Abstract:
In vivo dosimetry can be considered as a quality assurance tool that supplements port films, computational double checks and phantom studies. It is a process of monitoring the dose (skin, entrance, exit, mid plane and target) delivered to a patient during treatment and plays a role in relating the degree to which the delivered dose matches the prescribed dose. Dosimeters currently used for in vivo dosimetry are diodes, thermoluminescence dosimeters (TLDs), electronic portal imaging detectors (EPID) and radiochromic films.
In this research, GafChromic EBT2 film (GAF2) from International Special Products (ISP), USA, was used as the in vivo dosimeter due to its intrinsic characteristics such as tissue equivalent nature (Z = 6.96), wide dose range (0.1cGy – 8Gy), ease of handling, very small effective thickness( 0.29mm) and Two Dimensional (2D) dose distribution. The behaviors of both bare film and a specially constructed encapsulated film were considered in this research. The film was cut to 1cm×1 cm and exposed to known doses of radiation determined by ion chamber reading under reference conditions. The optical density (OD) was read with point densitometer and plotted against the corresponding dose to obtain the calibration curve. The response of the film was also determined for changes in field size, source to skin distance (SSD), gantry angle, trays and wedges. Correction factors were then determined for SSD, field size, gantry angle, wedges and tray since the clinical conditions may be different from the reference conditions. Also, skin dose as well as entrance dose calibration factor were determined for both bare and encapsulated GAF2. Knowledge of the surface film reading, calibration factor and correction factors were used in the determination of entrance dose, skin dose as well as percentage skin dose.
From a preliminary % skin dose assessment using both bare film and encapsulated film at reference irradiation conditions, encapsulated film was observed to expose the skin to a higher dose (68%) than bare film (23.7%) due to the bulkiness of the encapsulation material and was subsequently eliminated from being used for entrance dose measurements. Furthermore, percentage skin dose determination with bare film was observed to be mainly affected by field size (45%). By varying one irradiation parameter
and keeping all others at reference conditions, the maximum percentage entrance dose difference observed was 2.6% for a dose prescription of 200 cGy to dmax. This falls within the tolerance level of 5% set by accredited regulatory bodies. However, by varying all the irradiation parameters, the maximum percentage entrance dose difference was observed to be higher than the tolerance level. It is therefore recommended to carry out further studies on the use of GAF2 before being implemented as an in vivo dosimeter at KBTH by minimizing and being cognizant of the various levels of errors and uncertainties involved in the determination of the calibration curve, calibration factor and correction factors.