracy because of improved image quality.

Initial results for patient setup verification using transperineal ultrasound and cone beam CT in external beam radiation therapy of prostate cancer

BACKGROUND

Evaluation of set up error detection by a transperineal ultrasound in comparison with a cone beam CT (CBCT) based system in external beam radiation therapy (EBRT) of prostate cancer.

METHODS

Setup verification was performed with transperineal ultrasound (TPUS) and CBCT for 10 patients treated with EBRT for prostate cancer. In total, 150 ultrasound and CBCT scans were acquired in rapid succession and analyzed for setup errors. The deviation between setup errors of the two modalities was evaluated separately for each dimension.

RESULTS

A moderate correlation in lateral, vertical and longitudinal direction was observed comparing the setup errors. Mean differences between TPUS and CBCT were (-2.7 ± 2.3) mm, (3.0 ± 2.4) mm and (3.2 ± 2.7) mm in lateral, vertical and longitudinal direction, respectively. The mean Euclidean difference between TPUS and CBCT was (6.0 ± 3.1) mm. Differences up to 19.2 mm were observed between the two imaging modalities. Discrepancies between TPUS and CBCT of at least 5 mm occurred in 58 % of monitored treatment sessions.

CONCLUSION

Setup differences between TPUS and CBCT are 6 mm on average. Although the correlation of the setup errors determined by the two different image modalities is rather week, the combination of setup verification by CBCT and intrafraction motion monitoring by TPUS imaging can use the benefits of both imaging modalities.

Radiation Oncology--New Approaches in Squamous Cell Cancer of the Head and Neck

The many advances in radiotherapy for squamous cell cancer of the head and neck described in this article will have significant effects on the ultimate outcomes of patients who receive this treatment. The technological and clinical advances should allow one to maintain or improve disease control, while moderating the toxicity associated with head and neck radiation therapy.

Analysis of prostate bed motion using an endorectal balloon and cone beam computed tomography during postprostatectomy radiotherapy

Background

The authors conducted this prospective study to analyze the amount of interfractional prostate bed motion (PBM) and quantify its components with the use of an endorectal balloon (ERB).

Methods

A total of 1,348 cone beam computed tomography images from 46 patients who underwent postprostatectomy radiotherapy were analyzed. For the pilot image, electronic portal imaging, guided by skin marks was performed to ensure proper positioning and inflation of the ERB. Then, for bone matching, manual or automatic registration of the planning and each cone beam computed tomography was performed, based on the bony anatomy of the pelvis. Shifts (bony misalignment [BM]) in three directions were recorded at each treatment session. For prostate bed matching, manual matching was conducted based on the anterior rectal wall and the shift (PBM) was recorded. Total setup error was defined as the shift from the skin mark to the prostate bed matching, based on anterior rectal wall stretched by the ERB. PBM was defined as the difference between the total setup error and BM.

Results

Systematic errors for the total setup error were 1.0, 1.3, and 1.0 mm in the right–left, anterior–posterior, and superior–inferior directions, with random errors of 1.9, 2.4, and 1.9 mm, respectively. Systematic errors were 1.6, 1.6, and 0.3 mm for BM and 0.8, 1.1, and 0.9 mm for PBM, with random errors of 2.4, 2.5, and 1.1 mm for BM and 1.8, 2.2, and 1.9 mm for PBM.

Conclusion

The BM was the main component of the total setup error, suggesting that interfractional PBM was well controlled by the ERB device. Planning target volume margins of <5 mm were needed to include 95% of the interfractional variations when using an ERB.

Improving Image Quality of On-Board Cone-Beam CT in Radiation Therapy Using Image Information Provided by Planning Multi-Detector CT: A Phantom Study

Purpose

The aim of this study was to improve the image quality of cone-beam computed tomography (CBCT) mounted on the gantry of a linear accelerator used in radiation therapy based on the image information provided by planning multi-detector CT (MDCT).

Methods

MDCT-based shading correction for CBCT and virtual monochromatic CT (VMCT) synthesized using the dual-energy method were performed. In VMCT, the high-energy data were obtained from CBCT, while the low-energy data were obtained from MDCT. An electron density phantom was used to investigate the efficacy of shading correction and VMCT on improving the target detectability, Hounsfield unit (HU) accuracy and variation, which were quantified by calculating the contrast-to-noise ratio (CNR), the percent difference (%Diff) and the standard deviation of the CT numbers for tissue equivalent background material, respectively. Treatment plan studies for a chest phantom were conducted to investigate the effects of image quality improvement on dose planning.

