Tomo Direct to Deliver Static Angle Tomotherapy Treatments

Abstract

Radiation Therapy (RT) is a mainstay option for human cancer treatment, as it is employed as single agent or within combined modality approaches, with different timelines and aims (radical, neoadjuvant, adjuvant, palliation) and in almost all anatomic districts [1]. In recent years, significant developments have been introduced in RT planning a delivery, leading to improvements in terms of tumor control and normal tissue avoidance [2]. The Tomotherapy Hi-Art II system (Accuray Inc. , Sunnyvale, CA) is a RT platform able to deliver highly conformal intensity modulated radiotherapy (IMRT) plans within a helical geometry under image guidance (IGRT) [3]. A recent platform upgrade is named TomoDirect and it has been introduced in clinical practice in 2010, as a suitable solution for specific clinical context characterized by particular spatial relationships between target volumes (TVs) and organs at risk (OARs) allowing for beam arrangements constrained to a limited number of directions [4]. Tomotherapy delivers 6MV-IMRT through a rotating fan beam carved by a binary multileaf collimator (MLC), since the ring gantry rotates around the treatment couch as it progresses within the gantry bore across the delivery plane [4]. Beam modulation is obtained by a single row of binary leaves for each projected angle over a rotational interval of 7 degrees, up to a total of 51 projections for each gantry revolution. This approach provides robust conformality, abrupt dose fall off and reliable accuracy. However, since radiation is delivered all around the patient, a larger integral dose might become an issue, with low dose delivery (the so called ‘low dose bath’) to body segments that would only receive scatter radiation with a 3D conformal RT approach. During the optimization process, virtual structures named ‘blocks’ can improve dosimetry precluding radiation from specific volumes Thus, RT delivery is limited to fewer directions with a slender set of beamlets. Since the gantry rotational velocity remains constant, while the number of treatment directions decreases (with more gantry rotations needed to deliver the prescribed dose), this might result in unnecessary prolonged treatment time. Whenever the desired treatment is constrained to a limited number of incoming directions, this planning modality might represent an issue. TomoDirect (TD) combines static gantry positions, concomitant couch translation (along the craniocaudal direction) and MLC modulation. Specifically, while the patient is translated along the superior-inferior direction past the fixed fan beam path during delivery of each treatment field (defined as a fixed gantry angle and jaw width), the beam intensity is modulated by the binary collimator while the pitch (fraction of the jaw opening advanced by the treatment couch per evolution) regulates the degree of modulation in the cranio-caudal direction. Soon after the patient