**1. Introduction**

Lung cancer is the most common type of cancer in OECD (Organization for Economic Co-operation and Development) countries. It is responsible for over 1.38 million deaths annually [1]. There are currently several treatments for lung cancer, including surgery, radiotherapy, chemotherapy and palliative care. One of the most effective is radiotherapy. It is because malignant cells are eliminated via ionizing radiation and it is less invasive for the patient [2]. Nowadays, more than half of all cancer patients receive radiotherapy, either alone or in combination with surgery or chemotherapy [3]. However, radiotherapy has detrimental side effects, as healthy tissue is also affected by these usually high-dose-rate radiation beams.

Radiotherapy involves the use of controlled doses of high-intensity radiation to kill cancer cells or to reduce the size of tumors. In the case of lung cancer, radiotherapy is used as a main treatment in patients for whom surgery is impossible. It can also be applied before or after the surgery, as palliative therapy or to relieve blocked airways. Radiotherapy is generated outside the body, in a Linear Accelerator (LINAC), and is generally used to treat non-small cell tumor.

Regarding the side effects of radiotherapy, a distinction between acute and chronic effects is made. The former are those that appear during treatment and usually disappear within a few weeks, such as fatigue and skin reactions, among others. Chronic side effects appear months or even years after treatment and may be permanent.

One of the essential parts of external radiotherapy treatment is planning [4]. Before starting treatment, it is important to calculate the dose needed, the emission angles and other parameters that will allow the patient to receive the right amount of radiation without affecting adjacent areas.

Compared to other parts of the body, the lungs are in constant motion [5,6]. According to Seppenwoolde et al. the movement of the lung in this way is largely two-dimensional and it is possible to reproduce the movement of any part of the lung when the length and capacity of the breath is known.

Previous studies have shown that a human adult breathes between 16 and 20 times per minute, which means one breath (inhalation and exhalation) every 3–3.75 s [7]. Closer inspection of the lungs shows that not all points move at the same rate and, therefore, not at the same speed [8]. Nevertheless, the general movement can be equated to a hysteresis loop [9,10], as shown in Figure 1. This is why lung tumor irradiation cannot be concentrated in a fixed tumor position at each phase of the breathing cycle, and it must be applied with small margins of the target volume to cover the tumor position during the entire respiratory cycle. To irradiate the tumor, it is necessary to follow the movement of the tumor [11] to plan the radiotherapy treatment.

**Figure 1.** Hysteresis loop of lung tumors movement: (**A**) The orthogonal projections of the trajectories of the 21 tumors on (left) the coronal (LR-CC) and (right) the sagittal (AP-CC) plane is shown (reproduced with permission from [9]); and (**B**) One recreated hysteresis cycle that simulates the movement of the tumor is displayed.

Good planning will reduce the amount of radiation that healthy tissues receives, while reducing the side effects. To verify the treatment of sophisticated radiation techniques in lung cancer, a parametric simulator of the tumor movement is needed.

The imaging techniques currently available, such as the 4D Computer Tomography (4DCT), acquire images synchronized with respiratory movement [12,13]. This permits to track the tumor position in each phase of the cycle and extract a custom path from patients [14–19]. These images are taken with the patient immobilized, in the same way in which the future treatment will be applied in each of the sessions.

In this work, we present a proof of concept of a lung tumor movement simulator prototype, which considers that not every human has the same breathing characteristic curve. In addition it can be adapted to any possible specific height, width and speed of breathing movement. Although there are some commercial alternatives, they are not Open-Source. The Lung Tumor Movement Simulator was entirely created using 3D printed parts in order to achieve a customizable device that can be adapted

for specific cases. Furthermore, it was printed with an Open-Source 3D printer that makes it affordable for every research centre. With this device, individualized radiation treatment plans can be tested in advance.

### **2. Materials and Methods**

### *2.1. Linear Accelerator*

A Linear Accelerator (LINAC) is a device that is most commonly used to give external beam radiation therapy to patients with cancer. It supplies high-energy X-rays to the tumor region of the patient. These X-ray treatments can be designed to destroy cancer cells without affecting normal surrounding tissues.