Results

For the electron density phantom, the mean value of CNR was 17.84, 26.78 and 34.31 in CBCT, shading-corrected CBCT and VMCT, respectively. The mean value of %Diff was 152.67%, 11.93% and 7.66% in CBCT, shading-corrected CBCT and VMCT, respectively. The standard deviation within a uniform background of CBCT, shading-corrected CBCT and VMCT was 85, 23 and 15 HU, respectively. With regards to the chest phantom, the monitor unit (MU) difference between the treatment plan calculated using MDCT and those based on CBCT, shading corrected CBCT and VMCT was 6.32%, 1.05% and 0.94%, respectively.

Conclusions

Enhancement of image quality in on-board CBCT can contribute to daily patient setup and adaptive dose delivery, thus enabling higher confidence in patient treatment accuracy in radiation therapy. Based on our results, VMCT has the highest image quality, followed by the shading corrected CBCT and the original CBCT. The research results presented in this study should be able to provide a route to reach a high level of image quality for CBCT imaging in radiation oncology.

Comparison of setup accuracy of three different image assessment methods for tangential breast radiotherapy

Introduction

To compare the differences in setup errors measured with electronic portal image (EPI) and cone‐beam computed tomography (CBCT) in patients undergoing tangential breast radiotherapy (RT). Relationship between setup errors, body mass index (BMI) and breast size was assessed.

Methods

Twenty‐five patients undergoing postoperative RT to the breast were consented for this study. Weekly CBCT scans were acquired and retrospectively registered to the planning CT in three dimensions, first using bony anatomy for bony registration (CBCT‐B) and again using breast tissue outline for soft tissue registration (CBCT‐S). Digitally reconstructed radiographs (DRR) generated from CBCT to simulate EPI were compared to the planning DRR using bony anatomy in the V (parallel to the cranio‐caudal axis) and U (perpendicular to V) planes. The systematic (Σ) and random (σ) errors were calculated and correlated with BMI and breast size.

Results

The systematic and random errors for EPI (Σ_V = 3.7 mm, Σ_U = 2.8 mm and σ_V = 2.9 mm, σ_U = 2.5) and CBCT‐B (Σ_V = 3.5 mm, Σ_U = 3.4 mm and σ_V = 2.8 mm, σ_U = 2.8) were of similar magnitude in the V and U planes. Similarly, the differences in setup errors for CBCT‐B and CBCT‐S in three dimensions were less than 1 mm. Only CBCT‐S setup error correlated with BMI and breast size.

Conclusions

CBCT and EPI show insignificant variation in their ability to detect setup error. These findings suggest no significant differences that would make one modality considered superior over the other and EPI should remain the standard of care for most patients. However, there is a correlation with breast size, BMI and setup error as detected by CBCT‐S, justifying the use of CBCT‐S for larger patients.

One-year analysis of Elekta CBCT image quality using NPS and MTF

The image quality (IQ) of imaging systems must be sufficiently high for image-guided radiation therapy (IGRT). Hence, users should implement a quality assurance program to maintain IQ. In our routine IQ tests of the kV cone-beam CT system (Elekta XVI), image noise was quantified by noise standard deviation (NSD), which was the standard deviation of CT numbers measured in a small area in an image of an IQ test phantom (Catphan), and the high spatial resolution (HSR) was evaluated by the number of line-pairs (LPN) visually recognizable in the image. We also measured the image uniformity, the low contrast resolution, and the distances of two points for geometrical accuracy. For this study, we did an additional evaluation of the XVI data for 12 monthly IQ tests by using noise power spectrum (NPS) for noise, modulation transfer function (MTF) for HSR, and CT number-to-density relationship. NPS was obtained by applying Fourier analysis in a small area on the uniformity test section of Catphan. The MTF analysis was performed by applying the Droege-Morin (D-M) method to the line-pair bar regions in the phantom. The CT number-to-density relationship was obtained for insert materials in the low-contrast test section of the phantom. All the quantities showed a noticeable change over the one-year period. Especially the noise level improved significantly after a repair of the imager. NPS was more sensitive to the IQ change than NSD. MTF could provide more quantitative and objective evaluation of HSR. The CT number was very different from the expected CT number, but the CT number-to-density curves were constant within 5% except for two months. Since the D-M method is easy to implement, we recommend using MTF instead of LPN even for routine QA. The IQ of the imaging systems was constantly changing; hence, IQ tests should be periodically performed. Additionally, we found the importance of IQ tests after every service work, including detector calibration as well as preventive maintenance.

Quantification and Assessment of Interfraction Setup Errors Based on Cone Beam CT and Determination of Safety Margins for Radiotherapy

Introduction

To quantify interfraction patient setup-errors for radiotherapy based on cone-beam computed tomography and suggest safety margins accordingly.

Material and Methods

Positioning vectors of pre-treatment cone-beam computed tomography for different treatment sites were collected (n = 9504). For each patient group the total average and standard deviation were calculated and the overall mean, systematic and random errors as well as safety margins were determined.

