A Novel Collimator Concept for Fast Rotational IMRT

The 2D binary multileaf collimator (2D-bMLC) is a novel collimator concept specially dedicated to fast rotational radiotherapy treatments. The 2D-bMLC consists of individually controlled absorber channels arranged side by side to form a 2D aperture. The design, which leads to radiation fields characterized by a striped pattern, aims at very quick aperture modulations. Using a Monte Carlo (MC) method we established the dosimetric model of a theoretical flattening filter free medical linac equipped with a 2D-bMLC. We further developed a treatment planning system for rotational 2D-bMLC treatments with a new optimization method and a MC framework for exact calculation of the dose. In plan comparison studies, 2D-bMLC plans were calculated for various clinical indications and compared to clinically accepted IMRT and Helical Tomotherapy plans. The design of the 2D-bMLC might be especially sensitive to geometrical misalignments of the collimator on the one hand, and to intrafraction motion on the other hand. Both aspects have been investigated in additional MC studies. The results of the planning studies showed that the 2D-bMLC concept is in principal adequate for rotational radiotherapy treatments with potential delivery times considerably below those of IMRT techniques applied in the clinics today. Clinically acceptable 2D-bMLC plans with delivery times below 30 seconds were calculated for all investigated tumour sites. Dosimetric parameters were comparable to those of the reference plans. Already very small geometrical misalignment of the 2D-bMLC can cause severe under-dosage, and especially high demands on manufacture tolerances as well as on quality assurance will be necessary, if a 2D-bMLC should be produced for clinical use. Consideration of intrafraction motion, however, did not lead to significant changes in the accumulated doses calculated for prostate plans with standard fractionation

Planning benchmark study for SBRT of early stage NSCLC

Purpose The aim was to evaluate stereotactic body radiation therapy (SBRT) treatment planning variability for early stage nonsmall cell lung cancer (NSCLC) with respect to the published guidelines of the Stereotactic Radiotherapy Working Group of the German Society for Radiation Oncology (DEGRO). Materials and methods Planning computed tomography (CT) scan and the structure sets (planning target volume, PTV; organs at risk, OARs) of 3 patients with early stage NSCLC were sent to 22 radiotherapy departments with SBRT experience: each department was asked to prepare a treatment plan according to the DEGRO guidelines. The prescription dose was 3 fractions of 15 Gy to the 65% isodose. Results In all, 87 plans were generated: 36 used intensity-modulated arc therapy (IMAT), 21 used three-dimensional conformal radiation therapy (3DCRT), 6 used static field intensity-modulated radiation therapy (SF-IMRT), 9 used helical radiotherapy and 15 used robotic radiosurgery. PTV dose coverage and simultaneously kept OARs doses were within the clinical limits published in the DEGRO guidelines. However, mean PTV dose (mean 58.0 Gy, range 52.8-66.4 Gy) and dose conformity indices (mean 0.75, range 0.60-1.00) varied between institutions and techniques (p <= 0.02). OARs doses varied substantially between institutions, but appeared to be technique independent (p = 0.21). Conclusion All studied treatment techniques are well suited for SBRT of early stage NSCLC according to the DEGRO guidelines. Homogenization of SBRT practice in Germany is possible through the guidelines; however, detailed treatment plan characteristics varied between techniques and institutions and further homogenization is warranted in future studies and recommendations. Optimized treatment planning should always follow the ALARA (as low as reasonably achievable) principle