In vivo dose monitoring with plastic scintillator is feasible and will be tested on a small number of patients in the near future. Further development is required before they are ready for daily clinical use. For ease of use and transportation, the compact reading apparatus needs to be placed inside a more convenient support and housing that includes a reproducible connector mechanism similar to that used with the offline dosimeter array. With such a support, detector calibration would be required after 9 hours of exposure at a dose rate of 600 MU/min. At 5 patients per day each receiving about 2Gy, the detector will need to be calibrated once a year. In addition, software should be developed to facilitate image acquisition and processing, detector calibration and data storage.
With the plastic scintillator system described here, the measurement uncertainty was 3% (1 standard deviation) for an acquisition time of 150 ms, equivalent to a dose of 1.5cGy. The precision improved with increasing dose; at 200 cGy, the standard deviation was 0.4%. In contrast, the precision of diodes has been reported to be greater than 1% for doses between 32 to 385 cGy. Moreover, for TLDs irradiated to doses of ~300 cGy, the standard deviation has been reported as 1.5% for individual readings.
A real-time in vivo detector system offers all the advantages of an integrating, offline detector plus the capability to determine the exact moment when deviations between the planned and delivered doses occurred. We plan to use this information in our adaptive radiation therapy treatment procedure. The first use of the real-time dose information is as a warning mechanism. By monitoring dose continuously, we can detect any sudden deviation from the expected dose and if it exceeds a certain threshold we can stop the treatment, re-position the patient and resume. The second use of the plastic scintillators ystem is to evaluate if a change in the dose prescription or a re-planning is required between fractions. This is similar to what could be achieved with an integrating offline detector, but with the added knowledge of when discrepancies occur. This information can be used to determine if a single field should be adjusted or, if the patient motion is too large in a given direction, the margins should be changed.
Shorter acquisition times could be used, but at the cost of higher uncertainty per frame. In contrast, longer acquisition times or better light collection efficiency could further reduce the uncertainty per frame. Larger plastic scintillators could also be used to reduce the uncertainty per frame, but at the expense of spatial resolution. Smaller plastic scintillators are especially important if they are to be used in conjunction with medical devices to provide in vivo measurements; however, larger plastic scintillators could be used to measure entry and/or exit doses.