Abstract
Current commercially available planning systems with Monte Carlo (MC)‐based final dose calculation in IMRT planning employ pencil‐beam (PB) algorithms in the optimization process. Consequently, dose coverage for SBRT lung plans can feature cold‐spots at the interface between lung and tumor tissue. For lung wall (LW)‐seated tumors, there can also be hot spots within nearby normal organs (example: ribs). This study evaluated two different practical approaches to limiting cold spots within the target and reducing high doses to surrounding normal organs in MC‐based IMRT planning of LW‐seated tumors. First, “iterative reoptimization”, where the MC calculation (with PB‐based optimization) is initially performed. The resultant cold spot is then contoured and used as a simultaneous boost volume. The MC‐based dose is then recomputed. The second technique uses noncoplanar beam angles with limited path through lung tissue. Both techniques were evaluated against a conventional coplanar beam approach with a single MC calculation. In all techniques the prescription dose was normalized to cover 95% of the PTV. Fifteen SBRT lung cases with LW‐seated tumors were planned. The results from iterative reoptimization showed that conformity index (CI) and/or PTV dose uniformity (UPTV) improved in 12/15 plans. Average improvement was 13%, and 24%, respectively. Nonimproved plans had PTVs near the skin, trachea, and/or very small lung involvement. The maximum dose to 1cc volume (D1cc) of surrounding OARs decreased in 14/15 plans (average 10%). Using noncoplanar beams showed an average improvement of 7% in 10/15 cases and 11% in 5/15 cases for CI and UPTV, respectively. The D1cc was reduced by an average of 6% in 10/15 cases to surrounding OARs. Choice of treatment planning technique did not statistically significantly change lung V5. The results showed that the proposed practical approaches enhance dose conformity in MC‐based IMRT planning of lung tumors treated with SBRT, improving target dose coverage and potentially reducing toxicities to surrounding normal organs.PACS numbers: 87.55.de, 87.55.kh
Highlights
113 Altman et al.: Practical intensity-modulated radiation therapy (IMRT) Stereotactic body radiation therapy (SBRT) lung planningsurrounding lower-density lung tissue
Lung tumors treated by SBRT can be generally categorized in two groups: “island” type tumors (Fig. 1(a)) which are completely surrounded by lung parenchyma, and lung wall (LW)seated tumors (Fig. 1(b)), in which one side of the tumor is adjacent to the interface between the lung and the surrounding anatomical structures
One of the planning target volume (PTV) metrics did not show improvement — the conformity index (CI) increased in one case (Patient 8) by 11%, and the UPTV decreased by 6% in the other (Patient 15)
Summary
113 Altman et al.: Practical IMRT SBRT lung planningsurrounding lower-density lung tissue. This, in turn, yields a buildup of dose within the higher-density tumor tissue on the radiation beam entrance side of the tumor and/or a builddown of dose on the beam exit side of the tumor.[5,6,7] Dosimetrically, the tumor tissue near the tumor/lung interface becomes underdosed relative to the portions of the tumor which are more interior to this buildup or builddown region. The distribution of this underdosed region depends on the location of the tumor relative to nearby structures. In the SBRT lung treatment planning of either case, it is of pivotal importance to ensure that the tumor/lung interface region is not underdosed
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