planning target volume margin assessment using daily cone beam computed tomography image verification in prostate cancer patients

Introduction: conformal radiation techniques are widely used in prostate cancer treatment as it improves the therapeutic ratio. however, inter- and intra-fraction variations remain challenging, which reflects on the planning target volume (ptv) margin. this study aimed to determine the benefit of daily cone beam computed tomography (cbct) in reducing the  ptv margin.material and methods: 51 patients over 3 years were included in the study.  ptv was obtained by applying 8mm in left-right (lr) and anterior (a), 5mm in posterior (p) and 8mm in superior-inferior (si) directions of clinical target volumes (ctv). pelvic bones of cbct were matched with the planning ct scan and manually adjusted with soft tissue, assessing the total inter-fraction error. values were registered in left-right (x), superior-inferior (y), and anterior-posterior (z) directions.results: the median age was 71 years and  84.3% were in the high-risk group. the mean inter-fraction error (mp) was obtained in all three principal axes (x, y, z). the majority had shifts ≤ 0.3cm. none recorded a shift >1cm. the random errors were 0.44cm in lr (x), 0.36cm in si (y) and 0.40cm in ap (z) directions. the systematic errors were 0.05cm in lr (x), 0.04cm in si (y) and 0.04cm in ap (z) directions. the ptv margin was calculated using van herk’s formula and obtained as 0.68cm.conclusion: routine use of cbct significantly reduces the random and systematic errors and ensures safer dose escalation with reduced ptv margin.