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SU‐FF‐T‐73: Accounting for Tissue Heterogeneities in Head and Neck IMRT Plans Increases Planning Target Volume and Spinal Cord Doses
Author(s) -
AlHallaq H,
Bian J,
Wood A
Publication year - 2006
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.2240999
Subject(s) - head and neck , spinal cord , medicine , nuclear medicine , volume (thermodynamics) , physics , surgery , psychiatry , quantum mechanics
Purpose: Using TLD measurements in an anthropomorphic phantom, we previously demonstrated that the presence of heterogeneities causes an average 5.1% dose increase compared to the dose calculated by CORVUS' pencil‐beam algorithm without heterogeneity correction. In this treatment planning study, we investigated the dosimetric effects of heterogeneities on clinical head and neck IMRT plans. Method and Materials: Ten CORVUS plans for five nasopharynx (NP) and five base of tongue (BOT) tumors were recalculated using the convolution/superposition algorithm of Pinnacle 3 6.2b. In contrast to the CORVUS algorithm used in our clinic which assumes a homogeneous water‐equivalent patient, the convolution/superposition algorithm accounts for the effects of heterogeneities by converting CT numbers to electron densities. Dose volume histograms were compared for the Pinnacle 3 and CORVUS plans. To characterize tumor coverage, the D 95 and V 105 were calculated for the PTV. The maximum doses to the spinal cord were also compared. Results: The D 95 , V 105 , and spinal cord doses calculated by Pinnacle 3 following heterogeneity correction were larger than CORVUS doses for all ten patients. The D 95 increased by an average of 2.5% (2.9% for NP and 2.2% for BOT tumors). The V 105 increased by an average of 57.5% (60.2% for NP and 54.8% for BOT tumors). Increases in V 105 ranged from 28.5% to 71.9%. Spinal cord doses increased by an average of 4.5% (4.5% for NP and 4.6% for BOT tumors) with a range of 0.8% to 8.5%. Conclusion: PTV coverage was minimally changed but the dose inhomogeneity within the PTV increased. The increased PTV dose inhomogeneity was larger by 5.4% in the NP tumors compared to the BOT tumors. Spinal cord doses were systematically underestimated by CORVUS. Overall, the differences between NP and BOT treatment plans were minimal. This was probably due to the similarity of the initial PTV volumes for the two sites.