Comparative performance analysis of 2D and 3D gamma metrics for patient specific QA in VMAT using Octavius 4D with 2D-Array 1500.

INTRODUCTION: Gamma pass percentage (GPP) is the predominant metric used for Patient Specific Quality Assurance (PSQA) in radiation therapy. The dimensionality of the measurement geometry in PSQA has evolved from 2D planar to 3D planar, and presently to state-of-the-art 3D volumetric geometry. We aim to critically examine the performance of the three-dimensional gammas vis-à-vis the older gamma metrics of lower dimensionality to determine their mutual fungibility in PSQA, using clinically approved Volumetric Arc Therapy (VMAT) plans. METHODS AND MATERIALS: Gamma pass percentages derived from PSQA for VMAT plans using Octavius 4D phantom with 2D-Array 1500 and its proprietary software were recorded. 2D planar, 3D planar, and 3D volumetric gamma pass percentages were retrospectively extracted for multiple treatment plans at three sites, using three acceptance limits, and for two modes of normalization. The differences in mean pass percentages, and the pairwise correlation between geometries were calculated within limits of statistical significance. RESULTS: A significant increase in mean pass rates was observed from 2D planar to 3D planar geometries. The difference was less pronounced from 3D planar to 3D volumetric. 2D planar v/s 3D planar showed a significant degree of correlation among themselves, which was not seen against most of the 3D volumetric pass rates. CONCLUSION: The mean gamma pass rates show conclusive evidence of the benefits of shifting from 2D planar to higher dimensions measurement geometries, but the benefits of using 3D volumetric compared to 3D planar is not always unequivocal. The correlations show mixed results regarding the interdependence of pass percentages at different geometries.

Clinical audit of dose-escalated radical radiotherapy for advanced cervical carcinoma using a pragmatic protocol (3 fractions of 8 Gy HDR brachytherapy).

INTRODUCTION: Recent image-guided brachytherapy data suggests, dose-escalation to a cumulative EQD2 (equivalent dose delivered at 2 Gy/#) of ≥87 Gy is associated with significantly better disease control. We present a clinical audit of a pragmatic radical radiotherapy protocol for advanced cervical cancer, using fewer fractions of brachytherapy than in the presently most popular protocol. MATERIAL & METHODS: Between July 2015 and December 2018, 96 consecutive advanced cervical carcinoma patients were treated by pelvic external beam radiotherapy (EBRT) (50 Gy/25fractions/5 weeks) ± weekly intravenous chemotherapy followed by image guided high dose rate (HDR) brachytherapy, using intracavitary/interstitial/hybrid techniques (intended point A dose: 8 Gy/fractions) × 3 fractions (cumulative target EQD2 ≥ 86 Gy). Insertion was done individually for each fraction of treatment. RESULTS: All patients completed their intended radiation protocol. 93.8% patients achieved complete response, while 6.2% patients achieved only partial response; no patients had stable/progressive disease. Out of the patients with partial response, 4.2% (4 out of 5 cases) cases of central/nodal residual disease underwent salvage surgery. At a median follow up of 21 months, 8.3% (8) patients had local failure, 1.1% (1) had nodal failure and 3.1% (3) had distant failures. Median Failure Free Survival was 29 months (26.5-31.5 months). On follow up, 6.3% and 3.2% patients had grade 2 or worse rectal and bladder morbidities respectively. CONCLUSION: The protocol under study has been safe and effective in achieving dose-escalated radical chemoradiation in advanced cervical carcinoma. The use of fewer insertions of brachytherapy is logistically easier & can also be expected to improve compliance.

Dosimetric analysis of Deep Inspiratory Breath-hold technique (DIBH) in left-sided breast cancer radiotherapy and evaluation of pre-treatment predictors of cardiac doses for guiding patient selection for DIBH.

INTRODUCTION: The risk of radiotherapy-associated cardiovascular disease has been a concern for decades in breast cancer survivors. The objective of our study is to evaluate the dosimetric benefit of Deep Inspiratory Breath-hold technique (DIBH) on organs-at-risk (OAR) sparing in left-sided breast cancer radiotherapy and to find out pre-treatment predictors of cardiac doses for guiding patient selection for DIBH. MATERIAL AND METHODS: Pre-radiotherapy planning CT scans were done in Free Breathing (FB) and in DIBH [using Active Breathing Coordinator system (ABC™)] in 31 left sided breast cancer patients. 3DCRT plans were generated for both scans. Comparison of anatomical and dosimetric variables were done using paired t test and correlation was evaluated using Pearson correlation. Linear regression was used to get independent predictors of cardiac sparing and Receiver Operating Characteristic (ROC) curve analysis was done to find out the specific threshold of the predictors. RESULTS: There was a 39.15% reduction in mean heart dose in DIBH compared to FB (2.4 Gy vs 4.01 Gy) (p < 0.001), 19% reduction in maximum Left Anterior Descending (LAD) dose and a 9.9% reduction in ipsilateral lung mean dose (p = 0.036) with DIBH. A significant correlation was observed between reduction in Heart Volume in Field (HVIF) and Maximum Heart Depth (MHD) with reduction in mean heart dose. Reduction in HVIF (ΔHVIF) independently predicted cardiac sparing. CONCLUSION: DIBH leads to significant reduction in OAR doses and is suggested for all patients of left-sided breast cancer undergoing radiotherapy. However, HVIF and MHD predicted for cardiac sparing and threshold criteria of ΔHVIF and ΔMHD may be used by centres with high workload to select patients for DIBH.

Retrospective study on performance of constancy check device in Linac beam monitoring using Statistical Process Control.

AIM: To examine the application of Statistical Process Control (SPC) and Ishikawa diagrams for retrospective evaluation of machine Quality Assurance (QA) performance in radiotherapy. BACKGROUND: SPC is a popular method for supplementing the performance of QA techniques in healthcare. This work investigates the applicability of SPC techniques and Ishikawa charts in machine QA. MATERIALS AND METHODS: SPC has been applied to recommend QA limits on the particular beam parameters using the QUICKCHECK webline QA portable constancy check device for 6 MV and 10 MV flattened photon beams from the Elekta Versa HD linear accelerator (Linac). Four machine QA parameters - beam flatness, beam symmetry along gun target direction and left-right direction, and beam quality factor (BQF) - were selected for retrospective analysis. Shewhart charts, Exponentially Weighted Moving Average (EWMA) charts and Cumulative Sum (CUSUM) charts were obtained for each parameter. The root causes for a failure in machine QA were broken down into an Ishikawa diagram enabling the user to identify the root cause of error and rectify the problem subsequently. RESULTS: Shewhart charts and EWMA charts applied could detect loss in control in one variable in the 6 MV beams and in all four variables in 10 MV beams. CUSUM charts detected offsets in the readings. The Ishikawa chart exhaustively included the possible errors that lead to loss of control. CONCLUSION: SPC is proven to be effective for detection of loss in control in machine QA. The Ishikawa chart provides the set of probable root causes of machine error useful while troubleshooting.