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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Background Mega-voltage fan-beam Computed Tomography (MV-FBCT) holds potential in accurate determination of relative electron density (RED) and proton stopping power ratio (SPR) but is not widely available. Objective To demonstrate the feasibility of MV-FBCT using a medical linear accelerator (LINAC) with a 2.5 MV imaging beam, an electronic portal imaging device (EPID) and multileaf collimators (MLCs). Methods MLCs were used to collimate MV beam along z direction to enable a 1 cm width fan-beam. Projection data were acquired within one gantry rotation and preprocessed with in-house developed artifact correction algorithms before the reconstruction. MV-FBCT data were acquired at two dose levels: 30 and 60 monitor units (MUs). A Catphan 604 phantom was used to evaluate basic image quality. A head-sized CIRS phantom with three configurations of tissue-mimicking inserts was scanned and MV-FBCT Hounsfield unit (HU) to RED calibration was established for each insert configuration using linear regression. The determination coefficient ($${R}^{2}$$ R 2 ) was used to gauge the accuracy of HU-RED calibration. Results were compared with baseline single-energy kilo-voltage treatment planning CT (TP-CT) HU-RED calibration which represented the current standard clinical practice. Results The in-house artifact correction algorithms effectively suppressed ring artifact, cupping artifact, and CT number bias in MV-FBCT. Compared to TP-CT, MV-FBCT was able to improve the prediction accuracy of the HU-RED calibration curve for all three configurations of insert materials, with $${R}^{2}$$ R 2 > 0.9994 and $${R}^{2}$$ R 2 < 0.9990 for MV-FBCT and TP-CT HU-RED calibration curves of soft-tissue inserts, respectively. The measured mean CT numbers of blood-iodine mixture inserts in TP-CT drastically deviated from the fitted values but not in MV-FBCT. Reducing the radiation level from 60 to 30 MU did not decrease the prediction accuracy of the MV-FBCT HU-RED calibration curve. Conclusion We demonstrated the feasibility of MV-FBCT and its potential in providing more accurate RED estimation.

Implementing tighter intensity modulated radiation therapy (IMRT) quality assurance (QA) tolerances initially resulted in high numbers of marginal or failing QA results and motivated a number of improvements to our calculational processes. This work details those improvements and their effect on results. One hundred eighty IMRT plans analyzed previously were collected and new gamma criteria were applied and compared to the original results. The results were used to obtain an estimate for the number of plans that would require additional dose volume histogram (DVH)‐based analysis and therefore predicted workload increase. For 2 months and 133 plans, the established criteria were continued while the new criteria were applied and tracked in parallel. Because the number of marginal or failing plans far exceeded the predicted levels, a number of calculational elements were investigated: IMRT modeling parameters, calculation grid size, and couch top modeling. After improvements to these elements, the new criteria were clinically implemented and the frequency of passing, questionable, and failing plans measured for the subsequent 15 months and 674 plans. The retrospective analysis of selected IMRT QA results demonstrated that 75% of plans should pass, while 19% of IMRT QA plans would need DVH‐based analysis and an additional 6% would fail. However, after applying the tighter criteria for 2 months, the distribution of plans was significantly different from prediction with questionable or failing plans reaching 47%. After investigating and improving several elements of the IMRT calculation processes, the frequency of questionable plans was reduced to 11% and that of failing plans to less than 1%. Tighter IMRT QA tolerances revealed the need to improve several elements of our plan calculations. As a consequence, the accuracy of our plans have improved, and the frequency of finding marginal or failing IMRT QA results, remains within our practical ability to respond.

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