SU‐E‐T‐638: Influence of Dose Rate On Step and Shoot IMRT Plan Quality and Deliverability

Purpose: To study the influence of dose rate on step and shoot IMRT plan quality and deliverability. Methods: Five previously treated patients of carcinoma rectum (pre‐operative) were studied. Planning target volume (PTV) and organ at risk (OAR) i.e. bladder and bowel were contoured. Step and shoot IMRT plans (6MV X‐rays, 7‐beam, 45Gy/25fraction) were generated in Eclipse TPS at 300MU/min for Varian CL2300C/D linear accelerator. Then plan was re‐optimized by keeping dose volume constraints, priorities and all other parameters constant, only the dose rate was varied as follows: 400MU/min, 500MU/min and 600MU/min. Therefore, four different plans were generated for each case at 300– 600MU/min. Plan quality was analyzed by means of PTV maximum and mean‐doses, coverage index (CI=PTV covered by prescription dose/PTV) and OAR mean‐doses. Total monitor units (MUs) and treatment time required to deliver a plan was also noted. Deliverability of plans were verified with I' matriXX ion‐chamber array and compared with TPS dose‐plane using gamma index of 3% dose difference and 3mm distance to agreement criteria. Results: Maximum dose to the PTV was 5063.66±99.76cGy, 5083.56±104.54cGy, 5062.74±96.65cGy and 5062.72±92.13cGy for 300–600MU/min dose rates, respectively. Mean‐dose to PTV was 4659.08±26.05cGy, 4660.50±26.09cGy, 4656.34±25.78cGy and 4656.18±24.01cGy. The CI for PTV was 0.9520±0.020, 0.9528±0.022, 0.9497±0.023 and 0.9505±0.023. Mean‐dose to the bladder was 3644.78±452.68cGy, 3642.52±452.15cGy, 3641.56±450.71cGy and 3639.98±450.90cGy for 300‐600MU/min, respectively. Bowel mean dose was 1666.60±747.77cGy, 1665.46±747.49cGy, 1665.84±746.44cGy and 1665.50±744.97cGy. Total MUs were 1063±319, 1059±315, 1062±315 and 1080±353. Total time required to deliver a plan was 8.75±1.02min, 8.34±0.90min, 7.78±0.79min and 7.53±0.65min. Gamma pass rate was 99.86±0.08%, 99.80±4.04%, 99.74±0.10% and 99.61±0.28%. Conclusion: Plan quality in terms of PTV coverage, maximum and mean‐dose to OAR showed almost similar results for all dose rates. The time required to deliver a plan was reduced with increase in dose rate. However, gamma pass rate was slightly decreased with increase in dose rate.