J.P. Bangsgaard

Translational and rotational intra- and inter-fractional errors in patient and target position during a short course of frameless stereotactic body radiotherapy

Abstract Background. Implementation of cone beam computed tomography (CBCT) in frameless stereotactic body radiotherapy (SBRT) of lung tumours enables setup correction based on tumour position. The aim of this study was to compare setup accuracy with daily soft tissue matching to bony anatomy matching and evaluate intra- and inter-fractional translational and rotational errors in patient and target positions. Material and methods. Fifteen consecutive SBRT patients were included in the study. Vacuum cushions were used for immobilisation. SBRT plans were based on midventilation phase of four-dimensional (4D)-CT or three-dimensional (3D)-CT from PET/CT. Margins of 5 mm in the transversal plane and 10 mm in the cranio-caudal (CC) direction were applied. SBRT was delivered in three fractions within a week. At each fraction, CBCT was performed before and after the treatment. Setup accuracy comparison between soft tissue matching and bony anatomy matching was evaluated on pretreatment CBCTs. From differences in pre- and post-treatment CBCTs, we evaluated the extent of translational and rotational intra-fractional changes in patient position, tumour position and tumour baseline shift. All image registration was rigid with six degrees of freedom. Results. The median 3D difference between patient position based on bony anatomy matching and soft tissue matching was 3.0 mm (0–8.3 mm). The median 3D intra-fractional change in patient position was 1.4 mm (0–12.2 mm) and 2.2 mm (0–13.2 mm) in tumour position. The median 3D intra-fractional baseline shift was 2.2 mm (0–4.7 mm). With correction of translational errors, the remaining systematic and random errors were approximately 1°. Conclusion. Soft tissue tumour matching improved precision of treatment delivery in frameless SBRT of lung tumours compared to image guidance using bone matching. The intra-fractional displacement of the target position was affected by both translational and rotational changes in tumour baseline position relative to the bony anatomy and by changes in patient position.

A treatment planning and delivery comparison of volumetric modulated arc therapy with or without flattening filter for gliomas, brain metastases, prostate, head/neck and early stage lung cancer

Abstract Background. Flattening filter-free (FFF) beams are an emerging technology that has not yet been widely implemented as standard practice in radiotherapy centers. To facilitate the clinical implementation of FFF, we attempted to elucidate the difference in plan quality and treatment delivery time compared to flattening filter beams (i.e. standard, STD) for several patient groups. We hypothesize that the treatment plan quality is comparable while the treatment delivery time of volumetric modulated arc therapy (VMAT) is considerably shorter using FFF beams, especially for stereotactic treatments. Methods. A total of 120 patients treated for head and neck (H&N) tumors, high-grade glioma, prostate cancer, early stage lung cancer and intra-cranial metastatic disease (both single and multiple metastases) were included in the study. For each cohort, 20 consecutive patients were selected. The plans were generated using STD- and FFF-VMAT for both 6 MV and 10 MV, and were compared with respect to plan quality, monitor units and delivery time using Wilcoxon signed rank tests. Results. For H&N and high-grade gliomas, there was a significant difference in homogeneity index in favor for STD-VMAT (p < 0.001). For the stereotactic sites there were no differences in plan conformity. Stereotactic FFF-VMAT plans required significantly shorter delivery time compared to STD-VMAT plans (p < 0.001) for higher dose per fraction, on average 54.5% for 6 MV and 71.4% for 10 MV. FFF-VMAT generally required a higher number of MU/Gy (p < 0.001), on average 7.0% for 6 MV and 8.4% for 10 MV. Conclusion. It was generally possible to produce FFF-VMAT plans with the same target dose coverage and doses to organs at risk as STD-VMAT plans. Target dose homogeneity tended to be somewhat inferior for FFF-VMAT for the larger targets investigated. For stereotactic radiotherapy, FFF-VMAT resulted in a considerable time gain while maintaining similar plan quality compared to STD beams.

Reduced lung dose and improved inspiration level reproducibility in visually guided DIBH compared to audio coached EIG radiotherapy for breast cancer patients

Abstract Introduction. Patients with left-sided breast cancer with lymph node involvement have routinely been treated with enhanced inspiration gating (EIG) for a decade at our institution. In a transition from EIG to deep inspiration breath hold (DIBH) we compared the two techniques with focus on target coverage, dose to organs at risk and reproducibility of the inspiration level (IL). Material and methods. Twenty-four patients were computed tomography (CT) scanned with EIG and DIBH. For DIBH we used visual feedback and for EIG audio coaching, both during scan and treatment. Treatment plans for 50 Gy over 25 fractions were calculated. Seventeen of the patients were included in the analysis of reproducibility. They were audio coached for one minute before beam-on in DIBH at nine treatment sessions. These respiration curves were analysed with average maximum IL and standard deviation (SD) for the EIG part of the respiratory signal, and mean IL and SD for the DIBH. Comparison of dosimetric and respiration parameters were performed with the Wilcoxon signed rank-sum test. Results. In DIBH, the ipsilateral lung volume increased further compared to EIG (p < 0.0004, mean increase 11%). This lead to a 9% mean reduction (p = 0.002) of the ipsilateral lung volume receiving 20 Gy (V20 Gy). We found no other significant dosimetric differences between the two methods. The reproducibility of the IL was better with the DIBH method, observed as a significantly smaller SD in most patients (p < 0.04 for 16 of 17 patients). Conclusion. The DIBH method resulted in a significantly larger lung volume and lower ipsilateral lung V20 Gy compared to EIG. The IL for visually guided DIBH was more reproducible than audio-coached EIG. Based on these findings, the DIBH technique is our new breathing adaptation standard for radiotherapy of patients with left-sided breast cancer with lymph node involvement.

