Understanding Parameters Affecting Surface Dose to Reduce Skin Toxicity in Patients Treated with SBRT

Abstract

Skin toxicity has been reported with stereotactic body radiation therapy (SBRT) treatments. Skin dose is a complex function of many variables including beam energy, treatment distance (SSD), materials underlying beam path (t), and field size (FS). Various skin dose constraints have been reported and toxicity is noted despite meeting the published/proposed dose constraints predicted by the standard treatment planning systems. In vivo dosimetry assessments support that toxicity may be related to the bolus effect from the immobilization device. This study was undertaken to analyze the above parameters in the context of the BodyFix® (BF) system and their impact on skin dose. A custom BF device was created with three thicknesses (2.5, 5.0, 12.5 cm). Measurements were taken using solid water phantom with and without the BF using a PTW Markus parallel-plate chamber. The impact of BF on skin dose was investigated as a function of beam energy (6MV, 10MV, 10FFF), source to detector distance (SDD) (75, 85, 95cm), thickness (2.5, 7.5, 12.5cm), and depth (0, 1, 2, 3, 5mm & dmax) at clinically relevant field FS for SBRT (2x2 to 5x5cm2). Changes in percentage depth dose (PDD) with the same SDD were analyzed in the presence of BF, as well as relative skin dose (RSD), defined as dose with BF to dose without BF. The impact of BF on skin dose was significant for all beam energies as shown in Table. Comparing a 6MV beam without and with BF (6MV-BF), the PDD at 0mm depth (PDD0mm) ranged from 18.6% to 20.6% (61.0% to 83.5%) for all tested FS at 75cm SDD. The 10MV (10MV-BF) beam ranged from 10.8% to 12.6% (46.0% to 69.3%), and 10FFF (10FFF-BF) ranged from 16.6% to 17.7% (55.3% to 77.8%). RSD increased due to the presence of BF for all beams, increasing by 4-6x, 2-3x, and 1-2x at 0mm, 1mm, and 2mm depth, respectively. The effect of relative increase in RSD with BF decreased with depth. PDD decreased with larger BF thickness from 2.5-12.5 cm for all tested energies and FS. Compared to 2.5cm BF thickness, the PDD0mm decreased by ∼13% for 12.5cm thickness at 3x3cm2 FS for 10MV, indicating dose deposition is inside the BF for thicker BF. For 2.5cm of BF, more dose deposition is close to skin, resulting in increased skin dose. The RSD increased for all photon energies with increasing treatment distance. RSD at 0mm depth was 1.10 for 95cm SDD compared to 75cm SDD for t-2.5 cm & 2x2cm2. This increase is primarily due to the inverse square factor. BF parameters combined with dosimetric variables can create up to 6-fold increase in skin dose, which may result in acute skin toxicity. Using higher beam energy (up to 10x), larger BF thickness, and smaller SSD will help reduce skin dose in SBRT treatments. Accounting for immobilization devices and modifying planning parameters to reduce the bolus effect can potentially reduce skin dose and lead to lower toxicity.Abstract 3806; Table 13x3cm2PDD0mmPDD2mmPDD5mm6MV+BF72.9987.5897.256MV18.7260.0683.7110MV+BF55.3470.7184.4610MV10.6239.4364.0310FFF +BF63.5877.5188.9110FFF16.2550.4672.37 Open table in a new tab