Cobalt radiation therapy treatments with opposed plain fields and beam flattening filter

Abstract

Low LET radiations such as cobalt (1.25 MeV) and 6 MV linac megavoltage gamma/x radiations continue to be a basic facility in a busy radiation therapy department. When the beam profiles in water are carefully scrutinized, linac beams have flattened isodose curves compared to slightly rounded curves in Co-60 beams at all depths. It was speculated that to overcome slight non-uniformity in the edges, as well as slight variations inside the planning target volume (PTV), a beam flattening filter may be preferable to be used along with plain radiotherapy fields. A pilot study of clinical use of beam flattener in cobalt beam is reported. A flattening filter (FF) was locally made out of dental wax, for 20 x 20cm field, in the 780E Telecobalt machine (M/s MDS Nordian, Canada). Beam profiles with and without FF were measured using water phantom, to confirm the filter efficiency. Both manual and Radiation Field (RFA) methods in water phantom confirmed efficacy of wax filter. This flattened beam was used in the treatments of Uterine Cervix and Oesophagus cancers, (25 patients each) with a measured filter transmission factor 0.805 in treatment plans. All the patients completed the prescribed dose of 50Gy in 25 fractions for Ca Cervix and 50Gy/25 fractions or 50.4Gy/28 fractions for Ca Oesophagus; at 5 fractions/ week treatment regimens. Case records of these patients retrospectively verified did not reveal any untoward associated morbidity on routine clinical follow ups during treatment or immediate follow up. As multi-leaf collimated cobalt beam also has built in rounded beam profiles, outcome of this work implemented in clinical applications add to new research knowledge. As cervix, head and neck, oesophagus malignancies form significant percentage of radiation oncology treatments, results of this present work will benefit many conventional treatments. It is strongly recommended that the clinical efficacy of FF Cobalt treatments be investigated in a controlled clinical trial, by random allotments with FF plain fields, with output corrections.