Results

The systematic (and random errors) in the superior-inferior, left-right and anterior-posterior directions were: for prostate, 2.5(3.0), 2.6(3.9) and 2.9(3.9)mm; for prostate bed, 1.7(2.0), 2.2(3.6) and 2.6(3.1)mm; for cervix, 2.8(3.4), 2.3(4.6) and 3.2(3.9)mm; for rectum, 1.6(3.1), 2.1(2.9) and 2.5(3.8)mm; for anal, 1.7(3.7), 2.1(5.1) and 2.5(4.8)mm; for head and neck, 1.9(2.3), 1.4(2.0) and 1.7(2.2)mm; for brain, 1.0(1.5), 1.1(1.4) and 1.0(1.1)mm; and for mediastinum, 3.3(4.6), 2.6(3.7) and 3.5(4.0)mm. The CTV-to-PTV margins had the smallest value for brain (3.6, 3.7 and 3.3mm) and the largest for mediastinum (11.5, 9.1 and 11.6mm). For pelvic treatments the means (and standard deviations) were 7.3 (1.6), 8.5 (0.8) and 9.6 (0.8)mm.

Conclusions

Systematic and random setup-errors were smaller than 5mm. The largest errors were found for organs with higher motion probability. The suggested safety margins were comparable to published values in previous but often smaller studies.

Analysis of Dosimetric Impacts of Cone Beam Computed Tomography-Based Volumetric Modulated Arc Therapy Planning

OBJECTIVE

To quantify the Hounsfield unit (HU) variations between computed tomography (CT) and cone beam CT (CBCT) and study its impact on volumetric modulated arc therapy (VMAT) plans.

METHODS

HU number variations in CT and CBCT images were evaluated using the Catphan-504 phantom, and changes in seven different materials within the phantom (air, polymethylpentene, low-density polyethylene, polystyrene, acrylic, Delrin, and Teflon) were studied. The HU variations in half-fan and full-fan modes of CBCT were evaluated. The effect of variations in the shape of the body cross sections was assessed by reducing the body of the Catphan by 0.5 cm and 1.0 cm. CBCT-based VMAT plans in 27 patients (10 prostate, 10 brain, and 7 head and neck (HN)) were compared with corresponding CT-based plans. The dosimetric variations were assessed referring to different points on the dose volume histogram (D_5%, D_50%, and D_95% for PTVs and D_1%, D_max, and D_mean for organs at risk). The relative percentage of difference (ΔD (%)) between CT- and CBCT-based VMAT plans were examined on these points. To evaluate the dosimetric accuracy, dose distributions were compared using Omnipro-I'mRT software. The VMAT plans were evaluated based on 3 mm-3%, 2 mm-2%, and 1 mm-1% gamma criteria.

RESULTS

The HU difference in CT and CBCT was highest for air, Delrin, and Teflon, whereas the difference was less than 20 HU for the other materials. The dose volume histograms of both CT- and CBCT-based plans were in excellent agreement in both phantom and patients, except in HN cases where the difference was 7%. The average 3 mm-3% gamma pass points in brain, prostate, and HN patients were 97 ± 0.2%, 96 ± 0.06%, and 93.3 ± 1.1%, respectively. The gamma pass rates reduced to 88.8 ± 0.06%, 91 ± 0.04%, and 79 ± 6% in 2 mm-2%, and further declined to 76.6 ± 0.09%, 75.2 ± 0.5%, and 60 ± 6% using the stringent 1 mm-1% gamma criteria for brain, prostate, and HN cases, respectively.

CONCLUSION

Based on the results of this study, it is our belief that CBCT images can be used as a tool for evaluating the dosimetric variation in patient VMAT plans.

Towards clinical implementation of ultrafast combined kV-MV CBCT for IGRT of lung cancer : Evaluation of registration accuracy based on phantom study

PURPOSE

Combined kV-MV cone-beam CT (CBCT) is a promising approach to accelerate imaging for patients with lung tumors treated with deep inspiration breath-hold. During a single breath-hold (15 s), a 3D kV-MV CBCT can be acquired, thus minimizing motion artifacts and increasing patient comfort. Prior to clinical implementation, positioning accuracy was evaluated and compared to clinically established imaging techniques.

METHODS AND MATERIALS

An inhomogeneous thorax phantom with four tumor-mimicking inlays was imaged in 10 predefined positions and registered to a planning CT. Novel kV-MV CBCT imaging (90° arc) was compared to clinically established kV-chest CBCT (360°) as well as nonclinical kV-CBCT and low-dose MV-CBCT (each 180°). Manual registration, automatic registration provided by the manufacturer and an additional in-house developed manufacturer-independent framework based on the MATLAB registration toolkit were applied.

