Dosimetric evaluation of positioning uncertainties for blood-spinal irradiation

Modern treatment techniques as VMAT technique have been suggest to craniospinal irradiation and showed improvement in dose distribution. However, the evaluation of the dosimetric effect due to positioning errors has been reported with comparisons of only a few techniques and highlights only the advantages of the technique proposed by most authors. The objective of this study was to evaluate the influence of positioning errors along the longitudinal axis (3, 5 and 10 mm) for the VMAT technique and to compare the dose distribution of PTV and organs at risk (OARs, from English, organs at risk), with the other conformational techniques with gap (3D), with weekly gap change (3D_M) and with multiple subfields (FF). Different Craniospinal treatment techniques (3D, 3D_M, FF and VMAT) was planned in computed tomography images of a human phantom (Radon®) for the 36 Gy of dose. The thoracic isocenter was displaced 3, 5 and 10 mm in the longitudinal direction of the isocenter of the lumbar spine. The dose distribution was calculated by maintaining the same monitor units of the original schedules. Dosimetric values of planning target volume and risks organs were compared between the planning techniques without and with the simulated positioning error. With the addition of the positioning error, the VMAT technique showed less variation in minimum and maximum dose for PTV when compared with the other techniques. Their D2% PTV values were not higher than the doses of the other planning techniques. The same for covering the target volume of the spine for displacements not exceeding 5 mm. All treatment techniques, except VMAT, with the isocenter of the thoracic spine displaced over 5 mm had a maximum spinal cord dose higher than the limit dose suggested by the literature. The positioning errors affected the dose distribution in treatment volumes and risk organs for all techniques investigated in this study. VMAT was the least impaired technique in terms of absolute dose values for displacements not exceeding 5 mm.(author)