IMPACT OF SEGMENTS NUMBER REDUCTION IN IMRT PLANNING

Purpose: To investigate the planning and dosimetric advantages of direct aperture optimization (DAO) over beamlet optimization in IMRT treatment of head and neck (H/N) and prostate cancers. Materials and methods: a brain metastatic case with multiple lesions, five head and neck, as well as five prostate patients were planned using the beamlet optimizer in Elekta-Xio© ver 4.6 IMRT treatment planning system. Based on our experience in beamlet IMRT optimization, PTVs in brain were prescribed to 66Gy using 5 fields, PTVs in H/N plans were prescribed to 70 Gy delivered by 7 fields, and prostate PTVs were prescribed to 76 Gy using nine fields. In all plans, fields were set to be equally spaced. All cases were re-planed using Direct Aperture optimizer (DAO) in Prowess Panther© ver 5.01 IMRT planning system at same configurations and dose constraints. Plans were evaluated according to ICRU criteria, number of segments, number of monitor units and planning time. Results: In brain case, beamlet optimization was better than DAO for both GTVs and PTVs in 95% isodose coverage, and the hot area was about 7% more in beamlet plan than DAO plan. For OAR, results showed an improvement in OAR sparing up to more than 35% in rt. eye, lt. eye, rt. optic nerve and lt. optic nerve when using DAO for planning, while optic chiasma sparing was about 20% also in beamlet optimizer plan. For H/N plans, the near maximum dose (D2) and the dose that covers 95% (D95) of PTV has improved by 4% in DAO. For organs at risk (OAR), DAO reduced the volume covered by 30% (V30) inspinal cord, right parotid, and left parotid by 60%, 54%, and 53% respectively. This considerable dosimetric quality improvement achieved using 25% less planning time and lower number of segments and monitor units by 46% and 51% respectively. In DAO prostate plans, Both D2 and D95 for the PTV wereimproved by only 2%. The V30 of right femur, left femur and bladder were improved by 35%, 15% and 3% respectively. On the contrary, the rectum V30 got even worse by 9%. However, number of monitor units, and number of segments decreased by 20% and 25% respectively. Moreover the planning time reduced significantly too. Conclusion: DAO introduces considerable advantages over beamlet optimization for different sites of cancer, in regards to organ at risk sparing. While no significant improvement occurred in the PTV ICRU reporting dose.