RESULTS

Systematic setup error was reduced to 0.05 mm by high-precision phantom positioning with optical tracking. Stochastic mean displacement errors were 0.5 ± 0.3 mm in right-left, 0.4  ± 0.4 mm in anteroposterior and 0.0 ± 0.4 mm in craniocaudal directions for kV-MV CBCT with manual registration (maximum errors of no more than 1.4 mm). Clinical kV-chest CBCT resulted in mean errors of 0.2 mm (other modalities: 0.4-0.8 mm). Similar results were achieved with both automatic registration methods.

CONCLUSION

The comparison study of repositioning accuracy between novel kV-MV CBCT and clinically established volume imaging demonstrated that registration accuracy is maintained below 1 mm. Since imaging time is reduced to one breath-hold, kV-MV CBCT is ideal for image guidance, e.g., in lung stereotactic ablative radiotherapy.

Exposure Risks Among Children Undergoing Radiation Therapy: Considerations in the Era of Image Guided Radiation Therapy

Recent improvements in toxicity profiles of pediatric oncology patients are attributable, in part, to advances in the field of radiation oncology such as intensity modulated radiation (IMRT) and proton therapy (IMPT). While IMRT and IMPT deliver highly conformal dose to targeted volumes, they commonly demand the addition of 2- or 3-dimensional imaging for precise positioning--a technique known as image guided radiation therapy (IGRT). In this manuscript we address strategies to further minimize exposure risk in children by reducing effective IGRT dose. Portal X rays and cone beam computed tomography (CBCT) are commonly used to verify patient position during IGRT and, because their relative radiation exposure is far less than the radiation absorbed from therapeutic treatment beams, their sometimes significant contribution to cumulative risk can be easily overlooked. Optimizing the conformality of IMRT/IMPT while simultaneously ignoring IGRT dose may result in organs at risk being exposed to a greater proportion of radiation from IGRT than from therapeutic beams. Over a treatment course, cumulative central-axis CBCT effective dose can approach or supersede the amount of radiation absorbed from a single treatment fraction, a theoretical increase of 3% to 5% in mutagenic risk. In select scenarios, this may result in the underprediction of acute and late toxicity risk (such as azoospermia, ovarian dysfunction, or increased lifetime mutagenic risk) in radiation-sensitive organs and patients. Although dependent on variables such as patient age, gender, weight, body habitus, anatomic location, and dose-toxicity thresholds, modifying IGRT use and acquisition parameters such as frequency, imaging modality, beam energy, current, voltage, rotational degree, collimation, field size, reconstruction algorithm, and documentation can reduce exposure, avoid unnecessary toxicity, and achieve doses as low as reasonably achievable, promoting a culture and practice of "gentle IGRT."

Deep Inspiration Breath Hold-Based Radiation Therapy: A Clinical Review

Several recent developments in linear accelerator-based radiation therapy (RT) such as fast multileaf collimators, accelerated intensity modulation paradigms like volumeric modulated arc therapy and flattening filter-free (FFF) high-dose-rate therapy have dramatically shortened the duration of treatment fractions. Deliverable photon dose distributions have approached physical complexity limits as a consequence of precise dose calculation algorithms and online 3-dimensional image guided patient positioning (image guided RT). Simultaneously, beam quality and treatment speed have continuously been improved in particle beam therapy, especially for scanned particle beams. Applying complex treatment plans with steep dose gradients requires strategies to mitigate and compensate for motion effects in general, particularly breathing motion. Intrafractional breathing-related motion results in uncertainties in dose delivery and thus in target coverage. As a consequence, generous margins have been used, which, in turn, increases exposure to organs at risk. Particle therapy, particularly with scanned beams, poses additional problems such as interplay effects and range uncertainties. Among advanced strategies to compensate breathing motion such as beam gating and tracking, deep inspiration breath hold (DIBH) gating is particularly advantageous in several respects, not only for hypofractionated, high single-dose stereotactic body RT of lung, liver, and upper abdominal lesions but also for normofractionated treatment of thoracic tumors such as lung cancer, mediastinal lymphomas, and breast cancer. This review provides an in-depth discussion of the rationale and technical implementation of DIBH gating for hypofractionated and normofractionated RT of intrathoracic and upper abdominal tumors in photon and proton RT.

Emerging Role of MRI for Radiation Treatment Planning in Lung Cancer

Magnetic resonance imaging (MRI) provides excellent soft-tissue contrast and allows for specific scanning sequences to optimize differentiation between various tissue types and properties. Moreover, it offers the potential for real-time motion imaging. This makes magnetic resonance imaging an ideal candidate imaging modality for radiation treatment planning in lung cancer. Although the number of clinical research protocols for the application of magnetic resonance imaging for lung cancer treatment is increasing (www.clinicaltrials.gov) and the magnetic resonance imaging sequences are becoming faster, there are still some technical challenges. This review describes the opportunities and challenges of magnetic resonance imaging for radiation treatment planning in lung cancer.

