Fabry-Pérot based refractive index optical fiber sensor for measurement of oxygen concentration levels in hypoxic tumors during radiotherapy treatment Charusluk Viphavakit* a , DineshBabu Duraibabu a , Sinead O’Keeffe a , Elfed Lewis a a Optical Fibre Sensors Research Centre, University of Limerick, Limerick, Ireland ABSTRACT An optical fiber sensor based on Fabry-Pérot interferometer to measure the refractive index changes due to oxygenation level changes in hypoxic tumors for radiotherapy treatments is proposed. The sensors have an outer diameter of 220μm with a 20-30μm length air-cavity and a 30μm thickness end cap located at the tip of the sensor. The sensors are used to measure the phase change in the received optical spectrum when there is a change in refractive index using a Fast Fourier Transform based analysis method. The refractive index change is measured in order to determine the oxygenation level in hypoxic tumors. In this paper, different concentrations of iso-propanol solution are prepared to produce refractive index values between 1.3438 and 1.3655 in order to mimic the refractive index of hemoglobin. The sensors are coated with a 100 nm thick gold layer and a comparison is made with non-coated sensors. The coated sensors have a resolution in order of 10 -3 RIU. Keywords: Fabry-Pérot interferometer, refractive index sensors, hypoxic tumors. 1. INTRODUCTION Hypoxic tumors are solid tumors which are characterized by a lower oxygen level (0.3-2.4% of O 2 ) compared to the surrounding tissue (3.9-6.8% of O 2 ) [1-3]. They are commonly found as a form of cancer in lung, liver, stomach, colorectal, breast and prostate. The amount of oxygen in hypoxic tumors is related to the partial pressure of oxygen (pO 2 ) which can be used to distinguish the hypoxic tumors from non-hypoxic tumors and healthy tissue. The hypoxic tissue has a typical pO 2 value of 24-1.8mmHg while healthy tissue has a pO 2 value in the range 30-51.6 mmHg [1-3]. There exists a commercial sensor (OxyLite) [4] which has been developed to measure the pO 2 in tissue using a fluorescence quenching technique, but this cannot be used inside the human body due to FDA approval issues. Additionally, errors occur at high O 2 concentrations due to the non-linear relationship that exists in the sensor response at the higher O 2 concentration. Treatment of the hypoxic tumors can be undertaken by radiation therapy such as Intensity Modulated Radiation Therapy (IMRT) which applies the radiation dose directly to the tumors. In this case radiation dose has to be critically optimized in order to destroy the tumor cells but prevent damage to nearby healthy tissue especially organs. Hypoxic tumors are known to respond differently i.e. less favorably to radiation therapy than the non-hypoxic cases and this poses a significant problem when implementing radiotherapy oncology treatment in patients. Currently, there are no sensors available that are capable of measurement in close proximity to the tumor (in-vivo) and in real-time during radiation treatment i.e. as the radiation dose is delivered. This work is focused on developing a sensor to assist accurate diagnosis and real-time monitoring of radiation dose in the treatment of hypoxic tumors in prostate cancer. A Fabry-Pérot based optical fiber based sensor is presented in this paper which is capable of measurement of refractive index changes within tissue. Optical properties of light such as absorption and scattering in blood depend on the refractive index of erythrocytes (red blood cells) which is defined by the concentration of hemoglobin in the red blood cells [5]. Hemoglobin is a globular protein which transports the oxygen in blood and its refractive index is known to vary with oxygenation level [6]. Therefore, the oxygen level in the hypoxic tumors and surrounding healthy tissues can be indirectly measured from the refractive index of hemoglobin. According to the small size and compact structure of optical fiber sensors based on the Fabry-Pérot interferometer, they can be integrated with biopsy needles which is a medical tool routinely used in cancer diagnosis and therapy. Also in the case of Brachytherapy the sensors can be inserted within the standard Brachytherapy seed delivery needles. In addition, the optical fiber sensors are made of passive material which is glass and coated with gold which is biocompatible. Therefore, the sensor can be used as an *charusluk.v@gmail.com; phone 353 85-190-5846; www.ofsrc.ul.ie WK ,QWHUQDWLRQDO &RQIHUHQFH RQ 2SWLFDO )LEHU 6HQVRUV HGLWHG E\ <RXQJMRR &KXQJ :HL -LQ %\RXQJKR /HH -RKQ &DQQLQJ .HQWDUR 1DNDPXUD /LER <XDQ 3URF RI 63,( 9RO  . ā   63,( ā &&& FRGH ; ā GRL  3URF RI 63,( 9RO  .