Changes in chest wall thickness during four-dimensional CT in particle lung treatment planning

Abstract

ABSTRACT. Four-dimensional (4D) CT images for charged particle lung therapy were acquired using a 256 multislice CT without couch movement. The thickness of the posterior right chest wall changed with respiration with a water equivalent path length (WEL) of more than 5 mm over the mid-exhalation phase but the thickness of the left chest wall did not vary. State-of-the-art radiotherapy techniques in charged particle radiotherapy, such as intensity-modulated proton therapy (IMPT), have improved dose conformity to the tumour [1-3]. The Bragg peak positions for a charged particle beam can be changed by temporal variations in path length and density changes of the tissues traversed. Intrafractional target motion from respiratory and cardiac activity is a major source of treatment uncertainty, especially in the treatment of tumours of the chest and upper abdomen. In an effort to minimise uncertainties owing to respiratory motion, several treatment centres have developed image-guided radiotherapy (IGRT) and respiratory-gated radiotherapy techniques. These motion mitigation techniques have been proven to reduce targeting uncertainty and with several encouraging findings being reported [4], they have been incorporated into current radiotherapy strategies to improve local control. With the commercial availability of four-dimensional (4D) CT [5,6], several centres have now investigated the irradiation of thoracic and abdominal irradiation during free breathing. Most research has focused on range variation around the tumour itself and evaluated changes using different imaging technologies [7][8][9]. However, an external treatment beam may transit through normal tissues such as chest wall, pulmonary vessels, oesophagus, bone, heart and other critical structures before delivering the therapeutic dose to the tumour. The radiological path length has been shown to change owing to organ motion and thereby result in significantly diminished dose conformity.Here, we report a single case of variation in chest wall thickness on 4D lung CT, which did not correlate with respiratory phase and quantified the range variation in this case. Methods and materialsThe patient was a 65-year-old female who had signed an informed consent form for 4D CT scanning and treatment. The treated tumour measured 2.7 6 3.0 6 2.8 cm in size and was located in the right lower lobe. The respiratory period was 5.5 s. A 4D CT scan was generated to acquire volumetric CT data as a function of respiratory phase under free breathing conditions using 256 multislice CT. The patient was fixed on the patient bed under immobilisation in a body cast in accordance with routine practice in our centre. To minimise the effect of anxiety on perturbing the stable breathing pattern (respiratory cycle, baseline drifts, etc.), 4D CT scanning was carried out after a 10 min rest in the supine position on the CT bed. Scan conditions were slice collimation of 256 6 0.5 mm, 0.5 s per rotation and scan time of less than 6 s to obtain one respiratory cycle with no table movement, owing ...