Image-guided intensity-modulated radiotherapy of prostate cancer: Analysis of interfractional errors and acute toxicity

Purpose

The aim of the study was to estimate interfractional deviations in patient and prostate position, the impact of the frequency of online verification on the treatment margins, and to assess acute radiation reactions of high-dose external beam image-guided intensity-modulated radiotherapy (IG-IMRT) of localized prostate cancer.

Patients and methods

IG-IMRT was performed by daily online verification of implanted fiducial prostate markers using a megavoltage electronic portal imaging device (EPID). A total of 1011 image-guided treatment fractions from 23 consecutive unselected prostate cancer patients were analyzed. The median total dose was 79.2 Gy (range 77.4–81.0 Gy). Acute radiation reactions were assessed weekly during radiotherapy using the Common Terminology Criteria for Adverse Events (CTCAE) v.4.03.

Results

A relevant combined patient set-up and prostate motion population random error of 4–5 mm was observed. Compared to daily IGRT, image guidance every other day required an expansion of the CTV–PTV (clinical target volume–planning target volume) margin of 8.1, 6.6, and 4.1 mm in the longitudinal, vertical, and lateral directions, thereby, increasing the PTV by approximately 30–40 %. No grade 3 or 4 acute radiation reactions were observed with daily IG-IMRT.

Conclusion

A high dose with surprisingly low acute toxicity can be applied with daily IG-IMRT using implanted fiducial prostate markers. Daily image guidance is clearly superior to image guidance every other fraction concerning adequate target coverage with minimal margins.

Set-up errors in head and neck cancer patients treated with intensity modulated radiation therapy: Quantitative comparison between three-dimensional cone-beam CT and two-dimensional kilovoltage images

OBJECTIVES

To compare the patient set-up error detection capabilities of three-dimensional cone beam computed tomography (3D-CBCT) and two-dimensional orthogonal kilovoltage (2D-kV) techniques.

METHODS

3D-CBCT and 2D-kV projections were acquired on 29 head-and-neck (H&N) patients undergoing Intensity Modulated Radiotherapy (IMRT) on the first day of treatment (time 0) and after the delivery of 40 Gy and 50 Gy. Set-up correction vectors were analyzed after fully automatic image registration as well as after revision by radiation oncologists. The dosimetric effects of the different sensitivities of the two image guidance techniques were assessed.

RESULTS

A statistically significant correlation among detected set-up deviations by the two techniques was found along anatomical axes (0.60 < ρ < 0.72, p < 0.0001); no correlation was found for table rotation (p = 0.41). No evidence of statistically significant differences between the indications provided along the course of the treatment was found; this was also the case when full automatic versus manually refined correction vectors were compared. The dosimetric effects analysis revealed slight statistically significant differences in the median values of the maximum relative dose to mandible, spinal cord and its 5 mm Planning Organ at Risk Volume (0.95%, 0.6% and 2.45%, respectively), with higher values (p < 0.01) observed when 2D-kV corrections were applied.

CONCLUSION

A similar sensitivity to linear set-up errors was observed for 2D-kV and 3D-CBCT image guidance techniques in our H&N patient cohort. Higher rotational deviations around the table vertical axis were detected by the 3D-CBCT with respect to the 2D-kV method, leading to a consistent better sparing of organs at risk.

Different setup errors assessed by weekly cone-beam computed tomography on different registration in nasopharyngeal carcinoma treated with intensity-modulated radiation therapy

The study aimed to investigate the difference of setup errors on different registration in the treatment of nasopharyngeal carcinoma based on weekly cone-beam computed tomography (CBCT). Thirty nasopharyngeal cancer patients scheduled to undergo intensity-modulated radiotherapy (IMRT) were prospectively enrolled in the study. Each patient had a weekly CBCT before radiation therapy. In the entire study, 201 CBCT scans were obtained. The scans were registered to the planning CT to determine the difference of setup errors on different registration sites. Different registration sites were represented by bony landmarks. Nasal septum and pterygoid process represent head, cervical vertebrae 1–3 represent upper neck, and cervical vertebrae 4–6 represent lower neck. Patient positioning errors were recorded in the right–left (RL), superior–inferior (SI), and anterior–posterior (AP) directions over the course of radiotherapy. Planning target volume margins were calculated from the systematic and random errors. In this study, we can make a conclusion that there are setup errors in RL, SI, and AP directions of nasopharyngeal carcinoma patients undergoing IMRT. In addition, the head and neck setup error has the difference, with statistical significance, while patient setup error of neck is greater than that of head during the course of radiotherapy. In our institution, we recommend a planning target volume margin of 3.0 mm in RL direction, 1.3 mm in SI direction, and 2.6 mm in AP direction for nasopharyngeal cancer patients undergoing IMRT with weekly CBCT scans.

