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Influence of Dose Rate On Step and Shoot IMRT Plan Quality and Deliverability

S Sharma

S Sharma1*, D Manigandan2, A Biswas1, B Mohanty1, V Subramani1, S Chander1, P Julka1, G Rath1, (1) All India Institute of Medical Sciences (AIIMS), New Delhi, Delhi, (2) Sri Siddhivinayak Ganapaty Cancer Hospital (SSGCH), Miraj, Maharashtra,

SU-E-T-638 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

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.

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