Monitoring Radiotherapy with PET

The proton beam from the cyclotron is sent to a patient’s tumor and, as a secondary effect, creates radioactive isotopes that can be imaged:

We are investigating an in-room, stand-alone PET (soon PET-CT) system which could potentially provide a cost-effective, high-performance option for PET range verification. Compared to in-beam PET, an in-room PET system involves a significantly lower cost, is more flexible and technically much easier to implement. A full-ring detector geometry can be used which improves both sensitivity (solid angle for data collecting efficiency) and image quality. Compared to off-line PET, instead of transporting and repositioning the patient at a remote site for imaging, with an in-room scanner the acquisition can be initiated very shortly (∼2 min) after irradiation while short-lived nuclides are still abundant, and the overall signal intensity is be significantly higher than in the off-line scenario.

Treatment bed in the (a) proton treatment and (b) PET scan positions during an in-room phantom study. After beam delivery, the treatment bed was rotated and translated, and the phantom was inserted directly into the scanner for the PET scan.

In two repeated in-room PET studies, activity washout can be clearly observed in a 30-min scan. The coincidence count rate curve shows high activity immediately after irradiation (mostly ¹⁵O) decreased quickly as a function of time.

The new NeuroPET/CT scanner positioned to take data in the proton therapy room, along with CT and PET images showing the resolution:

Proton therapy offers a highly conformal dose delivery due to the Bragg peak. However, with material-dependent depths, and large doses at the end of range, there is greater sensitivity to the uncertainty in the proton range. Inaccurate planning can lead to range uncertainties and an under-dose to the tumor or over-dose to a critical organ. Better knowledge of the extent of dose range deviations from treatment plans is needed to optimize proton therapy on an individual level. We have explored a variety of methods to accurately assess the dose range from the endogenously produced PET activity.
Although the PET signal results from nuclear interactions, while the therapeutic dose arises mainly from electromagnetic interactions, these events occur in such tight proximity that it is possible for a correlation to be found.

One method we have explored (lead by Chul Hee Min in Radiation Oncology) is comparing the difference in the mid-point between 25% and 50% of the maximum activity in PET vs simulated PET for line-profiles along the beam direction. This strategy was employed on patient data for the entire distal surface of the activities, producing a map of under- and overshoot.

Reprinted from International Journal of Radiation Oncology*Biology*Physics, 86, Chul Hee Min, Xuping Zhu, Brian A. Winey, Kira Grogg, Mauro Testa, Georges El Fakhri, Thomas R. Bortfeld, Harald Paganetti, and Helen A. Shih, Clinical Application of In­Room Positron Emission Tomography for In Vivo Treatment Monitoring in Proton Radiation Therapy, 183-189, Copyright 2013, with permission from Elsevier.

Another range comparison method we are currently investigating involves comparing the endpoints of the PET activity and planned dose directly. The main advantage of this strategy is avoiding time-consuming MC simulations.

Dose (a) and MC-PET (b) distributions for patient A and profiles along the white lines (c). Endpoints for MC-PET and dose are indicated with vertical lines.