Sara Janek¹, Cathrine Jonsson², Roger Svensson¹, Rickard Holmberg¹, Anders Brahme¹

Introduction: To maintain a high quality in radiation therapy it is necessary to have a precise system for dosimetric verification. This is particularly important when IMRT is used where the beams must really be delivered accurately and the dose delivery of the treatment unit must be precisely and safely executed according to the treatment plan. The most effective way to achieve this would be by adaptive therapy where the delivered dose is really measured and possible deviations from the treatment plan are adjusted during proceeded treatment. This could be accomplished by using high energy photon beams. When using 50 MV photons (or at least above 20-30 MV) it is possible to generate a sufficiently activity of photonuclear reactions in the treated tissue to allow verification of the delivered dose distribution using a PET-CT of high resolution and high sensitivity.

Methods: The technique is based on the activation of body tissue by high energy bremsstrahlung beams resulting primarily in ¹¹C and ¹⁵O but also ¹³N, all positron-emitting radionuclides produced by photoneutron reactions in the nuclei ¹²C, ¹⁶O and ¹⁴N. PMMA and graphite phantoms as well as frozen animal tissue were irradiated to 5- 10 Gy and the induced positron activity distributions were measured off-line in a PET camera a couple of minutes after irradiation. The accelerator used was a Racetrack Microtron using 50 MV scanned photon beams. The PET measurements were performed with three different scanners, ECAT EXACT 921, ECAT EXACT HR and Biograph 64.

Results: Fused images consisting of CT- and PET images and planned dose distributions shows that the delivered dose distributions and the induced positron activity are not overall congruent. This is mainly due to tissue differences within the material and the fact that the absorbed dose distribution is determined by the secondary electrons set in motion by the whole photon spectrum whereas the PET distribution is the true tissue activation only from photons above the photonuclear cross section threshold energy. Since measured PET images change with time post irradiation, as a result of the different decay times of the radionuclides, the signals from activated ¹²C, ¹⁶O and ¹⁴N within the irradiated volume could be separated from each other. Oxygen, which constitutes about 2/3 of the total elemental composition in soft tissue, will dominate the PET signal in measurements taken a few minutes after the photon irradiation due to the short physical half-life of ¹⁵O and its high production cross section.

Conclusions: Based on the results obtained in this preliminary investigation a great value of a combined Radiotherapy-PET-CT unit is indicated in order to fully exploit the high activity signal from oxygen immediately after treatment. Furthermore, because of organ movements, the need of a RT-PET-CT unit is recommended in order to acquire PET-CT data in the same treatment position and avoid inaccuracies coming from the transport and repositioning of the patient in another diagnostic site. Such a diagnostic therapy unit for imaging the delivered dose distribution may become an ideal tool for adaptive IMRT dose delivery.