Image-guided radiation therapy in lymphoma management

Image-guided radiation therapy (IGRT) is a process of incorporating imaging techniques such as computed tomography (CT), magnetic resonance imaging (MRI), Positron emission tomography (PET), and ultrasound (US) during radiation therapy (RT) to improve treatment accuracy. It allows real-time or near real-time visualization of anatomical information to ensure that the target is in its position as planned. In addition, changes in tumor volume and location due to organ motion during treatment can be also compensated. IGRT has been gaining popularity and acceptance rapidly in RT over the past 10 years, and many published data have been reported on prostate, bladder, head and neck, and gastrointestinal cancers. However, the role of IGRT in lymphoma management is not well defined as there are only very limited published data currently available. The scope of this paper is to review the current use of IGRT in the management of lymphoma. The technical and clinical aspects of IGRT, lymphoma imaging studies, the current role of IGRT in lymphoma management and future directions will be discussed.

Characterization of a new transmission detector for patient individualized online plan verification and its influence on 6MV X-ray beam characteristics

PURPOSE

Online verification and 3D dose reconstruction on daily patient anatomy have the potential to improve treatment delivery, accuracy and safety. One possible implementation is to recalculate dose based on online fluence measurements with a transmission detector (TD) attached to the linac. This study provides a detailed analysis of the influence of a new TD on treatment beam characteristics.

METHODS

The influence of the new TD on surface dose was evaluated by measurements with an Advanced Markus Chamber (Adv-MC) in the build-up region. Based on Monte Carlo simulations, correction factors were determined to scale down the over-response of the Adv-MC close to the surface. To analyze the effects beyond dmax percentage depth dose (PDD), lateral profiles and transmission measurements were performed. All measurements were carried out for various field sizes and different SSDs. Additionally, 5 IMRT-plans (head & neck, prostate, thorax) and 2 manually created test cases (3×3cm(2) fields with different dose levels, sweeping gap) were measured to investigate the influence of the TD on clinical treatment plans. To investigate the performance of the TD, dose linearity as well as dose rate dependency measurements were performed.

RESULTS

With the TD inside the beam an increase in surface dose was observed depending on SSD and field size (maximum of +11%, SSD = 80cm, field size = 30×30cm(2)). Beyond dmax the influence of the TD on PDDs was below 1%. The measurements showed that the transmission factor depends slightly on the field size (0.893-0.921 for 5×5cm(2) to 30×30cm(2)). However, the evaluation of clinical IMRT-plans measured with and without the TD showed good agreement after using a single transmission factor (γ(2%/2mm) > 97%, δ±3% >95%). Furthermore, the response of TD was found to be linear and dose rate independent (maximum difference <0.5% compared to reference measurements).

CONCLUSIONS

When placed in the path of the beam, the TD introduced a slight, clinically acceptable increase of the skin dose even for larger field sizes and smaller SSDs and the influence of the detector on the dose beyond dmax as well as on clinical IMRT-plans was negligible. Since there was no dose rate dependency and the response was linear, the device is therefore suitable for clinical use. Only its absorption has to be compensated during treatment planning, either by the use of a single transmission factor or by including the TD in the incident beam model.

Accelerated Partial Breast Irradiation in Clinical Practice

Accelerated partial breast irradiation (APBI) has been under clinical investigation for more than 15 years. There are several technical approaches that are clinically established, e.g. brachytherapy, intraoperative radiotherapy (IORT), or external-beam radiotherapy. The understanding of the underlying biology, optimal technical procedures, patient selection criteria, and imaging changes during follow-up has increased enormously. After completion of several phase III trials using brachytherapy or IORT, APBI is currently increasingly used either in phase IV studies, registries, or in selected patients outside of clinical studies. Consensus statements about suitable patients are available from several international and national societies like ASTRO, ESTRO, and DEGRO. One may expect that 15-25% of patients undergoing breast-conserving surgery may qualify for APBI, i.e. patients with small invasive ductal breast cancer without clinical lymph node involvement.

An Investigation of the Feasibility and Utility of a Low-dose Cone-beam Computed Tomography Scan Protocol for Head and Neck Cancer Patients

INTRODUCTION

Routine use of cone-beam computed tomography (CBCT) scan protocols as part of the image guidance process (image-guided radiation therapy) has become an integral part of the practice of radiation therapists (RTs). Concerns regarding imaging dose as well as increased in-room time for patients led to reluctance among site group members to adopt CBCT for all radical head and neck cancer (HNC) patients at our institution. This investigation set out to assess the feasibility and utility of a revised CBCT scan protocol with the aim of supporting daily CBCT for HNC patients receiving radiation therapy.

