Evaluating aperture shape controller (ASC) in modulated radiation treatments

Abstract: To decrease aperture complexity, the aperture shape controller (ASC) is a recently introduced function in Eclipse external beam planning (Varian Medical Systems, Palo Alto, USA). ASC works within the photon optimizer, minimizing distances between adjacent leaf tips of the MLC. The user can define via settings, ranging from Very Low to Very High, at which level the ASC is allowed to influence the optimization. Varian also suggests using another feature, convergence mode (CM), when using high settings of ASC. Since both experience and results of planning with ASC are limited, this work aims to examine the effects of ASC use in clinical treatment planning by investigating plan quality. The project was divided into two sub-studies. Sub-study 1 investigated plan quality parameters of executed treatment plans extracted pre- and post-ASC introduction. In sub-study 2, plans were created with different ASC and CM settings to evaluate how these settings affected the same plan quality parameters. The plan quality parameters used for both sub-studies were Edge Area Metric (EAM), gamma evaluation comparing planned and delivered dose distributions and plan quality index (PQI). Executed treatment plans for cancer diagnoses, rectum, tonsil, and prostate, were extracted from the database ARIA at Sahlgrenska University Hospital for sub-study 1. The resulting comparison groups were ASC None and Very Low for the prostate and tonsil and Lower versus Higher for the rectum. The rectum groups Lower and Higher includes ASC settings Very Low, Moderate, and High, Very High. A system for calculating a plan quality index was developed using clinical objectives, data from clinically accepted plans, and consults from clinicians. In sub-study 2, 15 prostate group plans were re-planned with different ASC and CM. When re-planning, the objectives were to have similar dose distribution between plans of the same patient case and fulfill clinical objectives. Time spent planning was also recorded during sub-study 2. Results were evaluated using Mann-Whitney U-test and comparing group medians. The results of sub-study 1 indicated a difference in medians and results of the Mann-Whitney test for rectum EAM and prostate PQI. For EAM, results suggested that a higher ASC results in a lower EAM. In the PQI results, ASC Very Low resulted in a lower PQI compared to ASC None for prostate plans. The results were inconclusive for all other ASC group comparisons or showed no significant differences. In sub-study 2, results indicated that a lower MLC complexity is obtained when using a higher ASC setting. No difference was seen when comparing the EAM results of different CM settings. The gamma pass rate and PQI results showed no significant difference between groups of different ASC and CM settings. Results of recorded time spent planning in sub-study 2 showed higher variances between individual plans than between groups of other ASC and CM settings. This concluded that using ASC at a high level could decrease aperture complexity. The ASC usage did not seem to affect the treatment deliverability negatively. ASC was found in one sub-study to have a negative effect on dose distribution quality for prostate plans. No other results in dose distribution quality indicated a difference between ASC settings. In addition to these results, using ASC and CM was not found to impact time spent planning negatively.