Abstract
A dynamic lung tumor phantom was used to investigate the geometric reconstruction accuracy of a commercial four‐dimensional computed tomography (4D‐CT) system. A ball filled with resin, embedded in a cork cube, was placed on a moving platform. Various realistic antero‐posterior (AP) motions were programmed to reproduce the respiratory motion of a lung tumor. Several three‐dimensional (3D) CT and 4D‐CT images of this moving object were acquired and compared using different acquisition parameters. Apparent volume and diameter of the ball were measured and compared to the real values. The position of two points (the AP limits of the ball) during the motion in the coordinate system of the CT scanner were also compared with the expected values. Volume error was shown to increase with object speed. However, although the volume error was associated with intraslice artifacts, it did not reflect large interslice inconstancies in object position and should not be used as an indicator of image accuracy. The 3D‐CT gave a random position of the tumor along the phantom excursion; accuracy in the assessment of position by 4D‐CT ranged from 0.4 mm to 2.6 mm during extreme phases of breathing. We used average projection (AVE) and maximum intensity projection (MIP) algorithms available on the commercial software to create internal target volumes (ITVs) by merging gross tumor volume (GTV) images at various respiratory phases. The ITVs were compared to a theoretical value computed from the programmed ball excursion. The ITVs created from the MIP algorithm were closer to the theoretical value (within 12%) than were those created from the AVE algorithm (within 40%).PACS numbers: 87.53.Xd, 87.56.Mp, 87.57.Ce, 87.59.Fm
Highlights
The importance of respiratory motion in radiotherapy of tumors located in the chest has been reported in numerous publications.[1,2,3,4] It is commonly admitted that breathing-adapted radiotherapy should improve local control by increasing the accuracy of targeting—for example, for non-small-cell lung cancer.[5]. If no significant improvements in survival have yet been reported using these techniques, several studies have highlighted better dose conformity and a reduction in dose to organs at risk (OARs).(6–10)
If breath-hold is achieved at deep inspiration, dosimetric benefits have been observed for lung.[6]. In the gating technique, the patient is treated at a particular phase of free breathing, generally by placing an external marker on the patient’s chest and synchronizing beam delivery with the motion of the marker.[12,13,14] For this group of patients, the treatment plan should be based on a breathingadapted imaging technique
By performing a cross-comparison between spirometer and 4D-CT, we demonstrated how lung volume could be obtained accurately from either technique.[21]. On the other hand, Rietzel et al used a dynamic phantom animated with a sinusoidal motion to study the influence of superior–inferior (SI) motion on 4D-CT images
Summary
The importance of respiratory motion in radiotherapy of tumors located in the chest has been reported in numerous publications.[1,2,3,4] It is commonly admitted that breathing-adapted radiotherapy should improve local control by increasing the accuracy of targeting—for example, for non-small-cell lung cancer.[5] If no significant improvements in survival have yet been reported using these techniques, several studies have highlighted better dose conformity and a reduction in dose to organs at risk (OARs).(6–10) These new techniques that manage the patient’s respiratory motion during treatment are separated into two groups: breath-hold techniques[6,11] and gating techniques.[12] In the breath-hold technique, the patient is imaged and treated during a monitored breath-hold. In the case of therapy with a reduced target volume, small geometric uncertainties could lead to large dosimetric errors—underdosage of tumor and overdosage of OARs
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