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Background: The SIOPE-Brain Tumor Group recently published a guideline on craniospinal target volume delineation for highly conformal radiotherapy. In order to spare critical structures like e.g., the lens or cochlea, highly conformal techniques can underdose the cerebrospinal fluid (CSF) in the dural reflections around cranial and sacral nerves. The purpose of this study is to generate evidence for CSF extension within the dural sheaths of the cranial and sacral nerves in order to improve accuracy in target volume delineation. Material and methods: Ten healthy volunteers, age 21 till 41 years, underwent an MRI-scan of the skull-base and sacral plexus. To evaluate CSF extension, cT2-weighted images with fat suppression, low signal to noise ratio and little to no motion-related artifacts were used. Two observers measured the extension of CSF from the inner table of the skull for the cranial nerves, and outside the spinal canal for the sacral nerves. Results: CSF extension (mean distance [95% CI]) was visible within the dural sheaths surrounding the majority of the cranial nerves: optic nerve (40 mm [38-42]), trigeminal nerve (16 mm [15-19]), facial-vestibulocochlear nerve (11 mm [11-12]), glossopharyngeal-vagus-accessory nerve (7 mm [7-9]) and hypoglossal nerve (8 mm [7-9]). No CSF was observed outside the spinal canal at sacral level. No significant difference between both observers was measured. Conclusion: This study generates evidence for significant CSF extension outside the inner table of the skull. Despite the vicinity of the lens and cochlea, we therefore recommend the inclusion of both optic nerves and internal auditory canals in the clinical target volume for craniospinal irradiation when using highly conformal delivery techniques.

Introduction: Decreasing the radiation dose in the surgical area is important to lower the risk of wound complications when surgery and radiotherapy are combined for the treatment of spinal metastases. The purpose of this study was to compare the radiation dose in the surgical area for spinal metastases between single fraction external beam radiotherapy (EBRT), single fraction stereotactic body radiotherapy (SBRT) and single fraction SBRT with active sparing (SBRT-AS) of the posterior surgical area., Methods: Radiotherapy treatment plans for EBRT, SBRT and SBRT-AS of the posterior surgical area were created for 13 patients with spinal metastases. A single fraction of 8Gy was prescribed to the spinal metastasis in the EBRT plan. For the SBRT treatment plans, a single fraction of 18Gy was prescribed to the metastasis and 8Gy to the rest of the vertebral body. For the SBRT plan with active sparing the dose in the designated surgical area was minimized without compromising the dose to the organs at risk., Results: The median dose in the surgical area was 2.6Gy (1.6-5.3Gy) in the SBRT plan with active sparing of the surgical area compared to a median dose of 3.7Gy (1.6-6.3Gy) in the SBRT plan without sparing and 6.5Gy (3.5-9.1Gy) in the EBRT plans (p < .001). The radiation doses to the spinal metastases and organs at risk were not significantly different between the SBRT plan with and without sparing the surgical area., Conclusions: The radiation dose to the surgical area is significantly decreased with the use of SBRT compared to EBRT. Active sparing of the surgical area further decreased the mean radiation dose in the surgical area without compromising the dose to the spinal metastasis and the organs at risk.

Purpose/objective: Flank irradiation for Wilms' tumor (WT) is currently performed at our institute using a cone-beam computed tomography-guided volumetric modulated arc (VMAT CBCT ) workflow. By adding real-time magnetic resonance imaging (MRI) guidance to the treatment, safety margins could be reduced. The study purpose was to quantify the potential reduction of the planning target volume (PTV) margin and its dosimetric impact when using an MRI-guided intensity modulated radiation therapy (IMRT MRI ) workflow compared to the VMAT CBCT workflow., Material/methods: 4D-CT, MRI and CBCT scans acquired during preparation and treatment of 15 patients, were used to estimate both geometric, motion and patient set-up systematic (∑) and random (σ) errors for VMAT CBCT and IMRT MRI workflows. The mean PTV (PTV mean ) expansion was calculated using the van Herk formula. Treatment plans were generated using five margin scenarios (PTV mean ± 0, 1 and 2 mm). Furthermore, the IMRT MRI plans were optimized with a 1.5T transverse magnetic field turned-on to realistically model an MRI-guided treatment. Plans were evaluated using dose-volume statistics (p<.01, Wilcoxon)., Results: Analysis of ∑ and σ errors resulted in a PTV mean of 5 mm for the VMAT CBCT and 3 mm for the IMRT MRI workflows in each orthogonal direction. Target coverage was unaffected by the margin decrease with a mean V 95% =100% for all margin scenarios. For the PTV mean , an average reduction of the mean dose to the organs at risk (OARs) was achieved with IMRT MRI compared to VMAT CBCT : 3.4 ± 2.4% (p<.01) for the kidney, 3.4 ± 2.1% (p<.01) for the liver, 2.8 ± 3.0% (p<.01) for the spleen and 4.9 ± 3.8% (p<.01) for the pancreas, respectively., Conclusions: Imaging data in children with WT demonstrated that the PTV margin could be reduced isotropically down to 2 mm when using the IMRT MRI compared to the VMAT CBCT workflow. The former results in a dose reduction to the OARs while maintaining target coverage.

