Dosimetric Evaluation of Total Body Irradiation (TBI) Treatment by Volumetric Modulated Arc Therapy (VMAT) on the Coach

Abstract

Many different tecniques have been described for effective irradiation of the whole body and indeed improvements in both the irradation tecnique and physical dosimetry. Most of the current TBI procedures are based on tecniques established on linear accelerators that are used for conventional radiotherapy. The large photon fields are generally achieved by treating the patient at extended SSD with standart linear accelerators or with special dedicated machines. The variable SSD available at current radiotherapy facilities is partially responsible for the variation in TBI tecniques and procedures. Equipment Guidelines recommend the use of parallel opposed pairs of high-energy photon beams from 4 to 18 MV for TBI [2]. TBI tecniques typically comprise a combination of various opposing field setups in a sitting or lying patient position at very extended SSDs. The extended SSD tecnique using a single large field encompassing the entire patient is by far the simplest and the most prevalent TBI tecnique used today. It eliminates the dosimetry complications in regions of field junctions introduced by using multiple small fields and concern that cells circulating through the body could potentially receive a reduced dose. These tecniques use Standard radiotherapy linear accelerators (LINAC)s and rely on a maximum collimator setting, a large SSD, and beam divergence to produce the large irradiation field required for TBI. AAPM's TG-51 calibration protocol provides guidelines for dosimetry of high-energy photon beams [17]. Recent studies demonstrate the efficacy of helical tomotherapy for decreasing TBI treatment time and increasing homogeneity of delivered radiation [18]. Helical tomotherapy (HT) allows for irradiation of large target volume at nominal source axis distance (SAD), without additional requirements on treatment room size or shielding. HT consists of a linac mounted on a computed tomography (CT) gantry and delivers a fan beam (up to 5 cm x 40 cm) modulated with binary 64-leaf multileaf collimator (MLC). The beam source continuously rotates in the gantry while the couch is translated through the bore, which generates a helical pattern that can cover a treatment length of up to 160 cm. Because of their limited body length the tecnique seems especially eligible in juvenile patients. Journal of Biochemistry and Biophysics In our study, we investigated the use of Linac-Based TBI by VMAT at nominal SAD technique on the coach. Five TBI patient's treatment planning performed using Monaco5.1® treatment planning system with three different VMAT tecniques for each patient. First one was single arc VMAT tecnique, second one was dual arc VMAT tecnique and third one was 2 fields arc VMAT tecnique for one isocentre. The VMAT-TBI tecnique consisted of three isocentres and three overlapping arcs. Eight TBI patient's TPS were performed with the best VMAT technique which we determined. For treating patients, Versa HD® (Crawley, Elekta) linear accelerator with 6 MV, equipped with Agility® collimator system, XVI 5.0 cone beam CT was used as an image guided radiation therapy (IGRT) method for each VMAT delivery, also, Sentinel® (Sweeden, C-RAD) laser based surface guided radiotherapy (SGRT) device was used for patient positioning and monitoring. This is shown Figure 1. to 15 Gy given in 8 to 12 fractions over 4 days, with 2 to 3 treatments daily doses > 15 Gy have been shown to decrease relapse rate, but also increase the incidence of graft vs. host disease and decrease 2 years survival [8]. Low-dose TBI, with doses of 2 to 8 Gy given in 1 to 4 fractions in combination with chemotherapy, is an effective conditioning regimen for hematopoietic stem cell transplantation in patients who cannot tolerate myeloablation due to age or comorbidities [8,13,14]. A randomized study from Seattle in the setting of AML compared single dose TBI (10 Gy) to a fractionated schedule (2 Gy for six fractions). The last update of this trial showed significant superiority of the fractioanated scheme in terms of every free survival [15]. Another Seattle randomized trial of AML in first remission compared fractionated TBI doses of 12 Gy with 15.75 Gy, showing a decreased relapse rate from 35% to 12%, but at the expense of a significant increase in therapy related mortalit, resulting in no survival advantages to a higher radiation dose [16].