Abstract
PurposeThe purpose of this study is to investigate the dosimetric characteristics of volumetric modulated arc therapy (VMAT) with flattening filter-free (FFF) beams and assess the role of VMAT in the treatment of advanced nasopharyngeal carcinoma (NPC).MethodsTen cases of CT data were randomly selected from advanced NPC patients. Three treatment plans were optimized for each patient, RapidArc with FFF beams (RA-FFF), conventional beams (RA) and static gantry intensity-modulated radiation therapy (IMRT). The doses to the planning target volumes (PTVs), organs at risk (OARs), skin and normal tissue were compared. All the plans were delivered on a Varian TrueBeam linear accelerator and verified using the Delta4 phantom. Technical delivery parameters including the mean gamma score, treatment delivery time and monitor units (MUs) were also analyzed.ResultsAll the techniques delivered adequate doses to the PTVs. RA-FFF gave the highest D1% (dose received by 1% of the volume), but the poorest conformity index (CI) and homogeneity index (HI) among the PTVs except for the planning target volume of involved regional lymph nodes (PTV66) CI, which showed no significant difference among three techniques. For the planning target volume of the primary nasopharyngeal tumor (PTV70), RA-FFF provided for higher mean dose than other techniques. For the planning target volume receiving 60 Gy (PTV60) and PTV66, RA delivered the lowest mean doses whereas IMRT delivered the highest mean doses. IMRT demonstrated the highest percentage of target coverage and D99% for PTV60. RA-FFF provided for the highest doses to the brain stem, skin and oral cavity. RA gave the highest D1% to the right optic nerve among three techniques while no significant differences were found between each other. IMRT delivered the highest mean doses to the parotid glands and larynx while RA delivered the lowest mean doses. Gamma analysis showed an excellent agreement for all the techniques at 3%/3mm. Significant differences in the MUs were observed among the three techniques (p < 0.001). Delivery times for RA-FFF and RA were 152 ± 7s and 153 ± 7s, respectively, nearly 70% lower than the 493 ± 24s mean time for IMRT.ConclusionsAll treatment plans met the planning objectives. The dose measurements also showed good agreement with computed doses. RapidArc technique can treat patients with advanced NPC effectively, with good target coverage and sparing of critical structures. RA has a greater dosimetric superiority than RA-FFF.
Highlights
Radiotherapy treatment for nasopharyngeal carcinoma (NPC) is often difficult due to the location of several tumor-adjacent organs at risk (OARs) including the brain stem, spinal cord, parotid glands and optic nerves
All techniques met the planning requirement for delivering the prescribed dose to at least 95% of the planning target volumes (PTVs), and intensity-modulated radiation therapy (IMRT) demonstrated the highest percentage of target coverage (p = 0.002) and D99% (p = 0.006) for planning target volume receiving 60 Gy (PTV60)
RA-flattening filter-free (FFF) provided for higher D1% than other techniques and gave the poorest conformity index (CI) and homogeneity index (HI) for the PTVs except for the CI for PTV66, which showed no significant difference among the three techniques
Summary
Radiotherapy treatment for nasopharyngeal carcinoma (NPC) is often difficult due to the location of several tumor-adjacent organs at risk (OARs) including the brain stem, spinal cord, parotid glands and optic nerves. RapidArc is based on the volumetric modulated arc therapy (VMAT) technique and is developed to simultaneously optimize the multi-leaf collimator (MLC) shape, dose rate and gantry angle. It can obtain a dose distribution similar to the static gantry intensity-modulated radiation therapy (IMRT). The bremsstrahlung distribution from photons in the MeV energy range is strongly forward peaked and demonstrates both an energy and intensity variation of the primary photon fluence with emission angle To compensate for this variation, the flattening filter has been introduced in the treatment head of a medical accelerator, which results in an almost uniform dose at a certain depth. With the development of IMRT techniques in all its forms, the flattening filter in many cases becomes redundant because the MLC can be used to reach the desired dose distribution
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