METHODS

The project was performed in three phases. Phase 1 involved the experimental adjustment of CBCT scan protocol parameters in clinical use for HNC patients at our institution. An Elekta Synergy linear accelerator with kilovoltage CBCT capability and a RANDO head phantom were used for scan acquisition procedures. Image registration using bony anatomy was performed on two image sets generated using the current clinical scan protocol (HNS20) and an experimental modified scan protocol (MHNS20). Image registration results were compared by two investigators. Measurements of scan doses using a metal-oxide-semiconductor field-effect transistor and a Unidose meter were performed. Catphan phantom images were acquired using HNS20 and MHNS20 protocols. In phase 2, ten volunteer RTs performed image registration and matching processes on two image sets performed using HNS20 and MHNS20 protocols. RTs were unaware of the scan protocols used for image acquisition. A threshold of 3 mm was set (the current maximum couch shift allowance in the clinical HNC IGRT protocol) to compare the image registration data from HNS20 and MHNS20. In phase 3, after research ethics board approval, 10 HNC patients consented to the study. Two pretreatment CBCT scans were performed: scan 1 was acquired using MHNS20 protocol, and scan 2 was acquired using the HNS20 protocol. A threshold of 2 mm was set to compare the differences in couch shift data resulting from the image registration of the two image sets. Comparison of HNS20 and MHNS20 based on image registration results was performed.

RESULTS

In phase 1, radiation doses measured by the investigators on the left optical lens using a metal-oxide-semiconductor field-effect transistor and a Unidose meter indicated that the MHNS20 protocol would result in a lower dose to the left optical lens. In phase 2, shifts of the treatment table to achieve the planned isocentre, which were recorded after the image matching process, were within 3 mm in 80% of the RT procedures. In the y-axis (superior/inferior direction), 100% of the procedures were within 3 mm. In the z-axis (anterior/posterior) and x-axis (lateral), 90% of the procedures were within 3 mm. Qualitative data from a questionnaire completed by RTs after the image matching indicated that 50% of the RTs had no preference between the images sets in terms of visibility of structures. Forty percent of RTs had no preference regarding speed of matching or preference for registration between the image sets. When a preference was indicated, the HNS20 scan protocol was chosen by the RTs. In phase 3, couch shift data recorded after each CBCT scan were compared. All results in all three planes for 10 patients included in the study were within the 2-mm threshold.

CONCLUSIONS

The feasibility and clinical utility of a potential lower-dose CBCT scan protocol has been investigated. The modified protocol (MHNS20) produced image data acceptable within current practice using bony anatomy for registration purposes. The MHNS20 protocol also delivered lower doses to the left optical lens and therefore potentially to other pertinent structures. The actual delivered doses to patients during IGRT procedures using the MHNS20 may be different than those measured during this investigation.

Consensus and differences in primary radiotherapy for localized and locally advanced prostate cancer in Switzerland: A survey on patterns of practice

INTRODUCTION

External beam radiotherapy (EBRT), with or without androgen deprivation therapy (ADT), is an established treatment option for nonmetastatic prostate cancer. Despite high-level evidence from several randomized trials, risk group stratification and treatment recommendations vary due to contradictory or inconclusive data, particularly with regard to EBRT dose prescription and ADT duration. Our aim was to investigate current patterns of practice in primary EBRT for prostate cancer in Switzerland.

MATERIALS AND METHODS

Treatment recommendations on EBRT and ADT for localized and locally advanced prostate cancer were collected from 23 Swiss radiation oncology centers. Written recommendations were converted into center-specific decision trees, and analyzed for consensus and differences using a dedicated software tool. Additionally, specific radiotherapy planning and delivery techniques from the participating centers were assessed.

RESULTS

The most commonly prescribed radiation dose was 78 Gy (range 70-80 Gy) across all risk groups. ADT was recommended for intermediate-risk patients for 6 months in over 80 % of the centers, and for high-risk patients for 2 or 3 years in over 90 % of centers. For recommendations on combined EBRT and ADT treatment, consensus levels did not exceed 39 % in any clinical scenario. Arc-based intensity-modulated radiotherapy (IMRT) is implemented for routine prostate cancer radiotherapy by 96 % of the centers.

CONCLUSION

Among Swiss radiation oncology centers, considerable ranges of radiotherapy dose and ADT duration are routinely offered for localized and locally advanced prostate cancer. In the vast majority of cases, doses and durations are within the range of those described in current evidence-based guidelines.

