Dosimetry overview: keeping score on the scorekeepers.

Abstract

As a direct result of the use of the absorbed dose unit the 'Gray' as the gold standard for predicting response in external beam radiotherapy, the physicist role has been essential to clinical practice for many decades. However, although the dosimetry for internal emitters has proven useful in managing health physics concerns and diagnostic nuclear medicine, the relative success of correlating absorbed dose with response from radionuclide therapy has been limited. This overview presents the relative success and/or failure of model-based dosimetry for radionuclide therapy in comparison to results quoted for external beam therapy dosimetry. Using the standard MIRD formalism for macroscopic dosimetry, the marked non-uniform distribution of radionuclide in both tumor and normal tissue has resulted in limited correlation between computed absorbed dose and biological response in clinical trials. Several efforts are underway aimed at improving this dose-response correlation which include individualized patient specific dosimetry and selected biological parameters. The physicist role in helping the clinician determining which patients will succeed on given radionuclide therapy has been improved with simplified methods such as the assessment of tracer whole body absorbed dose on a per patient basis. The dose-response correlations are now in the moderate range of significance when individualized patient dosimetry is included. These correlations are expected to improve as unified treatment planning programs are instituted and standard methods of clinically based dosimetry are widely accepted and practiced universally.