One of the biggest problems in hadron therapy – where cancer cells are bombarded with charged particles such as protons — is how to monitor the radiation that hits healthy tissues around a tumour. Protons can deliver energy at great depths inside the human body because, unlike X-rays used in traditional radiotherapy, they interact weakly with the skin and shallow tissues. But the intensity and direction of the beam must be precisely targeted. Currently, doctors use computational methods based on the patient’s CT scans, but a sensor inside the body would be a better option.
Scientists coordinated by Beatrice Fraboni at the University of Bologna, and Alberto Quaranta at the University of Trento, have designed an organic, thin and flexible sensor that can do just that. The device, described in a study in Science Advances1, is as thin as 125 micrometers and covers a surface of 5 by 6 millimeters. It is made by depositing a layer of organic semiconductor, based on the hydrocarbon pentacene, on a flexible plastic substrate, and is equipped with two gold electrodes powered with a low-intensity electric field. When protons hit the sensor, electric charges are generated in the semiconductor, and travel to the electrodes producing a current which corresponds to the energy delivered by the protons.
In experiments at the LABEC ion beam centre of the Istituto Nazionale di Fisica Nucleare (INFN) in Florence, a sequence of proton pulses of different duration and intensity, with an energy of 5 MeV, were directed at the sensor. “The interaction between the semiconductor and the plastic substrate allows measurement of the dose delivered by each pulse, but also the total dose received after several pulses”, explains Ilaria Fratelli, PhD student at the University of Bologna, and first author of the study. This is not obvious, because at each pulse, the current in the semiconductor rises and then falls to a baseline which is higher than the previous one, potentially confounding the measure. But, by accumulating charge, the plastic substrate ‘memorises’ each new baseline and factors it in the measurement. Real-time detection of pulses is useful to adjust the beam at each session, while the total dose measure keeps track of the radiation received by the patient during the whole treatment.
“The advantage of this sensor is that it mimics the response of human tissue, and does not require a complex calibration procedure, unlike silicon-based devices”, explains Fraboni. The fabrication of these sensors is not expensive and does not require high-tech processes, and therefore could be easily scaled industrially. Its volume and power consumption are small, which could make it appealing also for protecting astronauts from space radiation during long-duration missions.
“It’s a very promising result”, says Marco Durante, director of the biophysics department of GSI Helmholtz Centre for Heavy Ion Research in Darmstadt, who was not involved in the research. “An organic flexible sensor could fit in a rectal balloon to monitor the treatment of prostate cancer, or inside the oral cavity to control the vital organs at risk when treating head and neck tumours.” He suggests a next step could be to test the sensor with higher energy protons, up to hundreds MeV, such as those used in clinical practice.