Small field electron beam dosimetry using MOSFET detector

Abstract

The dosimetry of very small electron fields can be challenging due to relative shifts in percent depth‐dose curves, including the location of dmax, and lack of lateral electronic equilibrium in an ion chamber when placed in the beam. Conventionally a small parallel plate chamber or film is utilized to perform small field electron beam dosimetry. Since modern radiotherapy departments are becoming filmless in favor of electronic imaging, an alternate and readily available clinical dosimeter needs to be explored. We have studied the performance of MOSFET as a relative dosimeter in small field electron beams. The reproducibility, linearity and sensitivity of a high‐sensitivity microMOSFET were investigated for clinical electron beams. In addition, the percent depth doses, output factors and profiles have been measured in a water tank with MOSFET and compared with those measured by an ion chamber for a range of field sizes from 1 cm diameter to 10 cm× 10 cm for 6, 12, 16 and 20 MeV beams. Similar comparative measurements were also performed with MOSFET and films in solid water phantom. The MOSFET sensitivity was found to be practically constant over the range of field sizes investigated. The dose response was found to be linear and reproducible (within ±1% for 100 cGy). An excellent agreement was observed among the central axis depth dose curves measured using MOSFET, film and ion chamber. The output factors measured with MOSFET for small fields agreed to within 3% with those measured by film dosimetry. Overall results indicate that MOSFET can be utilized to perform dosimetry for small field electron beam.PACS number: 87.55.Qr

Radiotherapy electron beams collimated by small tubular applicators: characterization by silicon and diamond diodes

“…Results are not fully comparable due to the dissimilar geometries and materials of applicators, however it is worth to mention that a large component of collimator-scattered electrons in the useful beam has been reported for those collimators as well as results of Monte Carlo simulations. On the contrary, the phenomenon of OFs larger than unity generally does not take place for similar electron energies and analogous small-size fields obtained by blocking the open fields using lead or Cerrobend cut-outs placed at the end of standard applicators (blocked fields), minimizing the wall-scattered electron component (Rashid et al 1990, Björk et al 2004, Amin et al 2011. In such cases OFs monotonically decrease below unity for field size below 10 × 10 cm 2 .…”

Penentuan Faktor Keluaran Berkas Elektron Lapangan Kecil Pada Pesawat Linear Accelerator

“…Kanker yang lokasinya dekat dengan organ sehat memerlukan terapi menggunakan lapangan yang kecil [2], [3]. Berkas elektron lapangan kecil digunakan untuk terapi klinis pada lokasi kanker permukaan untuk elektron energi rendah.…”

“…Ukuran lapangan (cutout) yang kecil dapat mempengaruhi berkas elektron sisi defleksi yang didominasi oleh beberapa Coulomb hamburan yang menyebabkan kurangnya keseimbangan hamburan elektron lateral (electron lateral-scatter equilibrium), sehingga terjadinya variasi keluaran mesin [3], [6]. Kurangnya keseimbangan lateral dapat terjadi lebih buruk lagi apabila dosimeter yang digunakan ukurannya lebih besar dari lapangan penyinaran, khususnya berkas elektron lapangan kecil.…”

The effects of incidence angle on film dosimetry and their consequences in IMRT dose verification

“…6,7 As a matter of fact, radiographic film is still considered as a reference detector for small field dosimetry in photon 8 and electron beams. 9 Recently radiographic EDR2 film was used as a reference next to a semiconductor-based method in the context of composite IMRT QA. 10 Basically, two QA approaches can be followed for treatment verification.…”