Technical Aspects Of Conventional Pituitary Radiotherapy

Conventional radiotherapy for pituitary adenoma is given with photons by external beam irradiation using a linear accelerator. Interstitial irradiation with insertion of radioactive sources (gold or yttrium) was practiced more than 20 yr ago and is no longer routinely employed. The practical steps in the planning and delivery of conventional external beam radiotherapy include immobilization, computed tomography (CT) and magnetic resonance imaging (MRI) scans for accurate localization of the tumor and computerized 3D planning for localized treatment delivery.

The precision of the entire treatment planning and delivery process is aided by the use of immobilization devices. The most frequently employed is an individually molded closely fitting plastic mask, which can be relocated with an accuracy of 2-5 mm. Increased precision can be achieved with relocatable frames with a relocation accuracy of 0.5-1 mm based on operator skill.

Treatment is given with the patient lying supine. Imaging to define the site of irradiation is performed in the same position, usually in the immobilization device used for treatment. During the planning process, the CT or MRI visible mass is delineated on individual CT/MRI slices on the planning computer, and this is defined as the gross tumor volume (GTV). Pituitary adenoma is best viewed with MRI either alone or combined with CT particularly to delineate cavernous sinus involvement. The margins of the volume (GTV) must also consider information about possible residual disease after surgery. Preoperative images and details of the operative procedure provide important additional information to aid the treatment planning process, particularly to distinguish postoperative changes from residual tumor. Based on the known technical uncertainty of planning, immobilization, and treatment delivery, a 3D margin is added to the GTV and, in conventional radiotherapy, this varies between 5 and 10 mm. The margin should be based on the actual measurement of uncertainty of the whole planning and treatment process specific to a radiotherapy center. GTV with a margin is defined as the planning target volume (PTV). The aim of conventional radiotherapy is to deliver homogeneous dose to the PTV and least dose to surrounding normal tissue.

Conventional radiotherapy uses three fixed radiation fields—an anterior oblique field aimed at the pituitary through the forehead and two lateral opposing beams aimed at the pituitary through the temporal regions. Individually shaped shielding blocks or leaves of a multileaf collimator are increasingly used to shield out normal structures to minimize irradiation of normal brain, and this is described as conformal radiotherapy.

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