The purpose of this study is to apply the principles of statistical process control (SPC) in the context of patient specific intensity‐modulated radiation therapy (IMRT) QA to set clinic‐specific action limits and evaluate the impact of changes to the multileaf collimator (MLC) calibrations on IMRT QA results. Ten months of IMRT QA data with 247 patient QAs collected on three beam‐matched linacs were retrospectively analyzed with a focus on the gamma pass rate (GPR) and the average ratio between the measured and planned doses. Initial control charts and action limits were calculated. Based on this data, changes were made to the leaf gap parameter for the MLCs to improve the consistency between linacs. This leaf gap parameter is tested monthly using a MLC sweep test. A follow‐up dataset with 424 unique QAs were used to evaluate the impact of the leaf gap parameter change. The initial data average GPR was 98.6% with an SPC action limit of 93.7%. The average ratio of doses was 1.003, with an upper action limit of 1.017 and a lower action limit of 0.989. The sweep test results for the linacs were ‐1.8%,0%, and +1.2% from nominal. After the adjustment of the leaf gap parameter, all sweep test results were within 0.4% of nominal. Subsequently, the average GPR was 99.4% with an SPC action limit of 97.3%. The average ratio of doses was 0.997 with an upper action limit of 1.011 and a lower action limit of 0.981. Applying the principles of SPC to IMRT QA allowed small differences between closely matched linacs to be identified and reduced. Ongoing analysis will monitor the process and be used to refine the clinical action limits for IMRT QA.PACS number: 87.55.Qr

Purpose: To assess the efficacy of placing a polyethylene glycol (PEG) spacing hydrogel in patients undergoing proton beam radiation therapy for prostate cancer. This study also aims to assess the effect on rectal radiation dose of prostateerectum separation in various anatomic planes.Methods and Materials: Seventy-two consecutive prostate cancer patients undergoing conventionally fractionated pencil beam scanning proton radiation therapy with and without hydrogel placement were compared. Magnetic resonance images taken after hydrogel placement measured prostateerectum separation and were correlated to rectal dosing and rectal toxicity. Univariate analysis of clinical variables and radiation dosing was conducted using nonparametric Wilcoxon rank-sum test with continuity correction between groups (hydrogel spacer vs controls). Spearman's rank correlation coefficient assessed relationships between the various anatomic dimensions of perirectal space and rectal radiation dosing.Results: Fifty-one patients had hydrogel placement before therapy and 21 did not. There was a 42.2% reduction in rectal dosing (mL 3 rectum) in hydrogel patients (P < .001). Increasing midline sagittal lift resulted in a greater mitigation of total rectal dose (P Z .031). The degree of prostate surface area coverage on coronal plane did not correlate with further reductions in rectal radiation dose (P Z .673). Patients who had PEG hydrogels placed reported more rectal side effects during treatment compared with those patients who did not (35.3% vs 9.5%, P Z .061). At median 9.5-month follow-up, there was no difference in reporting of grade 2 rectal toxicity between the 2 groups (7.7% vs 7.1%, P Z .145).Conclusions: Polyethylene glycol hydrogel placement before pencil proton beam radiation therapy for prostate cancer reduced rectal radiation dose. The most important factor reducing total rectal dose was the degree of sagittal midline separation created by the PEG hydrogel. This is the largest study with the longest follow-up to investigate hydrogel placement in the proton beam radiation setting.

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Purpose: The purpose of this study is to apply the principles of Statistical Process Control (SPC) in the context of patient specific IMRT QA to set clinic specific action limits and evaluate the impact of changes to the MLC calibrations on IMRT QA results. Methods: Ten months of IMRT QA data with 247 unique QAs collected on three beam matched LINACS were retrospectively analyzed with a focus on the Gamma Pass Rate (GPR) and the average ratio between the measured and planned doses. Initial control charts and action limits were calculated. Based on this data, changes were made to the leaf gap parameter for the MLCs to improve the consistency between LINACS. This leaf gap parameter is tested monthly using a MLC sweep test. A follow up data set with 119 unique QAs was used to evaluate the impact of the leaf gap parameter change. Results: The initial data average GPR was 98.6 % with an SPC action limit of 93.7%. The average ratio of doses was 1.003 with an upper action limit of 1.017 and a lower action limit of .989. The sweep test results for the LINACS were −1.8%, 0%, and +1.2% from nominal. After the adjustment of the leaf gap parameter all sweep test results were within .4% of nominal. Subsequently, the average GPR was 99.4 % with an SPC action limit of 97.3%. The average ratio of doses was 1.003 with an upper action limit of 1.017 and a lower action limit of .989. Conclusion: Applying the principles of SPC to IMRT QA allowed small differences between closely matched LINACS to be identified and reduced. Ongoing analysis will monitor the process and be used to refine the clinical action limits for IMRT QA.

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