Elsevier

Medical Dosimetry

Volume 32, Issue 1, Spring 2007, Pages 16-22
Medical Dosimetry

Intensity modulation with respiratory gating for radiotherapy of the pleural space

https://doi.org/10.1016/j.meddos.2006.10.002Get rights and content

Abstract

We wanted to describe a technique for the implementation of intensity-modulated radiotherapy (IMRT) with a real-time position monitor (RPM) respiratory gating system for the treatment of pleural space with intact lung. The technique is illustrated by a case of pediatric osteosarcoma, metastatic to the pleura of the right lung. The patient was simulated in the supine position where a breathing tracer and computed tomography (CT) scans synchronized at end expiration were acquired using the RPM system. The gated CT images were used to define target volumes and critical structures. Right pleural gated IMRT delivered at end expiration was prescribed to a dose of 44 Gy, with 55 Gy delivered to areas of higher risk via simultaneous integrated boost (SIB) technique. IMRT was necessary to avoid exceeding the tolerance of intact lung. Although very good coverage of the target volume was achieved with a shell-shaped dose distribution, dose over the targets was relatively inhomogeneous. Portions of target volumes necessarily intruded into the right lung, the liver, and right kidney, limiting the degree of normal tissue sparing that could be achieved. The radiation doses to critical structures were acceptable and well tolerated. With intact lung, delivering a relatively high dose to the pleura with acceptable doses to surrounding normal tissues using respiratory gated pleural IMRT is feasible. Treatment delivery during a limited part of the respiratory cycle allows for reduced CT target volume motion errors, with reduction in the portion of the planning margin that accounts for respiratory motion, and subsequent increase in the therapeutic ratio.

Introduction

Intensity-modulated radiation therapy (IMRT) has the potential to conform radiation doses tightly to target volumes. It can reduce normal tissue toxicity, relatively sparing parotid glands in head and neck neoplasms and rectum in prostate cancer, in comparison with standard techniques. In diseases with complex target volumes, IMRT generally results in superior dose distributions compared with those resulting from conventional radiation techniques.1, 2

The pleural space is a challenging disease site because it is adjacent to a large number of critical structures, complicated by organ motion. These structures include the ipsilateral and contralateral lung, the spinal cord, liver, kidneys, esophagus, and heart. Organ motion is potentially most problematic in the thorax,3 and organ motion with intensity-modulated delivery can lead to unreliable dosimetry. One approach to account for target and normal structure movement is gated delivery, which applies radiation only when the target is in a position or state with minimal residual motion.4 Pleural IMRT after extrapleural pneumonectomy (EPP) for patients with malignant pleural mesothelioma has been shown to be feasible.5, 6 However, treatment of pleural space with intact lung using gated IMRT has not been reported.

In this report we describe a gated pleural IMRT treatment technique, illustrated by a case of pediatric osteosarcoma with unilateral pleural metastases.

Section snippets

Patient

Our case is a 16-year-old male with a history of osteosarcoma of the right thigh. After a three-year disease-free interval, he developed spontaneous right-sided pneumothorax with two fairly large right pleural-based masses with a substantial right pleural effusion. He underwent several cycles of chemotherapy including high-dose methotrexate, ifosfamide, and etoposide with good response and resolution of the effusion, followed by resection of residual masses. Residual viable tumor with

Results

Respecting the normal tissue tolerances of the lungs and liver proved to be the most significant difficulty with treatment planning. Figure 2 shows the dose distribution in the right pleural space in an axial CT slice. The dose was escalated to the postchemotherapy high-risk volume (PTV2) while delivering a lower dose to the rest of the pleural space (PTV1). The majority of PTV1 (98.1%) was covered by the prescription dose (44 Gy), whereas the homogeneity factor (maximum dose/prescription dose)

Discussion

The objective was to irradiate the right pleural space to relatively high doses with SIB to areas with a high risk of harboring microscopic disease while sparing adjacent normal organs. Intensity modulation can be advantageous for large irregular targets with critical structures in close proximity,13 in this case, making it possible to produce a shell type isodose distribution that treats the entire pleural space while sparing a central core within the right lung volume.

The Memorial

References (33)

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