Deep inspiration breath hold radiotherapy for locally advanced lung cancer: Comparison of different treatment techniques on target coverage, lung dose and treatment delivery time

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A closer look at RapidArc® radiosurgery plans using very small fields

RapidArc® has become the treatment of choice for an increasing number of treatment sites in many clinics. The extensive use of multiple subfields in RapidArc® treatments presents unique challenges, especially for small targets treated in few fractions. In this work, very small static fields and subsequently RapidArc® and conventional plans for two targets (0.4 and 9.9 cm(3)) were investigated. Doses from static fields 1-4 MLC leaves (0.25-1.00 cm) wide, and larger fields with 1-4 MLC leaves closed in their centres, were measured using the portal dosimeter-based QA system EPIQA (v 1.3) and gafchromic film. RapidArc and conventional plans for two tumours were then measured using EPIQA, gafchromic EBT2 film and the phantom-based QA system Delta4. Eclipse 8.6 and 8.9, grid spacings of 1.25 and 2.50 mm and a Varian HD linac were used. For static fields one MLC leaf wide, the dose was underestimated by Eclipse by as much as 53% (v 8.6, 2.5 mm grid). Eclipse underestimated the dose downstream from a few MLC leaves closed in the centre of a large MLC field by as much as 30%. Eclipse consistently overestimated the width of the penumbra by about 100%. For the conventional plans, there was good agreement between the calculated and measured dose for the 9.9 cm(3) PTV, but a 10% underdose was observed for the 0.4 cm(3) PTV. For the RapidArc® plans, the measured dose for the 9.9 cm(3) PTV was in good agreement with the calculated one. However, for the 0.4 cm(3) PTV about 10% overdosing was detected (Eclipse v 8.6, 2.5 mm grid spacing). EPIQA data indicated that the measured dose profiles were overmodulated compared to the calculated one. The use of small subfields, typically a few MLC leaves wide, or larger fields with one or a few MLC leaves closed in its centre can result in significant errors in the dose calculation. The detector systems used vary in their ability to detect the discrepancies. Using a smaller grid size and newer version of Eclipse reduces the discrepancies observed in this work but does not eliminate them.

Geometric uncertainties in voluntary deep inspiration breath hold radiotherapy for locally advanced lung cancer

BACKGROUND AND PURPOSE Deep inspiration breath hold (DIBH) increases lung volume and can potentially reduce treatment-related toxicity in locally advanced lung cancer. We estimated geometric uncertainties in visually guided voluntary DIBH and derived the appropriate treatment margins for different image-guidance strategies. MATERIAL AND METHODS Seventeen patients were included prospectively. An optical marker-based respiratory monitoring with visual guidance enabled comfortable DIBHs, adjusted to each patients performance. All patients had three consecutive DIBH CTs at each of the treatment fractions 2, 16 and 31. DIBH reproducibility was evaluated as inter- and intra-fractional variations in lung volume, tumour position and differential motion between primary tumour and mediastinal lymph nodes. RESULTS Lung volume increased by median 60% in DIBH. Inter- and intra-fractional lung volume variations were median 2.1% and 1.1%, respectively. Inter- and intra-fractional uncertainties in 3D tumour position were 4.8 ± 2.8 mm and 1.7 ± 1.4 mm (mean ± SD). Inter- and intra-fractional differential motion was 4.8 ± 3.3 mm and 0.0 ± 1.1 mm. CONCLUSIONS For single targets, visually guided voluntary DIBH radiotherapy is highly reproducible provided an image-guidance strategy with tumour registration is performed. If the primary tumour is separated from the mediastinal lymph nodes, inter-fractional differential motion remains a challenge and margins must be adapted to reflect the image registration strategy.

Absorption measurements on a new cone beam CT and IMRT compatible tabletop for use in external radiotherapy

With the development of intensity-modulated radiotherapy (IMRT), intensity-modulated arc therapy (IMAT), volumetric single arc therapy (RapidArc) and cone beam CT (CBCT) in 3D conformal radiotherapy, a need for new homogeneous treatment tabletops has evolved. Therefore, a new homogenous carbon fibre rotatable treatment tabletop Contesse has been developed. The intension was to create a CT compatible treatment tabletop accommodating all the standard treatment areas without add-on devises or the need to reconfigure the tabletop in everyday practice. At the same time, the absorption was sought to be minimized. Measurements of absorption with an ionization chamber have been performed for the main treatment geometries at normal and oblique angles together with build-up measurements at normal incidence. Absorption and build-up properties were shown to be comparable to those of other treatment tabletops with similar fixation devices. No artefacts from the tabletop are seen on CBCT scans. In order to reveal the absorption characteristics of tabletops, careful measurements at oblique angles including fixation devices on all clinically used tabletops must be performed. The Contesse tabletop is CBCT compatible, has acceptable absorption and is presently being used in the clinic for all patients, including IMRT and kilovoltage CBCT.

