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Strategies for Image-guided Proton Therapy of Cancer
European Oncological Disease, 2007;1(2):116-8
During the past half century, an ongoing technological revolution in cancer imaging and radiation treatment has taken us ever closer to the goal of treating localised tumours without harming normal tissues. In his visionary 1946 paper,1 Harvard physicist Robert Wilson suggested that energetic protons could provide a nearly ideal form of radiotherapy. What makes protons the preferred particle type for radiotherapy is their inverted dose profile, called the Bragg peak, in combination with the ability to place the Bragg peak at any depth in the patient, to spread out the peak to cover larger volumes and to have zero dose behind the most distal peak position.
In 1989, Dr Wilson visited the first clinical proton treatment centre at Loma Linda University Medical Center in California, which was about to begin its clinical operations (the first patient was treated in October 1990). The reasons more than 40 years elapsed between his original idea to its full technical and practical realisation were manifold, but the lack of adequate image guidance, in both treatment planning and treatment itself, played a key role.
Computed tomography (CT) utilising kilovolt (kV) X-rays was not developed until the early 1970s. For the first time this imaging modality provided 3D information about tumour location and at the same time about the electron density distribution required to perform 3D dose calculations. Therefore, it was a natural choice to develop CT-based radiation treatment planning. Magnetic resonance imaging (MRI) entered the treatment planning scene about 10 years later and provided further details with respect to the geographical relationship between tumour and normal tissues, providing much higher spatial and contrast resolution than X-ray CT. It took another 15 years before positron emission tomography (PET), in particular its combination with X-ray CT, became available for radiation treatment planning and added another dimension to the ability to see tumours and to distinguish them from normal tissue, based on differential metabolism. For conformal radiation modalities such as proton therapy, imaging technology is equally important in guiding the delivery of radiation therapy. Image-guided radiation therapy (IGRT), respiratory gating and related technological advances are about to enter the treatment room in many radiotherapy facilities. The idea behind this is that modern imaging can help not only to detect and outline tumours during treatment planning, but also to ensure that the dose delivered to the tumour in the treatment room is accurate and precise.
Current Role of Image Guidance in Proton Treatment Planning
Treatment Planning Volumes
Most patients coming to a proton-radiotherapy centre for consultation have been diagnosed with localised, non-metastatic organ cancer, such as prostate or lung cancer, and seek proton therapy as their definitive therapy. In other instances, patients have received surgery as their firstline treatment and require post-operative radiation due to known residual tumour or suspected microscopic disease. Lastly, it is not uncommon to see previously irradiated patients who have failed their initial radiotherapy and reached tolerance dose in nearby organs or vital structures. In any of these cases, accurate imaging-based definition of treatment volumes is crucial. Standard radiation planning volumes, previously defined for photon therapy by the International Commission on Radiation Units and Measurements (ICRU),2,3 are also suitable for proton therapy. The ICRU planning volumes are: the gross tumour volume (GTV), including macroscopic tumours visible in imaging studies; the clinical target volume (CTV), containing suspected subclinical malignant disease; and the planning target volume (PTV), which expands GTV or CTV by a margin accounting for geometric uncertainties, including set-up errors and internal organ motion.
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