Zhe Jay Chen, PhD, DABR, FASTRO, FAAPM

An example of my research in basic radiation dosimetry involved the determination of dose rate constant (DRC) for radioactive sources used in interstitial brachytherapy. The DRC is a fundamental quantity that links the intrinsic dosimetry properties of a radioactive source to the proper fulfillment of prescription dose in patient dosimetry. An accurate determination of DRC has been regarded as one of the most important tasks in characterizing the basic properties of radioactive sources. However, accurate determination of DRC for the sources emitting photons of less than 50 keV has remained a challenge in radiation dosimetry because of the lack of a suitable absolute dosimeter for accurate measurement of doses near the source. Existing experimental techniques have large overall uncertainties on the order of 8-10% at one standard deviation and 15% at the 95% confidence level.

We have developed a general formalism for DRC that permitted detailed elucidation of the general properties underlying the determination of DRC. Based on this theoretical finding, we have subsequently developed a new photon spectrometry technique for accurate determination of the DRC of low-energy interstitial brachytherapy sources. This new technique eliminated many of the difficulties associated with the existing experimental techniques and has provided new and independent determinations of DRC for over twenty low-energy brachytherapy source models. Its application has led to the discovery of a 15% discrepancy in the DRC reported for a newly marketed cesium-131 source and has helped resolve a large discrepancy in the DRCs reported in literature for a novel polymer-encapsulated palladium-103 source. The photon spectrometry technique is efficient and robust. We are developing it into a national resource for DRC determination and for periodic quality assurance check of DRC.

An example of my research work in treatment plan optimization and evaluation involved the derivation of a new radiobiological formalism for biologically effective dose (BED) of permanent interstitial brachytherapy (PIB) using sources of different decay half-lives. In PIB, the cancer cells are subjected to continuous photon irradiation. Because tumor cell repopulation and sub-lethal damage repair occur simultaneously during dose delivery, the net cell kill and therefore the clinical efficacy of PIB depend not only on the delivered dose but also on the interplay between the temporal patterns of dose delivery and cellular kinetics.

The BED formula captures this interplay and has enabled systematic evaluation of the potential clinical impacts of using mixed sources on cancers presenting different biological properties. This formalism has also enabled us to systematically examine the radiobiological effects of prostate edema in PIB for early stage prostate cancer and many of the theoretical issues related to the design of an effective dose compensation approach for edema-induced dose deficits.

This latter application has provided some of the preliminary data for a R01 research project currently funded by NIH since September 2008 (R01CA134627-01 Prostate Edema in Permanent Interstitial Brachytherapy, PI: Zhe Chen, Ph.D). The R01 project aims to quantitatively characterize the dosimetric and radiobiologic effects of prostate edema and to develop effective therapeutic interventions so that the efficacy of PIB can be optimized for each individual prostate cancer patient.

Other examples of my research work have dealt with clinical dosimetry and quality assurance for radiation therapy techniques ranging from intensity-modulated radiation therapy (IMRT), total-skin electron therapy (TSET) for cultaneous T-cell lymphoma, total-body irradiation (TBI) for bone marrow transplant and image guidance in the planning, delivery and evaluation of radiation therapy.

• Optically stimulated luminescence dosimetry for in vivo verification of total-body irradiation (TBI) for bone marrow transplant
• Intervention strategies for effective management of edema-induced dose variations in permanent interstitial brachytherapy for prostate cancer
• Photon spectrometry for dosimetric characterization of low energy photon-emitting radioactive sources in interstitial brachytherapy
• Quantifying and managing the dosimetric effects of respiratory motion on image-guided stereotactic body radiosurgery for inoperable lung tumors
• Image-guided adaptive radiotherapy for prostate cancer