Purpose: ElastoGel Bolus has been gaining increasing popularity. It has a density of 1.2g/cm3, instead of the traditional 1.0g/cm3. This difference in density will have an effect on treatment beams traversing the bolus, especially electron beams. This study aims to quantify the difference for typical electron beam energies.
Methods: Two methods were used to quantify the dose difference caused by the higher density of ElastoGel. First, solid water phantoms were scanned with ElastoGel and Superflab boluses with thicknesses of 0.3, 0.5, and 1.0cm. Treatment plans with a 10x10cm2 field, 100MUs and 100cm SSD to the bolus surface were calculated for 6, 9, 12, 16, and 20MeV electron beams. A dose profile was taken from these beams along the central axis and 2cm off-axis. Second, 9 plans, previously treated using ElastoGel were retroactively analyzed. The bolus depths included 0.3, 0.5, and 1.0cm. The electron energies were 6, 6 and 9, 9, 12, and 16MeV. For each plan, the patient was scanned with no bolus and the bolus was then added in the treatment planning software. To simulate Superflab, the bolus was assigned a Hounsfield Unit of 0. For the purposes of this study, a PTV was created to match the 90% isodose volume for the plan. The bolus was then changed to simulate ElastoGel, with 300HU. PTV coverage was compared using mean dose, D_max, V5, V20, V80, V90, V95, and V100.
Results: The largest difference was seen for 6Mev with 1cm of bolus. For the solid water plans, past the buildup within the bolus, this combination had a dose difference of 0.3%. For patients, V95 and V100 changed by up to 4.2%. All other metrics changed by less than 1%.
Conclusion: ElastoGel Bolus presents little dosimetric influence and is not a risk if the density is assigned incorrectly.