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2003 Bram van Asselen

Intensity-modulated radiotherapy for breast and head-and-neck cancer

Intensity-modulated radiotherapy (IMRT) is an advanced form of radiotherapy; the beam intensity can thereby be modulated. IMRT can be used to create a highly conformal dose distribution around a tumor, while reducing the dose to the surrounding normal tissue. IMRT can also be used to deliver a heterogeneous dose within the tumor. A higher dose is delivered to areas of high risk, such as a part of the tumor containing more tumor cells. In this thesis, IMRT is applied to the irradiation of breast and oropharyngeal cancer.

An IMRT technique for irradiation of breast cancer has been developed, based on the on the division of the tangential fields in four segments shaped by a multi-leaf collimator. The shape of these segments was obtained from an equivalent path length map of the irradiated volume. Using this IMRT technique, a more homogeneous dose distribution was achieved compared to the conventional technique. Furthermore the dose to the lung was reduced. A more complex case is the treatment of oropharyngeal cancer. Different dose levels are delivered to the primary tumor, the volume containing subclinical malignant disease and the lymph nodes. The dose to the spinal cord, brain and parotid gland should be minimized. The relation between the quality of the treatment plan and the number of beams in combination with the number of segments was investigated in order to obtain a clinically acceptable and deliverable plan. Seven beams were sufficient to achieve acceptable dose homogeneity while the dose to the spinal cord and brain was acceptable. The dose to the parotid glands was reduced without compromising the dose to the targets.

Special attention should be paid to the localization of the tumor when using IMRT to assure that the dose is delivered at the right location. A margin, taken for geometrical uncertainties, is therefore added to prevent underdosage of the target volumes. Due to this margin, the dose to the surrounding healthy tissues increases. In a theoretical study it was shown that a realistic reduction of this margin might result in a reduction of the probability for radiation induced xerostomia of approximately 20%.

In order to reduce the margin, the reliability and toxicity of the use of implanted gold markers for position verification during the irradiation of head-and-neck cancer was investigated. These markers were visualized using an a-Si flat panel imager. No acute major complications were observed. In order to prevent movement of the markers, they should be deep-seated. The random error of the geometric uncertainties obtained without correction during radiotherapy was 1-2 mm.

The intrafraction motions of the larynx were investigated during radiotherapy using an a-Si flat panel imager. Motions occurred due to swallowing, breathing, or movement of the tongue. An extra margin of approximately 3.5 mm should be applied to take into account frequent motions, which are probably related to breathing.

Two different segmental IMRT techniques have been developed in this thesis for respectively irradiation of the breast and the oropharynx. Both techniques have been implemented clinically. Furthermore, the position verification and intra-fraction motion for head-and-neck irradiation have been investigated.

Email: B.vanAsselen[at]umcutrecht.nl