A modeling study of functional magnetic resonance imaging to individualize target definition of seminal vesicles for external beam radiotherapy.

Abstract Background: Pre-treatment magnetic resonance imaging (MRI) can give patient-specific evaluation of suspected pathologically involved volumes in the seminal vesicles (SV) in prostate cancer patients. By targeting this suspicious volume we hypothesize that radiotherapy is more efficient without introducing more toxicity. In this study we evaluate the concept of using MRI-defined target volumes in terms of tumor control probability (TCP) and rectal normal tissue complication probability (NTCP). Material and methods: Twenty-one high-risk prostate cancer patients were included. Pre-treatment CT images, T2 weighted (T2w) MRI and two multi-parametric MRI were acquired. Overlap between a suspicious volume in the SV observed on T2w images and a suspicious volume observed on either multi-parametric MRI was assumed to reflect a true malignant region (named ‘MRI positive’). In addition the entire SV on the CT-scan was delineated. Three treatment plans of 2 Gy ×39 fractions were generated per patient: one covering the MRI positive volume in SV and prostate with margin of 11 mm to the MRI positive in the SV and two plans covering prostate and SV using 11 and 7 mm SV margin, respectively. All plans were prescribed the same PTV mean dose. Rectal NTCP grade ≥2 was evaluated with the Lyman–Kutcher–Burman model and TCP was estimated by a logistic model using the combined MRI positive volume in SV and prostate as region-of-interest. Results: Fourteen of twenty-one patients were classified as MRI positive, six of which had suspicious volumes in all three MRI modalities. On average TCP for the plan covering prostate and the MRI positive volume was 3% higher (up to 11%) than the two other plans which was statistically significant. The increased TCP was obtained without increasing rectal NTCP grade ≥2. Conclusions: Using functional MRI for individualized target delineation in the SV may improve the treatment outcome in radiotherapy of prostate cancer without increasing the rectal toxicity.

Metal artefact reduction for accurate tumour delineation in radiotherapy

Background and purpose Two techniques for metal artefact reduction for computed tomography were studied in order to identify their impact on tumour delineation in radiotherapy. Materials and methods Using specially designed phantoms containing metal implants (dental, spine and hip) as well as patient images, we investigated the impact of two methods for metal artefact reduction on (A) the size and severity of metal artefacts and the accuracy of Hounsfield Unit (HU) representation, (B) the visual impact of metal artefacts on image quality and (C) delineation accuracy. A metal artefact reduction algorithm (MAR) and two types of dual energy virtual monochromatic (DECT VM) reconstructions were used separately and in combination to identify the optimal technique for each implant site. Results The artefact area and severity was reduced (by 48–76% and 58–79%, MAR and DECT VM respectively) and accurate Hounsfield-value representation was increased by 22–82%. For each energy, the observers preferred MAR over non-MAR reconstructions (p < 0.01 for dental and hip cases, p < 0.05 for the spine case). In addition, DECT VM was preferred for spine implants (p < 0.01). In all cases, techniques that improved target delineation significantly (p < 0.05) were identified. Conclusions DECT VM and MAR techniques improve delineation accuracy and the optimal of reconstruction technique depends on the type of metal implant.

OC-0153: Dual energy CT and iterative metal artefact reduction for accurate tumour delineation

Material and Methods: A set of teeth containing an amalgamfilled removable tooth and an artificial polycaprolactone tumour was placed in water and CT scanned (Siemens Somatom Definition AS) at 120 kVp, 80 kVp, and 140 kVp. The two latter scans were used to reconstruct virtual monochromatic (VM) images. All image sets were additionally reconstructed with metal artefact reduction (MAR) software (iMAR, Siemens Healthcare). The following 4 MAR reconstructions were studied: 1) 130 keV VM 2) 70 keV VM with MAR, 3) 120 kVp with MAR, 4) 130 keV VM with MAR. A conventional 120 kVp CT was also taken and a 120 kVp image where the metal tooth was removed was used as control. 3 oncologists and 2 radiologists contoured the tumour volume on all 6 image sets while blinded to the image reconstruction type. A 7th high-quality image of only the artificial tumour was contoured to obtain the true shape of the tumour. Maximal Hausdorff distances and DICE coefficients of the 5 delineated contours compared to the true contour was were used to quantify delineation accuracy in all 6 image sets. Statistically, a Friedman-test was used for primary comparisons and a Nemenyi-test is performed for pairwise post hoc analysis.