The HeartSpare Study (Stage I)

Patients undergoing scanning in v\_DIBH will be positioned supine with arms extended on a MedTec breast board. They will be monitored visually on screen and will be given a buzzer to press when they have taken a breath. CT images will be obtained during DIBH. If the patient needs to release their breath, they will be asked to press the buzzer a second time. Patients will be closely monitored by radiographers via CCTV.

Patients undergoing scanning in ABC\_DIBH will be positioned supine with arms extended on a MedTec breast board to which an activated-breathing control device (Elekta, Crawley, UK) has been attached. They will breathe through the device. During ABC\_DIBH, CT images will be obtained.

Patients undergoing scanning in a prone position will lie on a customised prone treatment platform. CT images will be acquired under free-breathing conditions.

The ABC unit allows for temporary and reproducible immobilization of internal thoracic structures by monitoring the patient's breathing cycle and implementing a breath hold at a predefined lung volume level. The device consists of a mouthpiece connected to a pneumotachometer via a bidirectional valve and tubing. The air flow across the pneumotachometer is integrated and displayed on a computer monitor. The computer also controls a scissor valve which, when enabled, restricts air flow through the system, enabling the breath hold phase. The mouthpiece and air filter are patient specific, as is a nose clip that is used to ensure that patients are breathing through the device.