PV QA 3 - Poster Viewing Q&A 3
Purpose/Objective(s):Optimal radiotherapy (RT) dose delivery to superficial and irregular target volumes poses a unique challenge. Herein we quantitatively evaluate material properties of common commercially available water-equivalent planar bolus (CB) and a tissue-equivalent, three-dimensional printed bolus (3DB). We also evaluate dosimetric outcomes for volumetric modulated arc therapy (VMAT) patient treatment in various disease sites for improved superficial dose delivery.
Materials/Methods:Patients requiring bolus were simulated per departmental protocol. After simulation, a virtual 3DB was contoured in treatment planning software and printed on a 3D printer using a commercially available, flexible, clear photopolymer. We analyzed the bolus density and optic properties using CT and laser, respectively. In vivo dosimetry was performed on an anthropomorphic phantom, then on patients, with calibrated metal-oxide semiconductor field-effect transistors (MOSFET). Dosimetric comparison was accomplished using dose-volume histograms (DVH). Homogeneity index (HI) and conformity index (CI) of delivered plans was calculated to 0.5 cm depth using standard definitions. Statistical analysis was performed with t-test and significance set at p < 0.05.
Results:We evaluated 15 patients from head and neck (H&N, 10), anal (4), and breast (1) disease sites. 3DB density (1.087 g/cm3, range 1.086-1.088) was significantly more uniform than CB (1.125 g/cm3, range 1.080-1.150). Relative optic clarity of 0.5 cm bolus was 300% better with 3DB vs CB, and the difference further increased with increasing bolus thickness in favor of 3DB. Compared with the prescription dose, phantom 3DB MOSFET was 104% vs 105% with CB. On average, patient 3DB MOSFET measurement of superficial dose was 99% of prescription over 129 measurements. The average maximum skin-to-bolus air distance (cm) was 0.6 vs 1.0 for 3DB vs PB, respectively. The average air gap volume (cc) normalized by bolus size (cm2) was less for 3DB vs PB. For complex H&N plans, 3DB HI and CI was significantly better compared with CB plans. No patient treated with 3DB experienced a clinically adverse event.
Conclusion:3DB is clinically safe, and effective in improving superficial dose confirmed with in vivo measurement. Superior 3DB clarity allows more efficient, reproducible bolus placement with fewer air gaps. 3DB density is significantly more uniform than PB, and homogeneity and conformity indices demonstrate improved dosimetric outcomes of complex and/or irregular superficial RT targets.
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