Abstract
Dose distributions in HN‐IMRT are complex and may be sensitive to the treatment uncertainties. The goals of this study were to evaluate: 1) dose differences between plan and actual delivery and implications on margin requirement for HN‐IMRT with rigid setup errors; 2) dose distribution complexity on setup error sensitivity; and 3) agreement between average dose and cumulative dose in fractionated radiotherapy. Rigid setup errors for HN‐IMRT patients were measured using cone‐beam CT (CBCT) for 30 patients and 896 fractions. These were applied to plans for 12 HN patients who underwent simultaneous integrated boost (SIB) IMRT treatment. Dose distributions were recalculated at each fraction and summed into cumulative dose. Measured setup errors were scaled by factors of 2–4 to investigate margin adequacy. Two plans, direct machine parameter optimization (DMPO) and fluence only (FO), were available for each patient to represent plans of different complexity. Normalized dosimetric indices, conformity index (CI) and conformation number (CN) were used in the evaluation. It was found that current 5 mm margins are more than adequate to compensate for rigid setup errors, and that standard margin recipes overestimate margins for rigid setup error in SIB HN‐IMRT because of differences in acceptance criteria used in margin evaluation. The CTV‐to‐PTV margins can be effectively reduced to 1.9 mm and 1.5 mm for CTV1 and CTV2. Plans of higher complexity and sharper dose gradients are more sensitive to setup error and require larger margins. The CI and CN are not recommended for cumulative dose evaluation because of inconsistent definition of target volumes used. For fractionated radiotherapy in HN‐IMRT, the average fractional dose does not represent the true cumulative dose received by the patient through voxel‐by‐voxel summation, primarily due to the setup error characteristics, where the random component is larger than systematic and different target regions get underdosed at each fraction.PACS numbers: 87.53.Kn, 87.53.Tf.
Highlights
39 Worthy et alRigid setup error effects for Head and neck (HN)-intensity-modulated radiation therapy (IMRT) major concern, as the delivered dose to the patient may deviate significantly from the approved plan, preventing the benefits of IMRT over 3D conformal radiotherapy (3DCRT) from being realized clinically, and potentially resulting in delivery of a worse treatment.In order to ensure that the adequate doses are delivered accurately to tumor volumes, geometric uncertainties need to be taken into account by adding a margin to the clinical target volume (CTV) to create a planning target volume (PTV).(4-5) Optimal treatment delivery requires that margins are defined properly such that tumor volumes receive adequate dose while simultaneously minimizing damage to surrounding healthy tissues
Static plan dose characteristics It is of interest to first describe the dose characteristics of the static direct machine parameter optimization (DMPO) and fluence optimization (FO) patient plans before discussing the setup error effects on the delivered dose distribution. Both DMPO and FO plans were created for the same subset of simultaneous integrated boost (SIB) HN-IMRT patients, and selected plan quality metrics for targets and OARs were compared.[19]. A summary of the average dose volume histograms (DVHs) dose metric results previously found is presented in Table 1, along with additional dose metric results for the mandible and parotid glands
We evaluated the dosimetric effects of measured patient setup errors on SIB HNIMRT treatments and found that conventional margin recipes significantly overestimate the margins needed for planning
Summary
39 Worthy et alRigid setup error effects for HN-IMRT major concern, as the delivered dose to the patient may deviate significantly from the approved plan, preventing the benefits of IMRT over 3DCRT from being realized clinically, and potentially resulting in delivery of a worse treatment.In order to ensure that the adequate doses are delivered accurately to tumor volumes, geometric uncertainties need to be taken into account by adding a margin to the clinical target volume (CTV) to create a planning target volume (PTV).(4-5) Optimal treatment delivery requires that margins are defined properly such that tumor volumes receive adequate dose while simultaneously minimizing damage to surrounding healthy tissues. With respect to patient interfraction setup errors, there are a number of margin recipes which have been reported and reviewed in the literature. These recipes can be used to calculate planning margin size using systematic (Σ) and random (σ) setup errors measured for a population of patients. Of the margin recipes proposed, two commonly used are those from van Herk et al[6] (M1 = 2.5Σ + 0.7σ) and Stroom et al[7] (M2 = 2.0Σ + 0.7σ) Both recipes were derived and validated based on analysis of 3D conformal treatments, and their validity in HN-IMRT planning is still being investigated. One primary goal of this work is to evaluate the dose deviations due to the rigid setup errors on simultaneous integrated boost (SIB) HN-IMRT treatments to determine appropriate margins
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