Room: AAPM ePoster Library
To compare the relative dose received to the lens during 2.5 MV and 6 MV imaging for a 35-fraction head and neck treatment using an anatomical phantom and Gafchromic film.
Evaluation of the surface dose for both 2.5 MV and 6 MV was performed in a solid water phantom using a PTW Advanced Markus ionisation chamber at central axis without a buildup cap. The phantom was placed at 100 cm source-to-surface distance (SSD) and a 10x10 cm field size was used. The machine output for both 2.5 and 6 MV was measured using a secondary-standard calibrated PTW 3001 Farmer-type ionisation chamber and adjusted to deliver 1 cGy per monitor unit under reference conditions.
A 1x1cm cut of film was placed on an anatomical head phantom and aligned to isocentre. To measure the dose at lens depth, 3 mm of moldable wax was placed over the film to simulate the eyelid and cornea tissue thickness. Using a jaw setting of 30x30 cm, 35 high-quality 2.5 MV images were acquired anteriorly at gantry 0 degrees in machine service mode and repeated for 6 MV without adjusting the phantom position. FilmQAPro software was used to analyse the film.
The surface dose was measured to be 61% and 27% of depth-maximum for 2.5 and 6 MV respectively. Film measurements showed an increase in lens dose of 36.5% when using 2.5 MV compared to 6 MV imaging.
A relative increase in dose to the lens were measured when 2.5 MV images were acquired anteriorly compared to 6 MV. This is due to the combination of higher surface dose and the lens being situated close to depth maximum of a 2.5 MV beam. This observation can be used to inform clinicians in the treatment planning process if lens dose is approaching tolerance.