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

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 [7-9]. 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 materials

The 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 × 3.0 × 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 × 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 to the fact that 256 multislice CT can acquire an approximately 12 cm longitudinal length in a single rotation. The CT scan was started manually by the radiological technologist who observed the respiratory monitor and acquired the first respiratory phase in the 4D CT as T10 (early exhalation). The 4D CT data set was subdivided into 10 phases based on respiratory amplitude and peak inhalation and exhalation were defined as T00 and T50, respectively.

The water-equivalent path length (WEL) was calculated by converting the geometric path length to an electron density for a charged particle beam [10]. The WEL calculation region begins at the point where the external beam enters the patient to a point at the mid-plane of the patient in the right–left (RL) and left–right (LR) directions in circular regions of interest (ROIs) of 30 mm diameter.

Results and discussion

4D CT images as a function of a respiratory phase are shown in Figure 1. Diaphragm position and tumour were moved down around the inhalation phase with the diaphragm attaining its highest position at T10 (around early inhalation), with that at T90 (end exhalation) being almost the same (marked as a light blue line). The correlation of organ motion in the thoracic and abdominal regions with respiratory phase is well known; geometrical shape at both phases was also closely similar. The thickness of the right-side posterior chest wall at T10 (marked with a white arrow in Figure 1a) was almost the same as that at T30, while those at T50 and T90 were thicker. The opposite situation arose in the anterior right chest wall with the right anterior chest wall thinner at T50 and T90 than at T10 and T30. These variations in chest wall thickness were not correlated with respiratory phase.

Figure 1.

Four-dimensional CT images in axial and coronal sections as a function of respiratory phase. Light blue and green lines show diaphragm position at T10 and gross tumour volume (GTV), respectively. Right-side chest wall thickness at T90 was thicker than that at T10, even though diaphragm position at the two phases was almost the same (light blue line).

In clinical situations, because the treatment beam angle can be selected at around 270°, quantification of WEL is important. WEL difference (ΔWEL) images were calculated by registered pixel-by-pixel subtraction of the WEL images at end inhalation (T90) from those at early exhalation (T10) in the LR and RL directions (Figure 2a,b). This gave a better understanding of ΔWEL owing to chest wall variation because tumour and anatomical position in T10 and T90 were similar. Red regions (positive ΔWEL value) in the anterior chest wall were due to chest wall positional variation between T10 and T90 in Figure 2a,b. Small blue regions were observed in Figure 2a; however, the ΔWEL map showed WEL values of less than –6 mm and less than 10 mm over ROI2 and ROI4, respectively, in the RL direction (Figure 2b). These positive and negative WEL variations were caused by chest wall thickness variation, as shown in Figure 1. However, they were not seen when the calculation was done in the LR direction.

Figure 2.

Water-equivalent path length difference (ΔWEL) images (T90 minus peak-exhalation T10) overlayed on sagittal images as a function of respiratory phase, with a calculation direction of left to right (a) and right to left (b). Four regions of interest (ROIs) were defined over the lung region to calculate the average pixel values enclosed. ΔWEL values (each phase (Tn) minus peak-exhalation (T10)) averaged in ROIs in (a) and (b) are plotted in (c) and (d), respectively.

Mean ΔWEL values enclosed in ROIs are plotted in Figure 2c,d as a function of respiratory phase. WEL variations of less than 3 mm in ROI1 and 1 mm in others were observed in the ΔWEL map with a LR calculation direction. In contrast, the ΔWEL map with an RL calculation direction showed WEL variation of less than 2 mm as a function of respiratory phase, but the magnitude of WEL values was increased over T30 in ROI2 (–3 mm WEL) and ROI4 (5 mm WEL), which remained.

In this study, we identified significant unintended variation in chest wall thickness in 4D CT data. Medical physicists should consider ways to minimise these variations, such as via repeat acquisition of 4D CT, even though this would increase patient radiation, and consideration of variation in coverage range at treatment planning, etc. We did not calculate dose distribution in this report because it is beyond the scope of this study, but these WEL variations might affect dose conformity to the target. Beam overshoot owing to a negative WEL extends the beam stopping position beyond the distal edge of the target, resulting in an increase in excessive dose to normal tissues beyond the target. In contrast, positive WEL variation causes undershoot, in which no particle beam is delivered to the distal side of the target, therefore positive WEL variation is accordingly more important than negative variation. Periodic motion, such as respiratory and cardiac motion, is well understood and a number of reports have described its minimisation by treatment planning, immobilisation, etc. [11-13]. Other factors (unintended motion) such as bowel gas movement remain difficult problems in clinical care. The elasticity of the chest wall and consequently its thickness with respiration may change owing to various neuropathic conditions or from injury or fibrosing conditions. Our present patient, who showed positive WEL variation uncorrelated with respiratory phase, is the first such case experienced in our 4D CT studies among several tens of patients. Although the patient number was limited to a single case, we consider that the present report represents a significant contribution to current efforts to improve charged particle therapy for patients.