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Impact of Different Intensity Levels On Step and Shoot IMRT Plan Quality and Deliverability

D Manigandan

D Manigandan1*, S Sharma2, A Biswas2, B Mohanty2, V Subramani2, S Chander2, P Julka2, G Rath2, (1) Sri Siddhivinayak Ganapaty Cancer Hospital (SSGCH), Miraj, Maharashtra, (2) All India Institute of Medical Sciences (AIIMS), New Delhi, Delhi

SU-E-T-636 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: To evaluate the impact of different intensity-levels 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, 45Gy/25fraction prescribed at 95% isodose) were created in Eclipse TPS for Varian CL2300C/D linear accelerator at 300MU/min. During optimization, dose volume constraints and priorities were kept constant and only intensity-levels were varied as follows: 5, 10 and 15. Plan quality was analyzed in terms of maximum and mean doses of PTV, coverage index (CI=PTV covered by prescription dose/PTV), heterogeneity index (HI)=D₅/D₉₅ (D₅=dose received by 5% of PTV and D₉₅=dose received by 95% of PTV), OAR mean-doses and normal tissue integral-dose (NTID) (liter-Gray). Total monitor units (MUs) required to deliver a plan was noted. Deliverability of treatment plans were verified with I-matriXX array and compared with TPS dose-plane using gamma index of 3% dose difference and 3mm distance to agreement (DTA) criteria.

Results: Maximum dose to PTV was 5219.06±54.55cGy, 5071.30±49.01cGy and 5053.26±71.85cGy for 5, 10 and 15 intensity-levels, respectively. Mean dose to the PTV was 4655.02±27.29cGy, 4664.00±23.29cGy and 4659.98±22.01cGy. The CI of PTV was 0.8573±0.07, 0.9661±0.02 and 0.9686±0.01 and HI of PTV was 1.0800±0.02, 1.0701±0.01, and 1.0677±0.01 for 5, 10 and 15 intensity-levels. Mean dose to bladder was 3491.88±365.15cGy, 3513.98±387.04cGy and 3501.93±380.91cGy. Bowel mean dose was 1352.05±365.20cGy, 1369.65±378.76cGy and 1369.53±375.30cGy. NTID (liter-Gray) was 144.64±11.88, 145.77±12.85 and 145.60±12.73. Total MU required to deliver a plan was 1060±307, 1081±309 and 1061±303. Gamma pass rate was 99.65±0.35%, 99.81±0.16% and 99.87±0.12%.

Conclusion:: PTV coverage and homogeneity increases with increase in intensity-level. Plan quality was better at intensity-level 5 in terms of OAR mean-doses, NTID and total MU. Gamma pass rates were improved with increase of intensity-level.

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