Room: Exhibit Hall
Purpose: The progressive resolution optimizer (PRO) of RapidArc volumetric modulated arc therapy generates varying field apertures alongside dose rate and gantry speed variations to arrive at variable MU per degree. The aim was to quantify relative merit of MU deprived plans against freely optimized baseline plans in terms of plan quality and probe for changes made to the free parameters of RapidArc.
Methods: Eight cases of carcinoma Hypopharynx were retrospectively planned in three sequential phases. Each of the three baseline plans was generated without using MU objective tool. Three plan replicas of baseline plans for each phase were re-optimized using the same tool intending to reduce MUs by 20%, 35%, and 50%. Intermediate dose feature along with MU tool was utilized to proceed with the baseline optimization for retaining a similar dose distribution but with lesser MU. Plan merit was quantified as an overall quality index, determined from individual dosimetric indices obtained from target and OARs separately.
Results: Across the increasing MU reduction strengths, MU per Gy values reduced invariably, for all phases with an overall range of 8.6% to 34.7%; mean dose rate decreased among plans of each phase, phase3 plans had higher values due to small field size and recorded greater reductions. A 20% MU reduction strength showed good trade-off with lower MUs yet offering equivalent plan quality while 50% strength degraded it, except for phase3. Low dose volume spread up till 5 Gy range was higher for baseline plans while at intermediate levels of 20 Gy and 35 Gy it was lower. Absolute delta dose rate variations were the least for baseline plans.
Conclusion: These results demonstrate a methodical approach to understand the tradeoffs based on the plan merits and adopt the best strategy while using MU tool with PRO3.
Not Applicable / None Entered.