RESUMO
Using Monte Carlo simulation and a realistic patient model, it is shown that the volume of healthy tissue irradiated at therapeutic doses can be drastically reduced using a combination of standard megavoltage and kilovoltage X-ray beams with a contrast agent previously loaded into the tumor, without the need to reduce standard treatment margins. Four-dimensional computed tomography images of 2 patients with a centrally located and a peripherally located tumor were obtained from a public database and subsequently used to plan robotic stereotactic body radiotherapy treatments. Two modalities are assumed: conventional high-energy stereotactic body radiotherapy and a treatment with contrast agent loaded in the tumor and a kilovoltage X-ray beam replacing the megavoltage beam (contrast-enhanced radiotherapy). For each patient model, 2 planning target volumes were designed: one following the recommendations from either Radiation Therapy Oncology Group (RTOG) 0813 or RTOG 0915 task group depending on the patient model and another with a 2-mm uniform margin determined solely on beam penumbra considerations. The optimized treatments with RTOG margins were imparted to the moving phantom to model the dose distribution that would be obtained as a result of intrafraction motion. Treatment plans are then compared to the plan with the 2-mm uniform margin considered to be the ideal plan. It is shown that even for treatments in which only one-fifth of the total dose is imparted via the contrast-enhanced radiotherapy modality and with the use of standard treatment margins, the resultant absorbed dose distributions are such that the volume of healthy tissue irradiated to high doses is close to what is obtained under ideal conditions.
Assuntos
Neoplasias Pulmonares/radioterapia , Radiocirurgia , Planejamento da Radioterapia Assistida por Computador , Radioterapia Guiada por Imagem , Relação Dose-Resposta à Radiação , Humanos , Neoplasias Pulmonares/diagnóstico , Método de Monte Carlo , Movimento (Física) , Órgãos em Risco , Intensificação de Imagem Radiográfica , Radiocirurgia/métodos , Dosagem Radioterapêutica , Radioterapia Guiada por Imagem/métodos , Resultado do TratamentoRESUMO
PURPOSE: To demonstrate, via Monte Carlo simulation, that an image obtained from the patient-generated scattered radiation forced to impinge on the detector from a known direction by means of parallel-focused grids, can be used to complement the information conveyed by the primary image, such that accurate stereoscopic three-dimensional localization of fiducial markers can be achieved in a single kV x-ray exposure. METHODS: A voxelized Zubal phantom was used to model the process of fiducial marker localization. The markers were represented as made of gold and cylindrical in shape with dimensions of 5 mm in length and 1 mm in diameter. Three such markers were placed in the Zubal phantom at the prostate level. Two gantry-mounted image acquisition geometries were modeled: a single kV imaging system and a dual kV-MV imaging system. The detector was modeled as a 30 cm × 40 cm Gd2 O2 S screen with a thickness of 0.2 cm and a resolution of 768 × 1024 pixels. The PENELOPE Monte Carlo code was used to calculate the absorbed dose in this detector imparted by the transmitted primary and directional scatter radiation. A grayscale conversion was then applied to obtain an image from which the positions of the markers were determined. Two parallel-focused grid geometries were modeled, one based on the standard lead-carbon fiber grids and a proposed modification using tungsten as the shielding material. Absorbed dose in the patient model was also calculated. RESULTS: It is shown that the combination of primary and directional scatter images provides the means for an accurate stereoscopic fiducial marker 3D localization in a single x-ray exposure, provided the antiscatter grids are made thick enough to allow radiation traveling only in a particular direction to reach the detector. For the proposed tungsten grid and the x-ray spectrum used in this work, grid ratios of 20 and thickness of 0.2 cm, provide the necessary shielding while for the standard lead grids, a ratio of at least 166 and a thickness of 2 cm are needed to obtain discernible directional scatter images. CONCLUSIONS: We have shown that it is in principle possible to determine the 3D position of fiducial markers in a single exposure by making use of the radiation scattered by the patient to form an image that complements the information obtained with the primary beam. The method here proposed requires minimal modification of existing clinical hardware.
Assuntos
Marcadores Fiduciais , Espalhamento de Radiação , Humanos , Método de Monte Carlo , IncertezaRESUMO
BACKGROUND: Therapies using Y-anti-CD20 or I-anti-CD20 have demonstrated their efficacy in patients with B-cell non-Hodgkin's lymphoma. Re is a radionuclide useful for radioimmunotherapy. AIM: To develop a procedure for efficient labelling of anti-CD20 with Re from lyophilized formulations to achieve high radiochemical yield, high specific activity and preservation of the molecular recognition after a simple kit reconstitution without further purification. METHODS: Re-anti-CD20 was prepared by a direct labelling method using sodium tartrate as a weak competing ligand. Different lyophilized formulations were prepared to optimize tartrate and stannous chloride concentration, pH and reaction time. To evaluate the biological recognition a comparative study of the in-vitro binding of Re-anti-CD20, I-anti-CD20 (positive control) and Re-anti-CEA (negative control) to normal B lymphocytes was performed. Biodistribution studies in normal mice were accomplished to assess the in-vivo Re-anti-CD20 complex stability. RESULTS: Re labelled anti-CD20 was obtained with high radiochemical purities (>97%) and high specific activity (0.5-0.7 GBq . mg) 1-1.5 h after addition of sodium perrhenate solution to a kit containing 4.4 muM anti-CD20, 4 mM anhydrous stannous chloride, and 140 mM dihydrate sodium tartrate at pH 4. The binding of Re-anti-CD20 to cells was in the same range as I-anti-CD20 (>80%) and was significantly different to cell binding of Re-anti-CEA (<10%). No evidence of free Re release was found at 2, 4 and 24 h after Re-anti-CD20 administration in mice. Lyophilized kits showed high stability during the storage at 4 degrees C for 6 months. CONCLUSIONS: Optimal reaction conditions were defined enabling high radiochemical purities of Re-anti-CD20 to be obtained routinely and therefore potentially useful in the treatment of non-Hodgkin's lymphoma.