Fifty-sixth annual meeting of the American association of physicists in medicine
SU-E-J-88: Margin Reduction of Level II/III Planning Target Volume for Image-Guided Simultaneous Integrated Boost Head-And-Neck Treatment
To investigate the feasibility of improved normal tissue sparing for head-and-neck (H' N) image-guided radiotherapy (IGRT) by employing tighter CTV-to-PTV margins for target level II/III though a GPU-based deformable image registration and dose accumulation framework.
Ten H' N simultaneous integrated boost cases treated on TomoTherapy were retrospectively analyzed. Weekly kVCT scans in addition to daily MVCT scans were acquired for each patient. Reduced margin plans were generated with 0- mm margin for level II and III PTV (while 3-5 mm margin for PTV1) and compared with the standard margin plan using 3-5mm margin to all CTV1-3 (reference plan). An in-house developed GPU-based 3D image deformation tool was used to register and deform the weekly KVCTs with the planning CT and determine the delivered mean/minimum/maximum dose, dose volume histograms (DVHs), etc.
Compared with the reference plans, the averaged cord maximum, the right and left parotid doses reduced by 22.7 %, 16.5 %, and 9 % respectively in the reduced margin plans. The V95 for PTV2 and PTV3 were found within 2 and 5% between the reference and tighter margin plans. For the reduced margin plans, the averaged cumulative mean doses were consistent with the planned dose for PTV1, PTV2 and PTV3 within 1.5%, 1.7% and 1.4%. Similar dose variations of the delivered dose were seen for the reference and tighter margin plans. The delivered maximum and mean doses for the cord were 3.55 % and 2.37% higher than the planned doses; a 5 % higher cumulative mean dose for the parotids was also observed for the delivered dose than the planned doses in both plans.
By imposing tighter CTV-to-PTV margins for level II and III targets for H' N irradiation, acceptable cumulative doses were achievable when coupled with weekly kVCT guidance while improving normal structure sparing.