Are three doses of stereotactic ablative radiotherapy (SABR) more effective than 30 doses of conventional radiotherapy?

In early stage non-small cell lung cancer (NSCLC) definitive radiation therapy is an appropriate alternative to surgery. Recent studies show, that in such patients hypofractionation schedules (for example 3 times 18 Gy or 5 times 12 Gy), can be safely applied, without causing severe toxicities and achieving high local control rates of up to 90% and more. In the last couple of years a lot of knowledge about the cancer biology, technical aspects, clinical outcomes and toxicities has been accumulated from different clinical trials. The purpose of this review is to summarize recent outcomes and developments in stereotactic radiation therapy for patients with early stage NSCLC.

Feasibility study of automated framework for estimating lung tumor locations for target-based patient positioning in stereotactic body radiotherapy

Objective. To investigate the feasibility of an automated framework for estimating the lung tumor locations for tumor-based patient positioning with megavolt-cone-beam computed tomography (MV-CBCT) during stereotactic body radiotherapy (SBRT). Methods. A lung screening phantom and ten lung cancer cases with solid lung tumors, who were treated with SBRT, were employed to this study. The locations of tumors in MV-CBCT images were estimated using a tumor-template matching technique between a tumor template and the MV-CBCT. Tumor templates were produced by cropping the gross tumor volume (GTV) regions, which were enhanced by a Sobel filter or a blob structure enhancement (BSE) filter. Reference tumor locations (grand truth) were determined based on a consensus between a radiation oncologist and a medical physicist. Results. According to the results of the phantom study, the average Euclidean distances of the location errors in the original, Sobel-filtered, and BSE-filtered images were 2.0 ± 4.1 mm, 12.8 ± 9.4 mm, and 0.4 ± 0.5 mm, respectively. For clinical cases, these were 3.4 ± 7.1 mm, 7.2 ± 11.6 mm, and 1.6 ± 1.2 mm, respectively. Conclusion. The feasibility study suggests that our proposed framework based on the BSE filter may be a useful tool for tumor-based patient positioning in SBRT.

Dose-dependent changes in renal (1)H-/(23)Na MRI after adjuvant radiochemotherapy for gastric cancer

PURPOSE

Combined radiochemotherapy (RCT) for gastric cancer with three-dimensional conformal radiotherapy (3D-CRT) results in ablative doses to the upper left kidney, while image-guided intensity-modulated radiotherapy (IG-IMRT) allows kidney sparing despite improved target coverage. Renal function in long-term gastric cancer survivors was evaluated with 3T functional magnetic resonance imaging (MRI) including diffusion-weighted imaging (DWI) and (23)Na imaging.

PATIENTS AND METHODS

Five healthy volunteers and 13 patients after radiotherapy were included: 11×IG-IMRT; 1×3D-CRT; 1× "positive control" with stereotactic body radiotherapy (SBRT) of a metastasis between the spleen/left kidney. Radiation doses were documented for the upper/middle/lower kidney subvolumes. Late toxicity was evaluated based on CTC criteria, questionnaire, and creatinine values. Morphological sequences, DWI images, and (23)Na images were acquired using a (1)H/(23)Na-tuned body-coil before/after intravenous water load (WL). Statistics for [(23)Na] (concentration) and apparent diffusion coefficient (ADC) values were calculated for upper/middle/lower renal subvolumes. Corticomedullary [(23)Na] gradients and [(23)Na] differences after WL were determined.

RESULTS

No major morphological alteration was detected in any patient. Minor scars were observed in the cranial subvolume of the left kidney of the 3D-CRT and the whole kidney of the control SBRT patient. All participants presented a corticomedullary [(23)Na] gradient. After WL, a significant physiological [(23)Na] gradient decrease (p < 0.001) was observed in all HV and IG-IMRT patients. In the cranial left kidney of the 3D-CRT patient and the positive control SBRT patient, the decrease was nonsignificant (p = 0.01, p = 0.02). ADC values were altered nonsignificantly in all renal subvolumes (all participants). Renal subvolumes with doses ≥ 35 Gy showed a reduced change of the [(23)Na] gradient after WL (p = 0.043). No participants showed clinical renal impairment.

CONCLUSIONS

Functional parameters of renal (23)Na MRI after gastric IG-IMRT are identical to those of healthy volunteers, in contrast to renal subvolumes after ablative doses in the control and 3D-CRT patient. While kidney doses to the cortex below 20-25 Gy in fractional doses of ~ 1 Gy in IG-IMRT (combined with intensified chemotherapy) do not seem to cause significant MRI morphological or functional alterations, doses of > 35 Gy in 1.5-2 Gy fractions clearly result in impairment.