Room: AAPM ePoster Library
Purpose: Knife (GK) Icon for frameless treatment is used to treat large brain tumors (? 4 cm in diameter) with planning target volume (PTV) margin expansion of 1 mm around the gross target volume (GTV) with an exception of 1.5 mm in the superior and inferior direction. Here, we sought to validate our PTV margin scheme for the following clinical scenarios (i) motion that is just below the threshold limit (1.5 mm) of high-definition motion management (HDMM) and (ii) frequent treatment interruptions with motion beyond HDMM threshold, with a reference scenario being no head motion.
Methods: Prime anthropomorphic head phantom mounted on a remote controlled translational stage is used in the above mentioned scenarios. After PTV1 and PTV2 were created within the phantom, the dosimetric evaluations were performed using ionization chamber, EBT3 film, and polymer gel and compared with GK Icon plan generated by Convolution algorithm.
Results: comparison of point dose measurements (1D) using ion chamber and calculated doses were within the overall measurement uncertainty (<3%) for both scenarios. Gamma analysis (2%/2mm) of 2D measurements using EBT3 film showed a passing rates of 82.6% and 71.5% for PTV1 and PTV2 in scenario(i), attributed to head motion of 1.5 mm HDMM in addition to positional accuracy of film dosimetry. Gamma analysis (2%/2mm) of 3D gel measurements showed passing rates >98% for all scenarios. The D95 values from gel were well above the planned doses for all scenarios.
Conclusion: concluded that our margin scheme is adequate for typical patient intrafractional motion. Our results suggests that volumetric dose distribution analysis using 3D dosimetry such as gel is more beneficial to validate PTV margins to ensure better target coverage due to intrafractional motion.