Abstract
Recently, intensity-modulated radiation therapy (IMRT) has been used for total-body irradiation (TBI). Since the planning target volume (PTV) for TBI includes the surrounding air, a dose prescription to the PTV provides high fluence to the body surface. Thus with just a small set-up error, the body might be exposed to a high-fluence beam. This study aims to assess which target volume should be prescribed the dose, such as a clinical target volume (CTV) with a margin, or a CTV that excludes the surface area of the skin. Three treatment plans were created for each patient: the 5-mm clipped plan (Plan A), the 0-mm margin plan (Plan B) and the 5-mm margin plan (Plan C). The CTV was the whole body. PTVs were the CTV with the exception of 5 mm from the skin surface in Plan A, equal to the CTV in Plan B, and the CTV with a 5 mm margin in Plan C. The prescribed dose was 12 Gy in six fractions. To assess the influence of the set-up error, dose distributions were simulated on computed tomography (CT) images shifted 2 pixels (= 4.296 mm), 5 pixels (= 10.74 mm) and 10 pixels (= 21.48 mm) in the lateral direction from the original CT. With a set-up error of 10.74 mm, V110% was 8.8%, 11.1% and 23.3% in Plans A, B and C, respectively. The prescription to the PTV containing the surrounding air can be paradoxically vulnerable to a high-dose as a consequence of a small set-up error.
Highlights
Total-body irradiation (TBI) has been performed as a conditioning regimen for allogeneic hematopoietic stem cell transplantation (HSCT)
This study aims to assess which target volume should be prescribed the dose, such as a clinical target volume (CTV) with a margin, or a CTV that excludes the surface area of the skin
Because the CTV comprises the whole body in total-body irradiation (TBI), the planning target volume (PTV) contains the surrounding air, and the dose prescription to the PTV provides a much higher fluence to the body surface
Summary
Total-body irradiation (TBI) has been performed as a conditioning regimen for allogeneic hematopoietic stem cell transplantation (HSCT). In inverse planning of IMRT, a high fluence is administered to tangential beam segments near the skin in order to counter the build-up region [8]. This is the reason for increasing the skin dose when the clinical target volume (CTV) is near the skin, such as in TBI. The body might be exposed to a high-fluence beam as a consequence of a small set-up error. This logic generates a hypothesis that a plan with a PTV margin can be paradoxically vulnerable to set-up errors in TBI by IMRT. The purpose of this study was to assess which target volume is acceptable—a CTV with a margin or a CTV that excludes the surface area of the skin—and how much set-up error is acceptable
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