Speedy in-room PET shows potential for planning proton therapy

In-room PET scanning during proton therapy is feasible for monitoring treatment ranges and helps overcome logistical issues inherent with transporting a patient to a PET scanner outside the treatment room, according to a small clinical trial published online Feb. 5 in the International Journal of Radiation Oncology*Biology*Physics.

“We found that the standalone in-room PET scanner on wheels could be promptly positioned after radiation delivery in as little as two minutes, and this represents a reasonable system that could be replicated at other proton facilities,” wrote Chul Hee Min, PhD, and colleagues from Massachusetts General Hospital in Boston.

The authors explained that proton beams can deliver high radiation doses to target tumor volumes while avoiding damage to adjacent healthy tissues, though uncertainty remains in converting CT numbers to stopping power when treatment planning.

“Gaining insight into the end range uncertainties is very important to optimizing treatment and patient safety in proton radiation therapy,” wrote Min and colleagues. To this end, PET activity along the beam passage has been studied to assess its correlation with proton dose distribution. However, the short half-life of positron emitters is a limitation when bringing patients to an off-line PET scanner outside the treatment room, explained the authors.

To assess the potential of an in-room PET scanner for treatment verification in proton therapy, Min conducted a study involving nine patients undergoing passive scattering proton therapy who had an in-room PET scan immediately before treatment. PET activity was compared with Monte Carlo model results.

“The measured PET images show overall good spatial correlations with [Monte Carlo] predictions,” wrote the authors. “Some discrepancies could be attributed to uncertainties in the local elemental composition and biological washout.”

Eight of the patients were treated with a single field, and the average range differences between PET measurements and CT-based Monte Carlo results were less than 5 mm. Root-mean-square deviations were 4 to 11 mm with PET-CT image co-registration errors of approximately 2 mm, according to the authors.

They also reported that short-length PET scans of five minutes can yield similar results to a 20-minute PET scan.

“The shortened PET acquisition time could provide benefits in the treatment room throughput and greater patient convenience in routine clinical application,” wrote the authors. “Our investigation proves that an in-room PET system can reduce the delay time, one of the major issues in off-line PET treatment verification, to two minutes while attaining a co-registration of the in vivo beam range within 5 mm.”