title: Feasibility of external beam radiation therapy to deep-seated targets with kilovoltage x-rays.
abstract: OBJECTIVE
  Radiation therapy to deep-seated targets is typically delivered with megavoltage x-ray beams generated by medical linear accelerators or60Co sources. Here, we used computer simulations to design and optimize a lower energy kilovoltage x-ray source generating acceptable dose distributions to a deep-seated target.
  
  
  METHODS
  The kilovoltage arc therapy (KVAT) x-ray source was designed to treat a 4-cm diameter target located at a 10-cm depth in a 40-cm diameter homogeneous cylindrical phantom. These parameters were chosen as an example of a clinical scenario for testing the performance of the kilovoltage source. A Monte Carlo (MC) model of the source was built in the EGSnrc/BEAMnrc code and source parameters, such as beam energy, tungsten anode thickness, beam filtration, number of collimator holes, collimator hole size and thickness, and source extent were varied. Dose to the phantom was calculated in the EGSnrc/DOSXYZnrc code for varying treatment parameters, such as the source-to-axis distance and the treatment arc angle. The quality of dose distributions was quantified by means of target-to-skin ratio and dose output expressed in D50(50% isodose line) for a 30-min irradiation in the homogeneous phantom as well as a lung phantom. Additionally, a patient KVAT dose distribution to a left pararenal lesion (~1.6 cm in diameter) was calculated and compared to a 15 MV volumetric modulated arc therapy (VMAT) plan.
  
  
  RESULTS
  In the design of the KVAT x-ray source, the beam energy, beam filtration, collimator hole size, source-to-isocenter distance, and treatment arc had the largest effect on the source output and the quality of dose distributions. For the 4-cm target at 10-cm depth, the optimized KVAT dose distribution generated a conformal plan with target-to-skin ratio of 5.1 and D50in 30 min of 24.1 Gy in the homogeneous phantom. In the lung phantom, a target-to-skin ratio of 7.5 and D50in 30 min of 25.3 Gy were achieved. High dose conformity of the 200 kV KVAT left pararenal plan was comparable to the 15 MV VMAT plan. The volume irradiated to at least 10% (<240 cGy) of the prescription dose was 2.2 × larger in the 200 kV KVAT plan than in the 15 MV VMAT plan, but considered clinically insignificant.
  
  
  CONCLUSIONS
  This study demonstrated that conformal treatments of deep-seated targets were achievable with kilovoltage x-rays with dose distributions comparable to MV beams. However, due to the larger volumes irradiated to clinically tolerated low doses, KVAT x-ray source usage for deep-seated lesions will be further evaluated to determine optimal target size.
authors.affiliation: Department of Physics and Astronomy, University of Victoria, PO Box 1700 ST CSC, Victoria, BC, V8W 2Y2, Canada.
authors.forename: Magdalena
authors.initials: M
authors.lastname: Bazalova-Carter
authors.affiliation: Sirius Medicine LLC, Box 414, Half Moon Bay, CA, 94019, USA.
authors.forename: Michael D
authors.initials: MD
authors.lastname: Weil
authors.affiliation: Department of Physics and Astronomy, University of Victoria, PO Box 1700 ST CSC, Victoria, BC, V8W 2Y2, Canada.
authors.forename: Dylan Yamabe
authors.initials: DY
authors.lastname: Breitkreutz
authors.affiliation: Triple Ring Technologies, 39655 Eureka Dr, Newark, CA, 94560-4806, USA.
authors.forename: Brian P
authors.initials: BP
authors.lastname: Wilfley
authors.affiliation: Department of Radiation Oncology, Stanford University, 875 Blake Wilbur Dr, Stanford, CA, 94305-5847, USA.
authors.forename: Edward E
authors.initials: EE
authors.lastname: Graves
mesh_terms: D003198:Computer Simulation; D005240:Feasibility Studies; D006801:Humans; D009010:Monte Carlo Method; D019047:Phantoms, Imaging; D011829:Radiation Dosage; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted; D014964:X-Ray Therapy / Q000295:instrumentation / Q000379:methods*
keywords: Monte Carlo; kilovoltage photon beams; radiation therapy