The linear accelerator uses microwave technology to accelerate electrons and then allows these electrons to collide with a heavy metal target to produce high-energy X-rays [20]. These high energy X-rays are shaped as they exit the machine to target the tumor in the patient. The beam is shaped by a multi-leaf collimator that is incorporated into the head of the LINAC. The patient lies on a movable treatment couch and lasers are used to make sure that the patient is placed in the desired position. The ionizing beam is emitted from a part of the accelerator called a gantry, which can be rotated around the patient. Moreover, radiation can be delivered to the tumor from any angle, by rotating the gantry and by moving the treatment couch.

Because lungs are in constant motion, the tumor is constantly changing its position and this movement should be taken in account. This movement of the tumor is always studied before starting the treatment (Figure 1). To increase the LINACs accuracy and avoid irradiating healthy areas, there is a technique called tumor tracking [14] that is useful to analyze the movement of an internal lung tumor. Based on these tumor tracking studies, and to improve customized radiotherapy plan verification, the Lung Tumor Movement Simulator was created. It provides a good simulation of tumor movement and a good radiation measure because it holds a dosimeter in the Tip. In this case the Tip was designed to contain a Landauer OSL nanoDot™ (Landauer, Inc., Greenwood, IL, USA).

#### *2.2. Electromechanical Components*

To give motion to the mechanism, a pair of NEMA 17 stepper motors was used. The stepper motors have a minimum step angle of 1.8 degrees (200 steps/revolution). Each phase needs 280 mA to 7.4 V, allowing a torque of 650 g-cm (9 oz-in). To drive both stepper motors, a pair of stepper controllers DRV8825 (Texas Instruments, Dallas, TX, USA) were used, which features adjustable current limiting and six microstep resolutions (down to 1/32-step).

In cyclic or rotating movements, it is important to know the starting point of the rotation. For this reason, endstops are needed. Endstops sensors are electronic components that function as switches, sending signals when an element is placed in a certain position. There are various types such as mechanical, optical and magnetic. In the prototype two optical endstops are used because they can be activated by some element of the mechanism (Figure 2, element 3 and 9). At the same time, this type of sensor avoids the possible rebounds that may appear in mechanical sensors.

#### *2.3. Microcontroller*

The Arduino Mega 2560 (Smart Projects, Turin, Italy & SparkFun Electronics, Boulder, CO, USA) development board is a printed circuit that allows the use of a microcontroller ATMEGA2560 (Microchip Technology Inc., Chandler, AZ, USA) [21]. Arduino is commonly used in a high variety of research fields due to its versatility and low cost [22–25].

This microcontroller controls 54 digital Input/Output pins, 15 pulse width modulation pins, and 16 analog pins, and is able to automate any system. Documentation and software are open source and available at the Arduino website. The programming software is based on the C/C++ language and the power supply of this development board can be powered through USB or main supply from 5 V to 12 V.

**Figure 2.** (**A**) Real Lung Tumor Movement Simulator prototype; and (**B**) Schematic parts of the prototype. See Table 1 for details.


**Table 1.** Prototype parts.

All of these features makes Arduino perfect for this project, but, to connect the drivers to Arduino, which control the stepper motors, an Arduino-Shield was needed. The Reprap Arduino Mega Pololu Shield (RAMPS) is a board specially designed for the use of the selected drivers. This can control up to five stepper motors and support the connection of various endstops. The RAMPS board can only be used together with an Arduino Mega 2560 board on which it is placed by inserting the male connectors of the RAMPS into female Arduino pins.

#### *2.4. Manufacture and Material*

The entire project was thought to be printable in any commercial or Open-Source 3D printer. A Prusa i3 MK2 with a precision of 50 microns per layer height, and a printing surface of 10,500 cm3 (25 × 21 × 20 cm or 9.84 × 8.3 × 8 in) was used to print all of the parts. The entire prototype was manufactured with poly(lactic acid) (PLA) material. This material was chosen because it achieves the specifications that are need for the prototype [26]. It is light and strong enough to enable the mechanism to function smoothly. Furthermore, it is biodegradable and environmentally friendly.
