Abstract
BACKGROUND: A simultaneous integrated boost (SIB) may result in increased out-of-field (D(OOF)) and photoneutron (H(PN)) doses in volumetric modulated arc therapy (VMAT) for prostate cancer (PCA). This work therefore aimed to compare D(OOF) and H(PN) in flattened (FLAT) and flattening filter-free (FFF) 6‑MV and 10-MV VMAT treatment plans with and without SIB. METHODS: Eight groups of 30 VMAT plans for PCA with 6 MV or 10 MV, with or without FF and with uniform (2 Gy) or SIB target dose (2.5/3.0 Gy) prescriptions (CONV, SIB), were generated. All 240 plans were delivered on a slab-phantom and compared with respect to measured D(OOF) and H(PN) in 61.8 cm distance from the isocenter. The 6‑ and 10-MV flattened VMAT plans with conventional fractionation (6- and 10-MV FLAT CONV) served as standard reference groups. Doses were analyzed as a function of delivered monitor units (MU) and weighted equivalent square field size A(eq). Pearson's correlation coefficients between the presented quantities were determined. RESULTS: The SIB plans resulted in decreased H(PN) over an entire prostate RT treatment course (10-MV SIB vs. CONV -38.2%). Omission of the flattening filter yielded less H(PN) (10-MV CONV -17.2%; 10-MV SIB -22.5%). The SIB decreased D(OOF) likewise by 39% for all given scenarios, while the FFF mode reduced D(OOF) on average by 60%. A strong Pearson correlation was found between MU and H(PN) (r > 0.9) as well as D(OOF) (0.7 < r < 0.9). CONCLUSION: For a complete treatment, SIB reduces both photoneutron and OOF doses to almost the same extent as FFF deliveries. It is recommended to apply moderately hypofractionated 6‑MV SIB FFF-VMAT when considering photoneutron or OOF doses.