Dose delivery accuracy of therapeutic photon and electron beams at low monitor unit settings.

Purpose: Dose delivery accuracy at low monitor units (LMU) was evaluated for photon and electron beams. Knowledge of this study is required for few dosimetric applications and to know the dose delivered to the patient when the treatment is delivered with few monitor units (MU).

Material And Methods: Dose measurements were carried out for photon and electron beams with 0.6 cm(3) PTW ion chamber in white polystyrene phantom at D(max) with a field size of 10 x 10 cm(2) at 100 cm FSD. The relative dose, which is the ratio of dose delivered per MU at the testing to that of the calibration condition, was found out.

Results: Significant deviation (+20% to +25%) in dose delivery was noticed for photon and electron beams (+39% to +45%) at LMU settings. Slightly higher inaccuracy in dose delivery was noticed for 6-MV compared to 18-MV photons. The deviation in dose delivery for electron beams was found to be energy-independent and the pattern of variation was similar for all electron energies.

Conclusion: The dose delivery accuracy at LMU settings has to be ascertained before implementing conformal and IMRT (intensity- modulated radiotherapy) techniques. When there is dose nonlinearity, the treatment delivered with multiple small MU settings can result in significant error in dose delivery.