PET imaging for treatment verification of ion therapy
The favourable physical properties of ion beams in comparison to conventional photon and electron external radiation treatment modalities offer improved conformality for the delivery of the prescription dose to the tumour and better sparing of surrounding critical structures and healthy tissues. However, full clinical exploitation of this advantage is still hampered by uncertainties in the localisation of the distal dose fall-off within the patient. Thus, tools for in-vivo confirmation of the actual beam delivery and, in particular, of the ion range in the patient would be highly beneficial.
During treatment, positron emitters like 15O (half-life T½=121.8 s) and 11C (T½=1223 s) are produced in nuclear interactions between the impinging therapeutic radiation and the irradiated tissue. Detection of this small (0.3-1 kBq/Gy/ml) irradiation-induced transient radioactivity via Positron Emission Tomography (PET) may offer a valuable tool for verification of the delivered treatment. In particular, the comparison of the measured activity with its expectation based on the intended dose application serve as an in-vivo, non-invasive range validation method of the whole treatment planning and delivery chain, potentially calling for plan adaptation in case of detected discrepancies. For technical implementation, dedicated PET imaging during irradiation (in-beam) requires the development of customized, limited angle detectors with data acquisition synchronized with the beam delivery. Alternatively, commercial PET or PET/CT devices installed in close proximity to the treatment site may enable detection of the residual activation from long-lived emitters shortly after treatment (offline).
After a brief introduction to the principles of PET monitoring of ion therapy, this talk will review two examples of clinical implementation and results using a customized in-beam PET scanner for verification of carbon ion therapy at GSI Darmstadt, Germany [1], as well as a commercial off-line PET/CT tomograph for post-radiation imaging of proton treatments at Massachusetts General Hospital, Boston, USA [2]. Challenges as well as pros and cons of the two approaches will be discussed and, in particular, a quantitative comparison of the measurable PET signal depending on the imaging strategy and ion beam delivery system will be presented.
[1] Enghardt W, Parodi K. et al, Dose quantification from in-beam
positron emission tomography Radiother Oncol. (2004) 73 S96-8.
[2] Parodi K., Paganetti et al, Patient study on in-vivo verification of
beam delivery and range using PET/CT imaging after proton therapy Int.
J. Radiat. Oncol. Biol. Phys. (2007), in press
