Following the clinical introduction of the Elekta Unity MR-linac, there is an urgent need for development of dosimetry protocols and tools, not affected by the presence of a magnetic field. This work presents a benchmarking methodology comprising 2D/3D passive dosimetry and involving on-couch adaptive treatment planning, a unique step in MR-linac workflows. Two identical commercially available 3D-printed head phantoms (featuring realistic bone anatomy and MR/CT contrast) were employed. One phantom incorporated a film dosimetry insert, while the second was filled with polymer gel. Gel dose-response characteristics were evaluated under the Unity irradiation and read-out conditions, using vials and a cubic container filled with gel from the same batch. Treatment plan for the head phantoms involved a hypothetical large C-shape brain lesion, partly surrounding the brainstem. An IMRT step-and-shoot 7-beam plan was employed. Pre-treatment on-couch MR-images were acquired in order for the treatment planning system to calculate the virtual couch shifts and perform adaptive planning. Absolute 2D and relative 3D measurements were compared against calculations related to both adapted and original plans. Real-time dose accumulation monitoring in the gel-filled phantom was also performed. Results from the vials and cubic container suggest that gel dose-response is linear in the dose range investigated and signal integrity is mature at the read-out timings considered. Head phantom 2D and 3D measurements agreed well with calculations with 3D gamma index passing rates above 90% in all cases, even with the most stringent criteria used (2 mm/2%). By exploiting the 3D information provided by the gel, comparison also involved DVHs, dose-volume and plan quality metrics, which also reflected the agreement between adapted and delivered plans within  ±4%. No considerable discrepancies were detected between adapted and original plans. A novel methodology was developed and implemented, suitable for QA procedures in Unity. TPS calculations were validated within the experimental uncertainties involved.

In single-isocenter stereotactic radiosurgery/radiotherapy (SRS/SRT) intracranial applications, multiple targets are being treated concurrently, often involving non-coplanar arcs, small photon beams and steep dose gradients. In search for more rigorous quality assurance protocols, this work presents and evaluates a novel methodology for patient-specific pre-treatment plan verification, utilizing 3D printing technology. In a patient’s planning CT scan, the external contour and bone structures were segmented and 3D-printed using high-density bone-mimicking material. The resulting head phantom was filled with water while a film dosimetry insert was incorporated. Patient and phantom CT image series were fused and inspected for anatomical coherence. HUs and corresponding densities were compared in several anatomical regions within the head. Furthermore, the level of patient-to-phantom dosimetric equivalence was evaluated both computationally and experimentally. A single-isocenter multi-focal SRS treatment plan was prepared, while dose distributions were calculated on both CT image series, using identical calculation parameters. Phantom- and patient-derived dose distributions were compared in terms of isolines, DVHs, dose-volume metrics and 3D gamma index (GI) analysis. The phantom was treated as if the real patient and film measurements were compared against the patient-derived calculated dose distribution. Visual inspection of the fused CT images suggests excellent geometric similarity between phantom and patient, also confirmed using similarity indices. HUs and densities agreed within one standard deviation except for the skin (modeled as ‘bone’) and sinuses (water-filled). GI comparison between the calculated distributions resulted in passing rates better than 97% (1%/1 mm). DVHs and dose-volume metrics were also in satisfying agreement. In addition to serving as a feasibility proof-of-concept, experimental absolute film dosimetry verified the computational study results. GI passing rates were above 90%. Results of this work suggest that employing the presented methodology, patient-equivalent phantoms (except for the skin and sinuses areas) can be produced, enabling literally patient-specific pre-treatment plan verification in intracranial applications.

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Purpose: To investigate and quantify the impact of dose‐calculation grid‐size on dose volume histogram (DVH) for small targets, such as those in stereotactic radiosurgery (SRS). Methods: A cohort of ten patients with multiple metastasis was used in this planning study. All patients were treated with SRS and had target volumes ranging from 0.1 to 4.3cc. The Varian eclipse planning system was used for planning using a mono‐isocentric VMAT technique with four arcs. Each plan was calculated twice using a 1×1×1 mm3 and 3×3×3 mm3 dose grid respectively. Moreover, we used an open source DICOM‐RT viewer to read the raw‐RDose data (1×1×1 mm3 resolution) and were subsequently rescaled by linear interpolation to dose grids of 1.5×1.5×1.5, 2×2×2, 2.5×2.5×2.5 and 3×3×3 mm3. DVHs for all targets were recalculated and inter‐compared for the native and post‐processed resolutions to quantify the effect on DVH of dose grid resolution and correlate that with the PTV volume size. Results: DVHs shape and numerical values have a significant dependence on dose grid size resulting to a reduced Dmean, Dmax and Dmin with volume averaging effect being the main reason for these changes. Moreover, the magnitude of the DVH alteration is inversely proportional to tumor volume size. As the dose grid increases from 1×1×1 to 3×3×3 mm3, the DVH‐derived Dmean value is underestimated by ∼4, ∼7 and ∼10 % for tumors with volume sizes of 1.3, 0.5 and 0.1 cc respectively. Conclusion: The dose grid size that is used for the final dose calculation is affecting significantly the DVHs of small PTVs. The higher the dose grid size, the more noticeable is the DVH alteration, especially for tumors with a volume less than 1cc. For SRS planning, a dose grid of 1×1×1 mm3 should be used, especially for tumors with a volume < 1cc, to avoid erroneous DVHs. UTHSCSA, division of medical physics, has a research grant with Brainlab

Purpose: To validate dose calculation and delivery accuracy of a recently introduced mono‐isocentric technique for the treatment of multiple brain metastases in a realistic clinical case. Methods: Anonymized CT scans of a patient were used to model a hollow phantom that duplicates anatomy of the skull. A 3D printer was used to construct the phantom of a radiologically bone‐equivalent material. The hollow phantom was subsequently filled with a polymer gel 3D dosimeter which also acted as a water‐equivalent material. Irradiation plan consisted of 5 targets and was identical to the one delivered to the specific patient except for the prescription dose which was optimized to match the gel dose‐response characteristics. Dose delivery was performed using a single setup isocenter dynamic conformal arcs technique. Gel dose read‐out was carried out by a 1.5 T MRI scanner. All steps of the corresponding patient's treatment protocol were strictly followed providing an end‐to‐end quality assurance test. Pseudo‐in‐vivo measured 3D dose distribution and calculated one were compared in terms of spatial agreement, dose profiles, 3D gamma indices (5%/2mm, 20% dose threshold), DVHs and DVH metrics. Results: MR‐identified polymerized areas and calculated high dose regions were found to agree within 1.5 mm for all targets, taking into account all sources of spatial uncertainties involved (i.e., set‐up errors, MR‐related geometric distortions and registration inaccuracies). Good dosimetric agreement was observed in the vast majority of the examined profiles. 3D gamma index passing rate reached 91%. DVH and corresponding metrics comparison resulted in a satisfying agreement between measured and calculated datasets within targets and selected organs‐at‐risk. Conclusion: A novel, pseudo‐in‐vivo QA test was implemented to validate spatial and dosimetric accuracy in treatment of multiple metastases. End‐to‐end testing demonstrated that our gel dosimetry phantom is suited for such QA procedures, allowing for 3D analysis of both targeting placement and dose.