Multi-institutional comparison of computer-based independent dose calculation for intensity modulated radiation therapy and volumetric modulated arc therapy

The BeamSplitter - An algorithm providing the dose per control point of radiation therapy treatment plans

PURPOSE

Commercial radiation therapy treatment planning systems (TPS) provide the three-dimensional time-integrated planned dose distribution. A four-dimensional (4D) dose calculation is essential to minimise dose-rate effects on pacemaker or in total body irradiation treatment or for time-dependent patient-specific quality assurance. We introduce the BeamSplitter, an algorithm in a commercial TPS generating 4D dose calculation.

METHODS

Automation was performed with the Eclipse Scripting Application Programming Interface (ESAPI, v16, Varian Medical System). The accumulated and reference dose distributions of twenty-one patients treated in 2022 at our institution were compared. The patients were selected to cover a broad range of modulated delivery techniques and plan complexity, quantified with the average leaf pair opening (ALPO). The dose distributions were compared with gamma passing rates (γPR). Moreover, the mean absolute percentage error (MAPE) between accumulated and reference dose metrics (planning target volume (PTV) D99% / D95% in stereotactic / non-stereotactic treatment, organs at risk (OAR) D5cc / D0.03 cc, and mean dose) was calculated. Dose accumulation was validated by point dose measurement.

RESULTS

The ALPO ranged from 11.6 mm to 77 mm. The accumulated and reference dose distributions were similar (γPR0mm1% > 90 % in all patients). The MAPE between reference and accumulated dose metrics was lower than 1 % in all cases. The average dose difference through time between the accumulated and measured dose was lower than our clinical tolerance of 3 %.

CONCLUSIONS

The BeamSplitter is the first validated algorithm generating a 4D dose calculation of radiation therapy treatment plans within a commercial TPS.

Development of an Immobilization Device for Total Marrow Irradiation

PURPOSE

A body frame dedicated to total marrow (lymph node) irradiation (TMI/TMLI) could minimize patient motion during the potentially extended beam-on time with this technique. We present the development of a dedicated immobilization system for TMI/TMLI using volumetric modulated arc therapy.

METHODS AND MATERIALS

Since 2010, 59 adult patients were treated with TMI/TMLI using a multi-isocenter volumetric modulated arc therapy technique. Two computed tomographies (CTs) were required (1 head-first supine and 1 feet-first supine) to cover the whole volume. For the first 10 patients, 2 standard commercial frames with personalized masks (with/without personalized vacuum cushion for the lower extremities) were used without specific interfixation (frame A). For the next 49 patients a homemade 3-frame immobilization system was adopted (frame B), where each frame was interlocked with the next one and thermoplastic masks used to fix the patient. The effectiveness of the 2 immobilization systems was assessed by offline/online matching between daily cone beam CT of each isocenter and the simulation CTs.

RESULTS

Mean offline shifts for frame A were 3 to 12 mm in anterior-posterior, 2 to 5 mm in cranilal-caudal, and 2 to 6 mm in left-right directions. Larger shifts were found for feet-first supine series (shifts up to 23 mm). In frame B, mean offline shifts were 1 to 4 mm in anterior-posterior, 1 to 4 mm in cranial-caudal, and 1 to 4 mm in left-right directions. Mean online adjustments were -1 ± 4 mm in anterior-posterior, 0 ± 2 mm in cranial-caudal, and 0 ± 4 mm in left-right directions.

CONCLUSIONS

The patient positioning shifts for TMI/TMLI irradiation were mitigated by a homemade immobilization system and the use of individualized masks.

Multi-institutional comparison of secondary check of treatment planning using computer-based independent dose calculation for non-C-arm linear accelerators

PURPOSE

This report covers the first multi-institutional study of independent monitor unit (MU)/dose calculation verification for the CyberKnife, Vero4DRT, and TomoTherapy radiotherapy delivery systems.

METHODS

A total of 973 clinical treatment plans were collected from 12 institutions. Commercial software employing the Clarkson algorithm was used for verification after a measurement validation study, and the doses from the treatment planning systems (TPSs) and verification programs were compared on the basis of the mean value ± two standard deviations. The impact of heterogeneous conditions was assessed in two types of sites: non-lung and lung.

RESULTS

The dose difference for all locations was 0.5 ± 7.2%. There was a statistically significant difference (P < 0.01) in dose difference between non-lung (-0.3 ± 4.4%) and lung sites (3.5 ± 6.7%). Inter-institutional comparisons showed that various systematic differences were associated with the proportion of different treatment sites and heterogeneity correction.

CONCLUSIONS

This multi-institutional comparison should help to determine the departmental action levels for CyberKnife, Vero4DRT, and TomoTherapy, as patient populations and treatment sites may vary between the modalities. An action level of ±5% could be considered for intensity-modulated radiation therapy (IMRT), non-IMRT, and volumetric modulated arc radiotherapy using these modalities in homogenous and heterogeneous conditions with a large treatment field applied to a large region of homogeneous media. There were larger systematic differences in heterogeneous conditions with a small treatment field because of differences in heterogeneity correction with the different dose calculation algorithms of the primary TPS and verification program.

A multi-institutional study of independent calculation verification in inhomogeneous media using a simple and effective method of heterogeneity correction integrated with the Clarkson method

In inhomogeneous media, there is often a large systematic difference in the dose between the conventional Clarkson algorithm (C-Clarkson) for independent calculation verification and the superposition-based algorithms of treatment planning systems (TPSs). These treatment site-dependent differences increase the complexity of the radiotherapy planning secondary check. We developed a simple and effective method of heterogeneity correction integrated with the Clarkson algorithm (L-Clarkson) to account for the effects of heterogeneity in the lateral dimension, and performed a multi-institutional study to evaluate the effectiveness of the method. In the method, a 2D image reconstructed from computed tomography (CT) images is divided according to lines extending from the reference point to the edge of the multileaf collimator (MLC) or jaw collimator for each pie sector, and the radiological path length (RPL) of each line is calculated on the 2D image to obtain a tissue maximum ratio and phantom scatter factor, allowing the dose to be calculated. A total of 261 plans (1237 beams) for conventional breast and lung treatments and lung stereotactic body radiotherapy were collected from four institutions. Disagreements in dose between the on-site TPSs and a verification program using the C-Clarkson and L-Clarkson algorithms were compared. Systematic differences with the L-Clarkson method were within 1% for all sites, while the C-Clarkson method resulted in systematic differences of 1-5%. The L-Clarkson method showed smaller variations. This heterogeneity correction integrated with the Clarkson algorithm would provide a simple evaluation within the range of -5% to +5% for a radiotherapy plan secondary check.