A comparison of a moderately hypofractionated IMRT planning technique used in a randomised UK external beam radiotherapy trial with an in-house technique for localised prostate cancer

Abstract

To compare the radiotherapy technique used in a randomised trial with VMAT and an in-house technique for prostate cancer. Techniques are evolving with volumetric modulated arc therapy (VMAT) commonly used. The CHHiP trial used a 3 PTV forward planned IMRT technique (FP_CH). Our centre has adopted a simpler two PTV technique with locally calculated margins. 25 patients treated with FP_CH to 60Gy in 20 fractions were re-planned with VMAT (VMAT_CH) and a two PTV protocol (VMAT_60/52 and VMAT_60/48). Target coverage, conformity index (CI), homogeneity index (HI), monitor units (MU) and dose to the rectum, bladder, hips and penile bulb were compared. PTV coverage was high for all techniques. VMAT_CH plans had better CI than FP_CH (p≤ 0.05). VMAT_60/52/48 plans had better CI than VMAT_CH. FP_CH had better HI and fewer MU than VMAT (p≤ 0.05). More favourable rectum doses were found for VMAT _CH than FP_CH (V48.6, V52.8, V57, p≤ 0.05) with less difference for bladder (p≥ 0.05). Comparing VMAT_CH to VMAT_60/52/48 showed little differences for the bladder and rectum but VMAT_CH had larger penile bulb doses (V40.8, V48.6, mean, D2, p≤ 0.05). Femoral head doses (V40.8) were similarly low for all techniques (p=≥ 0.05). VMAT produced more conformal plans with smaller rectum doses compared to FP_CH albeit worse HI and more MU. VMAT_60/52 and VMAT_60/48 plans had similar rectal and bladder doses to VMAT_CH but better CI and penile bulb doses which may reduce toxicity.