Adaptive replanning strategies accounting for shrinkage in head and neck IMRT.
OBJECTIVE
Significant anatomic and volumetric changes occur in head and neck cancer patients during fractionated radiotherapy, and the actual dose can be considerably different from the original plan. The purposes of this study were (1) to evaluate the differences between planned and delivered dose, (2) to investigate margins required for anatomic changes, and (3) to find optimal replanning strategies.


METHODS
Eleven patients, each with one planning and six weekly helical CTs, were included. Intensity-modulated radiotherapy plans were generated using the simultaneous integrated boost technique. Weekly CTs were rigidly registered to planning CT before deformable registration was performed. The following replanning strategies were investigated with different margins (0, 3, 5 mm): midcourse (one replan), every other week (two replans), and every week (six replans). Doses were accumulated on the planning CT for comparison of various dose indices for target and critical structures.


RESULTS
The cumulative doses to targets were preserved even at the 0-mm margin. Doses to cord, brainstem, and mandible were unchanged. Significant increases in parotid doses were observed. Margin reduction from 5 to 0 mm led to a 22% improvement in parotid mean dose. Parotid sparing could be preserved with replanning. More frequent replanning led to better preservation; replanning more than once a week is unnecessary.


CONCLUSIONS
Shrinkage does not result in significant dosimetric difference in targets and critical structures, except for the parotid gland, for which the mean dose increases by approximately 10%. The benefit of replanning is improved sparing of the parotid. The combination of replanning and reduced margins can provide up to a 30% difference in parotid dose.
Wu
Qiuwen
Q
Department of Radiation Oncology, William Beaumont Hospital, Royal Oak, MI, USA. Qiuwen.Wu@Duke.edu
Chi
Yuwei
Y
Chen
Peter Y
PY
Krauss
Daniel J
DJ
Yan
Di
D
Martinez
Alvaro
A
D001933:Brain Stem / Q000528:radiation effects; D019583:Dose Fractionation, Radiation; D006258:Head and Neck Neoplasms / Q000000981:diagnostic imaging / Q000473:pathology* / Q000532:radiotherapy*; D006801:Humans; D008334:Mandible / Q000528:radiation effects; D010306:Parotid Gland / Q000528:radiation effects; D011829:Radiation Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D050397:Radiotherapy, Intensity-Modulated / Q000379:methods*; D012189:Retrospective Studies; D013116:Spinal Cord / Q000528:radiation effects; D013997:Time Factors; D036542:Tomography, Spiral Computed; D047368:Tumor Burden / Q000528:radiation effects*