Feasibility of using intermediate x-ray energies for highly conformal extracranial radiotherapy.
OBJECTIVE
To investigate the feasibility of using intermediate energy 2 MV x-rays for extracranial robotic intensity modulated radiation therapy.


METHODS
Two megavolts flattening filter free x-rays were simulated using the Monte Carlo code MCNP (v4c). A convolution/superposition dose calculation program was tuned to match the Monte Carlo calculation. The modeled 2 MV x-rays and actual 6 MV flattened x-rays from existing Varian Linacs were used in integrated beam orientation and fluence optimization for a head and neck, a liver, a lung, and a partial breast treatment. A column generation algorithm was used for the intensity modulation and beam orientation optimization. Identical optimization parameters were applied in three different planning modes for each site: 2, 6 MV, and dual energy 2/6 MV.


RESULTS
Excellent agreement was observed between the convolution/superposition and the Monte Carlo calculated percent depth dose profiles. For the patient plans, overall, the 2/6 MV x-ray plans had the best dosimetry followed by 2 MV only and 6 MV only plans. Between the two single energy plans, the PTV coverage was equivalent but 2 MV x-rays improved organs-at-risk sparing. For the head and neck case, the 2 MV plan reduced lips, mandible, tongue, oral cavity, brain, larynx, left and right parotid gland mean doses by 14%, 8%, 4%, 14%, 24%, 6%, 30% and 16%, respectively. For the liver case, the 2 MV plan reduced the liver and body mean doses by 17% and 18%, respectively. For the lung case, lung V 20, V 10, and V5 were reduced by 13%, 25%, and 30%, respectively. V 10 of heart with 2 MV plan was reduced by 59%. For the partial breast treatment, the 2 MV plan reduced the mean dose to the ipsilateral and contralateral lungs by 27% and 47%, respectively. The mean body dose was reduced by 16%.


CONCLUSIONS
The authors showed the feasibility of using flattening filter free 2 MV x-rays for extracranial treatments as evidenced by equivalent or superior dosimetry compared to 6 MV plans using the same inverse noncoplanar intensity modulated planning method.
Department of Radiation Oncology, University of California Los Angeles, California 90095.
Peng
P
Dong
Department of Radiation Oncology, University of California Los Angeles, California 90095.
Victoria
V
Yu
Department of Radiation Oncology, University of California Los Angeles, California 90095.
Dan
D
Nguyen
Department of Radiation Oncology, University of California Los Angeles, California 90095.
John
J
Demarco
RadiaBeam Technologies, Santa Monica, California 90404.
Kaley
K
Woods
RadiaBeam Technologies, Santa Monica, California 90404.
Salime
S
Boucher
Department of Radiation Oncology, University of California Los Angeles, California 90095.
Daniel A
DA
Low
Department of Radiation Oncology, University of California Los Angeles, California 90095.
Ke
K
Sheng
D005240:Feasibility Studies; D006801:Humans; D009010:Monte Carlo Method; D009369:Neoplasms / Q000532:radiotherapy; D010315:Particle Accelerators; D011874:Radiometry; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted; D050397:Radiotherapy, Intensity-Modulated / Q000295:instrumentation / Q000379:methods*; D015203:Reproducibility of Results; D012371:Robotics; D012886:Skull; D014965:X-Rays