The present thesis is focused on the development and application of narrow scanned high energy photon beam for radiation therapy. The introduction of physically and biologically optimized intensity modulated radiation therapy (IMRT) requires a flexible and accurate dose delivery method to maximize the treatment outcome. Narrow scanned photon beams is a fast option for IMRT since it is not dependent on mechanically moving heavy collimator leafs and largely independent of the complexity of the desired dose distribution. Scanned photon beams can be produced by scanning an electron beam of low emittance, incident on a thin bremsstrahlung target of low atomic number. The large fraction of high energy electrons that are transmitted through the target has to be removed by a strong purging magnet. In the thesis a strong purging magnet, coupled with a magnetic scanning magnet, is presented for an intrinsic electron energy of 50 - 75 MeV and a source to isocenter distance of 75 cm. The available scan area at isocenter can be as large as 43 x 40 cm2 for an incident electron energy of 50 MeV and 28 x 40 cm2 at 75 MeV. By modifying the existing treatment head of the racetrack microtron MM50, it was possible to experimentally produce relevant dose distributions with interesting properties from 50 MV scanned narrow photon beams while deflecting the transmitted electrons onto a simplified electron stopper. The deflection of the transmitted electrons was studied both experimentally and by the Monte Carlo method. With high energy photons, treatment verification is possible through PET-CT imaging of the positron annihilations induced by photonuclear reactions in the patient. Narrow scanned high energy photon beams is the ideal beam quality since the activation efficiency and the effective photon energy will be more uniform than the filtered photon beam from a full range bremsstrahlung target. The induced 11C activity 50 MV by scanned narrow photon beams was measured using PET-CT imaging and compared with Monte Carlo simulations. The combination of fast flexible dose delivery with treatment verification using PET-CT imaging makes narrow high energy scanned photon beams a very interesting treatment modality for biologically optimized adaptive radiation therapy. / At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 3: Submitted. Paper 4: Submitted.

For classical and intensity modulated radiation therapy of deep-seated tumors, high-energy photons are the optimal radiation modality from an integral dose point of view. By using narrow scanned beams the treatment outcome can be improved substantially by delivering biologically optimized intensity modulated distributions often with sharp dose gradients. This requires using photons with energies well above 15 MV enabling verification of the treatment delivery in vivo by PET/CT imaging in a manner not previously possible. This new technique is based on the production of positron emitting radionuclides when the incoming high-energy photons interact through photonuclear reactions with the body tissues. The produced radionuclides, commonly 11C, 15O and 13N can then be monitored by PET and the distribution of activated nuclei show exactly where the radiation has penetrated the patient. In the subcutaneous fat, present in all humans, a high induced activity produces a perfect visualization of the location and even the intensity modulation of the incident beams. The reason for this is the high carbon content in combination with a low biological perfusion in fat tissues. Errors in the patient positioning such as setup errors or misplacement of the beams will thus show up in the PET images as a deviation from the actual radiation treatment plan. Interestingly, the imaged activity distribution from the subcutaneous fat also visualizes how the dose delivery can be deformed when the patient is erroneously positioned on the treatment couch as seen on the cover figure. Furthermore, the different half-lives of the produced radionuclides (20 min, 2 min, and 10 min, for 11C, 15O and 13N, respectively) allows for analysis of the dynamic behavior of tissue activity with the possibility of retrieving information such as tissue composition, biological and physical half-lives. The present thesis shows that considerable clinical information regarding the treatment delivery with high-energy photon beams can be obtained using PET/CT imaging. Although the study is based on the use of 50 MV photons the method may apply for beams with energies &gt; 20 MV at higher doses. / <p>At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 2: Submitted. Paper 3: Submitted. Paper: Manuscript.</p>