Motion Effects on Spatially Fractionated Lattice Therapy

Abstract

The practice of spatially fractionated radiation therapy (SFRT), such as grid and lattice, has been shown to be effective in managing large-size tumors for palliation or more recently for medium-sized tumors with definitive intents. The main feature that differentiates SFRT from standard radiation therapy is the deliberately high degree of dose heterogeneity in the gross tumor volume (GTV). The key parameter in assessing an SFRT plan is the valley-to-peak dose ratio (VPDR), which can be defined as a simple dose ratio between the low- and high-dose region in the tumor. The belief is that the healthy tissues in the low-dose regions of the tumor would serve as centers of tissue repair, while the high dose would kill the cancerous cells and induce the bystander effects. However, the compartments of low- and high-dose regions can be washed out due to motion, for example in cases when the disease is at or near the diaphragm. This work aims at examining motion effects on VPDR and equivalent uniform dose (EUD) in SFRT plans. This work focuses on the effects of sinusoidal motion in lattice therapy, a 3D version of SFRT. A lattice VMAT plan with 6X was generated using the treatment planning system. Dose vertices were placed in a body-centered tetragonal lattice in a virtual water phantom. Each vertex was 1 cm3 and received 15 Gy to at least half of its volume. The volume ratio between the lattice and the GTV is about 3%. The distance between the two nearest vertice centers is 3 cm. A sinusoidal motion was introduced in the direction along the line connecting the two nearest neighbors and was binned into 10 phases with equal time intervals. The location of the phantom in each phase was determined by its average amplitude. The effect of the motion was assessed from the sum of all the plans in the 10 bins, each being scaled down by one-tenth of the prescribed dose. Dose coverage between the static and the sum plan is compared. Their difference in the VPDR and normal tissue damage with EUD are evaluated. The EUD was calculated based on Niemierko's formalism. The LQ model was used to estimate cell survival for normal tissues with a and b values of 0.366/Gy and 0.188 Gy2, respectively. It is common to have the VPDR at around 1/3 or lower. Our study shows that it increases from 0.26 to 0.55 for the 1.5 cm motion and the EUD for normal tissue from 5 to 7 Gy. See Table 1 for more results. A large VPDR can reduce the ability of healthy tissues in the low-dose area for repair and a lower maximum dose in the vertices may diminish the bystander effects. Motion may increase VPDR and normal tissue damage. Motion techniques such as gating or tracking can be used on disease sites subjected to respiratory motion.