Use of planar kV vs. CBCT in evaluation of setup errors in oesophagus carcinoma radiotherapy

PTV margin calculation for head and neck patients treated with VMAT: a systematic literature review

Aim:

The intent of the review was to identify different methodological approaches used to calculate the planning target volume (PTV) margin for head and neck patients treated with volumetric arc therapy (VMAT), and whether the necessary factors to calculate the margin size with the selected formula were used.

Materials and Methods:

A comprehensive, systematic search of related studies was done using the Hydi search engine and different databases: MEDLINE, PubMed, CINAHL, ProQuest (Nursing and Allied Health), Scopus, ScienceDirect and tipsRO. The literature search included studies published between January 2007 and December 2020. Eligibility screening was performed by two reviewers.

Results:

A total of seven studies were found. All the reviewed studies used the Van Herk formula to measure the PTV margin. None of the studies incorporated the systematic errors of target volume delineation in the PTV equation. Inter-fraction translational errors were assessed in all the studies, whilst intra-fraction errors were only included in the margin equation for two studies. The studies showed great heterogeneity in the key characteristics, aims and methods.

Findings:

Since systemic errors from target volume delineation were not considered and not all studies assess intra-fraction errors, PTV margins may be underestimated. The recommendations are that studies need to determine the effect of target volume variance on PTV margins. It is also recommended to compare PTV margin results using various formulas.

Low dose cone beam CT for paediatric image-guided radiotherapy: Image quality and practical recommendations

PURPOSE

Cone beam CT (CBCT) is used in paediatric image-guided radiotherapy (IGRT) for patient setup and internal anatomy assessment. Adult CBCT protocols lead to excessive doses in children, increasing the risk of radiation-induced malignancies. Reducing imaging dose increases quantum noise, degrading image quality. Patient CBCTs also include 'anatomical noise' (e.g. motion artefacts), further degrading quality. We determine noise contributions in paediatric CBCT, recommending practical imaging protocols and thresholds above which increasing dose yields no improvement in image quality.

METHODS AND MATERIALS

Sixty CBCTs including the thorax or abdomen/pelvis from 7 paediatric patients (aged 6-13 years) were acquired at a range of doses and used to simulate lower dose scans, totalling 192 scans (0.5-12.8 mGy). Noise measured in corresponding regions of each patient and a 10-year-old phantom were compared, modelling total (including anatomical) noise, and quantum noise contributions as a function of dose. Contrast-to-noise ratio (CNR) was measured between fat/muscle. Soft tissue registration was performed on the kidneys, comparing accuracy to the highest dose scans.

RESULTS

Quantum noise contributed <20% to total noise in all cases, suggesting anatomical noise is the largest determinant of image quality in the abdominal/pelvic region. CNR exceeded 3 in over 90% of cases ≥ 1 mGy, and 57% of cases at 0.5 mGy. Soft tissue registration was accurate for doses > 1 mGy.

CONCLUSION

Anatomical noise dominates quantum noise in paediatric CBCT. Appropriate soft tissue contrast and registration accuracy can be achieved for doses as low as 1 mGy. Increasing dose above 1 mGy holds no benefit in improving image quality or registration accuracy due to the presence of anatomical noise.

Evaluation of Ultra-low-dose Paediatric Cone-beam Computed Tomography for Image-guided Radiotherapy

AIMS

In image-guided radiotherapy, daily cone-beam computed tomography (CBCT) is rarely applied to children due to concerns over imaging dose. Simulating low-dose CBCT can aid clinical protocol design by allowing visualisation of new scan protocols in patients without delivering additional dose. This work simulated ultra-low-dose CBCT and evaluated its use for paediatric image-guided radiotherapy by assessment of image registration accuracy and visual image quality.

MATERIALS AND METHODS

Ultra-low-dose CBCT was simulated by adding the appropriate amount of noise to projection images prior to reconstruction. This simulation was validated in phantoms before application to paediatric patient data. Scans from 20 patients acquired at our current clinical protocol (0.8 mGy) were simulated for a range of ultra-low doses (0.5, 0.4, 0.2 and 0.125 mGy) creating 100 scans in total. Automatic registration accuracy was assessed in all 100 scans. Inter-observer registration variation was next assessed for a subset of 40 scans (five scans at each simulated dose and 20 scans at the current clinical protocol). This subset was assessed for visual image quality by Likert scale grading of registration performance and visibility of target coverage, organs at risk, soft-tissue structures and bony anatomy.

RESULTS

Simulated and acquired phantom scans were in excellent agreement. For patient scans, bony atomy registration discrepancies for ultra-low-dose scans fell within 2 mm (translation) and 1° (rotation) compared with the current clinical protocol, with excellent inter-observer agreement. Soft-tissue registration showed large discrepancies. Bone visualisation and registration performance reached over 75% acceptability (rated 'well' or 'very well') down to the lowest doses. Soft-tissue visualisation did not reach this threshold for any dose.

