International Journal of Radiation Oncology*Biology*Physics
Physics contributionConformal radiotherapy (CRT) planning for lung cancer: analysis of intrathoracic organ motion during extreme phases of breathing
Introduction
Optimization of three-dimensional conformal radiotherapy of intrathoracic tumors is based largely on improvement of the localization of target volumes. Radiotherapy beams are defined on the basis of static computed tomography (CT) acquisitions, by taking into account the patient’s predictable movements from one session to the next and organ/tumor motion during breathing (1). In the absence of precise data, the width of the margins is estimated arbitrarily, potentially resulting in either excessive exposure of normal tissues or insufficient target volume coverage 2, 3, 4, 5.
In the context of the lung, the estimated target volume comprises the clinical target volume (CTV), together with the uncertainties related to internal organ motion during breathing and uncertainties related to placement of irradiation fields and patient movements (6). For some authors, tumor movements correspond to a modification of the macroscopic tumor volume, whereas for others, they are an integral part of the definition of the estimated target volume. This controversy is purely theoretical, because as the final objective is to define a volume that is entirely included in the 95% isodose (5).
ICRU Report No. 62 distinguishes an internal target volume, taking into account movements of the CTV during breathing and a supplementary volume by addition of an external margin related to errors of irradiation beam positioning (6). Few data are available to define these volumes 4, 7, 8. The main objective of this study was to evaluate the amplitude of maximum intrathoracic organ motion during breathing. We also tried to determine whether the use of a personalized immobilization device had an impact on the amplitude of these respiratory movements.
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Patients
Twenty patients treated for inoperable non-small-cell lung cancer were included in the study: 10 patients treated at the Institut Curie with a personalized alpha cradle immobilization and an armrest above the head, and 10 patients treated at Tenon Hospital, with the arms above the head held with a Posirest device below the arms. Table 1 presents the characteristics of these 20 patients.
Thirty-five measurements were performed per patient and per acquisition. The patients from the two centers
Measurement comparisons of thoracic displacements at deep inspiration and deep expiration
Table 2, Table 3, Table 4, Table 5 summarize the amplitudes of the various thoracic displacements at each institution. In the overall study population, the greatest displacements were observed for the diaphragm in the craniocaudal direction, with mean values between inspiration and expiration of 34.25 ± 20.4 mm and a maximum amplitude of 67.8 mm (Table 2).
The smallest displacements were observed for the lung apices (Table 3). The differences between inspiration and expiration in the three
Discussion
Movements of the tumor and intrathoracic organs during breathing are a potential cause of failure of radiotherapy for lung cancers 11, 12. These movements lead to uncertainty responsible for insufficient coverage of the limits of the CTV and, consequently, the delivery of an insufficient dose to destroy tumor cells 2, 4, 7.
Four approaches can be proposed to reduce the uncertainties related to respiratory movements (13):
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Measure tumor movements, to take them into account in the final choice of
Conclusion
Movements of thoracic structures during extreme phases of breathing are considerable, particularly in regions close to the diaphragm. Identification and quantification of these movements are necessary to define the safety margin around the clinical target volume. A personalized immobilization device molded around the patient, combined with an armrest, appears to effectively reduce apical and lateral displacements, resulting in a reduction of the volume of healthy lung irradiated. Measurement
Acknowledgements
The authors thank Drs. F. Reboul, R. Garcia, and R. Oozeer at Clinique Sainte Catherine (Avignon, France) for their technical support in the conception of our immobilization device.
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