TLD-100H accuracy for kilovoltage cone-beam CT out-of-field measurements

Full text: The use of imaging, such as kilovoltage (kV) cone-beam CT (CBCT), during radiotherapy increases the dose to organs surrounding the target volume. Knowledge of this dose is important in order to assess the benefits and risks of the additional images. The thermoluminescent dosimeter (TLO) TLD-I OOH is a high sensitivity TLO making it ideal for the measurement of out-of-field image radiation dose. This study investigated the accuracy of TLO-I OOH chips in the peripheral region of a kV CBCT. A 0.6 cc farmer ionisation chamber and TLD-IOOH chips were irradiated with a 120 kV, 112 mAs CBCT in static mode. The static kV CBCT has a 26.5 x 26.5 cm2 field-of-view. Ion chamber and TLO response was measured at depths 0.5 and I cm in solid water, at the following distances of 0, 13.25, 14.25, 15.25, 16.25, 17.25 and 18.25 cm from the central axis. With the exception of one point, agreement of 5% was determined between the TLD-JOOH chips and the ion chamber. The large difference at the point 13.25 cm is attributable to the high dose gradient, as this position is immediately at the field edge. Conclusion The accuracy of the TLD- JOOH chips in the peripheral region of a kV CBCT was found to be within 5% from I to 5 cm from the field edge. This work will enable the accurate measurement of kV CBCT dose using TLO-IOOH chips to assess dose to organs both inside and outside the treatment field.