CONCLUSION

Ultra-low-dose CBCT was accurately simulated and evaluated in patient data. Patient scans simulated down to 0.125 mGy were appropriate for bony anatomy set-up. The large dose reduction could allow for more frequent (e.g. daily) image guidance and, hence, more accurate set-up for paediatric radiotherapy.

Setup Error Assessment and Correction in Planar kV Image- Versus Cone Beam CT Image-Guided Radiation Therapy: A Clinical Study of Early Breast Cancer Treated With External Beam Partial Breast Irradiation

Objective:

To compare differences in setup error assessment and correction between planar kilovolt images and cone beam computed tomography images for external beam partial breast irradiation during free breathing.

Methods:

Nineteen patients who received external beam partial breast irradiation after breast-conserving surgery were recruited. Interfraction setup error was acquired using planar kilovolt images and cone beam computed tomography. After online setup correction, the residual error was calculated, and the setup error was compared. The residual error and setup margin were quantified for planar kilovolt and cone beam computed tomography images.

Results:

The largest setup error was observed in the anteroposterior direction for both cone beam computed tomography and planar kilovolt imaging (−1.45 mm, 1.74 mm). The cone beam computed tomography–based setup error (systematic error [Σ]) was less than the planar kilovolt images based on Σ in the anteroposterior direction (–1.2 mm vs 2.00 mm; P = .005), and no significant differences were observed for random error (σ) in 3 dimensions (P = .948, .376, .314). After online setup correction, cone beam computed tomography significantly reduced the residual setup error compared with planar kilovolt images in the anteroposterior direction (Σ: −0.20 mm vs 0.50 mm, P = .008; σ: 0.45 mm vs 1.34 mm, P = .002). The cone beam computed tomography–based setup margin was smaller than the planar kilovolt image-based setup margin in the anteroposterior direction (−1.39 mm vs 5.57 mm, P = .003; 0.00 mm vs 3.20 mm, P = .003).

Conclusions:

Discrepancy between the setup errors observed with planar kilovolt and cone beam computed tomography was obvious in the anteroposterior direction. Compared to cone beam computed tomography, the elapsed treatment time was smaller when the initial alignment used kilovolt planar imaging. Whether using planar kilovolt or cone beam computed tomography, residual errors can be reduced to 1.5 mm for external beam partial breast irradiation procedures.

Value of Three-Dimensional Imaging Systems for Image-Guided Carbon Ion Radiotherapy

Carbon ion radiotherapy (C-ion RT) allows excellent dose distribution because of the Bragg Peak. Compared with conventional radiotherapy, it delivers a higher dose with a smaller field. However, the dose distribution is sensitive to anatomical changes. Imaging technologies are necessary to reduce uncertainties during treatment, especially for hypofractionated and adaptive radiotherapy (ART). In-room computed tomography (CT) techniques, such as cone-beam CT (CBCT) and CT-on-rails are routinely used in photon centers and play a key role in improving treatment accuracy. For C-ion RT, there is an increasing demand for a three-dimensional (3D) image-guided system because of the limitations of the present two-dimensional (2D) imaging verification technology. This review discusses the current imaging system used in carbon ion centers and the potential benefits of a volumetric image-guided system.

Cone-Beam CT-based position verification for oesophageal cancer: Evaluation of registration methods and anatomical changes during radiotherapy

Purpose

To evaluate different registration methods, setup margins and number of corrections for CBCT-based position verification for oesophageal cancer and to evaluate anatomical changes during the course of radiotherapy treatment.

Methods

From 50 patients, 440 CBCT-scans were registered automatically using a soft tissue or bone registration algorithm and compared to the clinical match. Moreover, relevant anatomical changes were monitored. A sub-analysis was performed to evaluate if tumour location influenced setup variations. Margin calculation was performed and the number of setup corrections was estimated. Results were compared to a patient group previously treated with MV-EPID based position verification.

Results

CBCT-based setup variations were smaller than EPID-based setup variations, resulting in smaller setup margins of 5.9 mm (RL), 7.5 mm (CC) and 4.7 mm (AP) versus 6.0 mm, 7.8 mm and 5.5 mm, respectively. A reduction in average number of setup corrections per patient was found from 0.75 to 0.36. From all automatically registered CBCT-scans, a clipbox around PTV and vertebras combined with soft tissue registration resulted in the smallest setup margins of 5.9 mm (RL), 7.7 mm (CC), 4.8 mm (AP) and smallest average number of corrections of 0.38. For distally located tumours, a setup margin of 7.7 mm (CC) was required compared to 5.6 mm for proximal tumours. Reduction of GTV volume, heart volume and change in diaphragm position were observed in 16, 10 and 15 patients, respectively.

Conclusions

CBCT-based set-up variations are smaller than EPID-based variations and vary according to tumour location. When using kV-CBCT a large variety of anatomical changes is revealed, which cannot be observed with MV-EPID.