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Abstract
In prostate IMRT planning, the planning target volume (PTV), extended from a clinical target volume (CTV), often contains an overlap air volume from the rectum, which poses a problem inoptimization and prescription. This study was aimed to establish a planning method for such a case. There can be three options in which volume should be considered the target during optimization process; PTV including the air volume of air density (‘airOpt’), PTV including the air volume of density value one, mimicking the tissue material (‘density1Opt’), and PTV excluding the air volume (‘noAirOpt’). Using 10 MV photon beams, seven field IMRT plans for each target were created with the same parameter condition. For these three cases, DVHs for the PTV, bladder and the rectum were compared. Also, the dose coverage for the CTV and the shifted CTV were evaluated in which the shifted CTV was a copied and translated virtual CTV toward the rectum inside the PTV, thus occupying the initial position of the overlap air volume, simulating the worst condition for the dose coverage in the target. Among the three options, only density1Opt plan gave clinically acceptable result in terms of target coverage and maximum dose. The airOpt plan gave exceedingly higher dose and excessive dose coverage for the target volume whereas noAirOpt plan gave underdose for the shifted CTV. Therefore, for prostate IMRT plan, having an air region in the PTV, density modification of the included air to the value of one, is suggested, prior to optimization and prescription for the PTV. This idea can be equally applied to any cases including the head and neck cancer with the PTV having the overlapped air region. Further study is being under process.
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Fig. 1.
(a) The extended PTV from CTV has an overlap air region in the rectum area. (b) One of the possible states of the CTV movement from e.g, setup errors. In this case, the density of original ‘air’ region occupied by the CTV is that of the tissue, not of the air.
Fig. 2.
Axial dose distributions for (a) airOpt, (b) density1Opt, and (c) noAirOpt plans. The plan 'noAirOpt' shows a tight conformation to the target excluding the overlap air region.
Fig. 3.
DVHs of PTVs after optimization and prescription of 78 Gy to each optimization target. The density of the included air region was ‘air’ for airOpt and density 1 for density1Opt. The noAirOpt plan had the air excluded during the optimization. The vertical line was drawn for the 78 Gy.
Fig. 4.
DVHs for the bladders and rectums. All lines were obtained under condition of air density irrespective of the optimization condition.
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