Purpose: Conventional techniques (3D-CRT) for craniospinal irradiation (CSI) are still widely used. Modern techniques (IMRT, VMAT, TomoTherapy ® , proton pencil beam scanning [PBS]) are applied in a limited number of centers. For a 14-year-old patient, we aimed to compare dose distributions of five CSI techniques applied across Europe and generated according to the participating institute protocols, therefore representing daily practice., Material and Methods: A multicenter (n = 15) dosimetric analysis of five different techniques for CSI (3D-CRT, IMRT, VMAT, TomoTherapy ® , PBS; 3 centers per technique) was performed using the same patient data, set of delineations and dose prescription (36.0/1.8 Gy). Different treatment plans were optimized based on the same planning target volume margin. All participating institutes returned their best treatment plan applicable in clinic., Results: The modern radiotherapy techniques investigated resulted in superior conformity/homogeneity-indices (CI/HI), particularly in the spinal part of the target (CI: 3D-CRT:0.3 vs. modern:0.6; HI: 3D-CRT:0.2 vs. modern:0.1), and demonstrated a decreased dose to the thyroid, heart, esophagus and pancreas. Dose reductions of >10.0 Gy were observed with PBS compared to modern photon techniques for parotid glands, thyroid and pancreas. Following this technique, a wide range in dosimetry among centers using the same technique was observed (e.g., thyroid mean dose: VMAT: 5.6-24.6 Gy; PBS: 0.3-10.1 Gy)., Conclusions: The investigated modern radiotherapy techniques demonstrate superior dosimetric results compared to 3D-CRT. The lowest mean dose for organs at risk is obtained with proton therapy. However, for a large number of organs ranges in mean doses were wide and overlapping between techniques making it difficult to recommend one radiotherapy technique over another.

Background and Purpose: To assess intra- and inter-fraction motion uncertainties, due to displacements of the tumor bed (TB) and organs at risk (OAR), as well as intra- and inter-fraction patient set-up uncertainties, due to positioning variations, during image-guided radiation therapy (IGRT) in children with Wilms' tumor., Material and Methods: Four-dimensional computed tomography (4D-CT) and daily pre- and post-treatment cone-beam CT (CBCT)-scans of 15 patients (average 4, range 1-8 years) undergoing flank irradiation after nephrectomy were analyzed. TB (marked by four surgical clips) and OAR motion uncertainties were quantified by displacements of the center of mass in all orthogonal directions. Translational and rotational bone off-sets were recorded for patient set-up uncertainties assessment in all orthogonal directions. The average results, systematic and random errors were computed., Results: Average intra- and inter-fraction motion uncertainties were ≤1.1 mm (range: [-6.9;7.9] mm) for the TB and ≤3.2 mm (range: [-9.1;9.6] mm) for the OAR. Average intra- and inter-fraction patient set-up uncertainties were ≤0.1 mm (range: [-3.3;4.8] mm) and ≤0.9° (range: [0.0;2.8°]). Both motion and patient set-up uncertainties were larger for the cranio-caudal direction. Calculated systematic and random errors were ≤2.4 mm for the motion uncertainties and ≤0.8 mm/0.7 ° for the patient set-up uncertainties., Conclusions: Average motion and patient set-up uncertainties during radiotherapy treatment were found to be limited. However, uncertainties were larger for the cranio-caudal direction and outliers were found in all orthogonal directions. When having available 4D-CT and CBCT information, the use of patient-specific and anisotropic safety margin expansions is advised for both target volume and OAR.