**1. Introduction**

Lung cancer continues to be the leading cause of cancer death in the United States, and over 158,000 lung cancer deaths were estimated in 2015 [1]. Radiation is one of the major treat‐ ment modalities for lung cancer treatment. Because of proton beams' finite range, proton beam therapy (PBT) has been increasingly used for lung cancer. Compared to 3D conformal or intensity modulated photon radiation (IMRT), proton beams can better spare the lung, esophagus, heart, cord, and other normal tissues while delivering the same or higher dose to the treatment target [2–4]. The dosimetric advantage of proton therapy could lead to potential better tumor control and less toxicity. Proton beams provide a superior dose distribution due to their finite ranges, but where they stop in the tissue is very sensitive to anatomical change. To ensure optimal target coverage and normal tissue sparing in the presence of geometrical

variations, such as tumor shrinkage and other anatomical changes, plan adaptation is often needed in proton therapy of lung cancer.

The chapter aims at illustrating the rationale and process in adaptive proton treatment of lung cancers, as well as the strategies and practical considerations in plan adaptation, with a focus on the use of uniform scanning proton beams.

### **2. Proton therapy system**

Depending on how proton beams are spread out laterally and in depth, there are mainly three proton delivery systems in clinical use: passive scattering proton therapy (PSPT), uniform scanning proton therapy (USPT), and pencil beam scanning (PBS). In PSPT, the proton beam is spread out laterally by a static scatterer (or double scatterers) located in the beam axis, and the beam modulation in depth is typically achieved by using a rotating range modulation wheel, which is composed of multiple steps of various thicknesses. Both USPT and PBS proton therapy use scanning magnets to sweep proton beams laterally and deliver the dose to a target volume layer by layer at various depths using proton beams of various energies. The main dif‐ ference between USPT and PBS is that proton beams are scanned continuously with a uniform intensity in a zigzag pattern at a fixed frequency for each energy layer in USPT, while deliv‐ ered with various beam intensities from one spot to another or continuously for each layer in PBS. PBS can be further divided into single field uniform dose (SFUD) delivery, which deliv‐ ers a uniform dose to the target for each field, and multiple field uniform dose (MFUD), which delivers a heterogeneous dose to the target for each field but achieves a homogeneous com‐ bined dose from all fields. MFUD is also called intensity modulated proton therapy (IMPT).

Since our main focus for this chapter is USPT, a detailed description of a USPT system at our center is described below. The proton therapy center is equipped with an IBA Cyclotron (IBA, Louvain‐la‐Neuve, Belgium), which accelerates proton beams to approximately 230 MeV before they are extracted to treatment rooms through a beam transportation system. The proton beam passes through an energy degrader, which can lower the energy when necessary, and an energy selection system (ESS) is then transported to a nozzle in the treatment room. After entering the nozzle, the proton beam will first pass through a first scatterer, which broadens the beam laterally to achieve the desired spot size at isocenter. The beam then passes through a range modulator wheel, which does not rotate continuously for uniform scanning beam delivery and mainly serves as an energy degrader. Together with the first scatterer, the modulator wheel lowers the proton energy to deliver a peak dose layer by layer in depth. The beam is scanned laterally with a constant frequency by two scanning magnets in a zigzag pattern to deliver a uniform dose for a near rectangular scanning area. It then passes through the main and backup ionization chambers that monitor the proton dose. At the end of the nozzle is a snout that holds an aperture and a compensator and can translate along the beam axis to achieve variable snout to isocenter positions. An aperture is used to collimate the beam to the treatment target later‐ ally, and a range compensator is used to conform the proton penetration to the distal boundary of the treatment target. More details on this system were described by Zheng *et al.* [5]. **Figure 1** shows a schematic diagram of the uniform scanning nozzle at our proton therapy center.

**Figure 1.** A schematic diagram of the uniform scanning nozzle at the ProCure Proton Therapy Center in Oklahoma City. Proton beams (P) go through a first scatterer (A), a range modulator wheel (B), two scanning magnets (C and D), the main and backup monitor unit ionization chambers (E), a snout (F), an aperture (G), a range compensator (H), and stop at the patient (I). The nozzle has a distance of about 290 cm between the first scatterer and the isocenter, and 211 cm between the effective source and the isocenter. (From Zheng *et al.* [5]).
