On the Nonequilibrium of Radiation-Induced Bystander Effects in Tumor Surface and Its Implications in Radiation Therapy

R. Vaitheeswaran^1,2, K. J. Maria Das³

¹Philips Radiation Oncology Systems, Philips India Ltd., Bangalore, India.
²Research & Development Center, Bharathiar University, Coimbatore, India.
³Department of Radiotherapy, Sanjay Gandhi Post Graduate Institute of Medical Sciences, Lucknow, India.

DOI: 10.4236/ijmpcero.2015.43025                

Abstract

This work aims to theoretically show the development of a nonequilibrium of radiation-induced bystander effect (RIBE) under steep dose gradient regions that typically occur in the field edges of a beam. We applied the kinetics model proposed by (McMahon et al. 2013) for in vivo conditions coupled with a hypothesis called “Layer-limited bystander signaling (LLBS)” to demonstrate 1) an enhancement in TCP (i.e. Enhanced TCP or ETCP) due to bystander signals, 2) the development of nonequilibrium of RIBE under steep dose gradient regions and 3) the reduction in ETCP in the surface of Clinical Target Volume (CTV) due to the non-equilibrium of RIBE. We incorporated the elements of RIBE directly in the existing Poisson LQ model available in Pinnacle3 TPS (Version 9.10.0) to compute the percentage reduction of ETCP in the tumor surface due to nonequilibrium of RIBE. The percentage improvement in TCP obtained in tumor surface by accounting for RIBE is about 46% lower than that obtained in the interior of the tumor. This suggests that relatively more number of cancerous cells might survive in the vicinity of tumor surface. The result obtained from the study is indicative of an additional uncertainty component associated with radiation treatment. Hence, this paper suggests that the radiation treatments employing steep dose gradients could be biophysically different in many ways.

Keywords

Bystander Effect, TCP, RIBE, CTV, IMRT, Dose Gradients

Conflicts of Interest

The authors declare no conflicts of interest.

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