Investigation of the reliability, accuracy, and efficiency of gated IMRT delivery with a commercial linear accelerator

Transferability of patients for radiation treatment between unmatched machines

Purpose

The feasibility of transferring patients between unmatched machines for a limited number of treatment fractions was investigated for three‐dimensional conformal radiation therapy (3DCRT) and volumetric modulated arc therapy (VMAT) treatments.

Methods

Eighty patient‐plans were evaluated on two unmatched linacs: Elekta Versa HD and Elekta Infinity. Plans were equally divided into pelvis 3DCRT, prostate VMAT, brain VMAT, and lung VMAT plans. While maintaining the number of monitor units (MUs), plans were recalculated on the machine not originally used for treatment. Relative differences in dose were calculated between machines for the target volume and organs at risk (OARs). Differences in mean dose were assessed with paired t‐tests (p < 0.05). The number of interchangeable fractions allowable before surpassing a cumulative ±5% difference in dose was determined. Additionally, patient‐specific quality assurance (PSQA) measurements using ArcCHECK for both machines were compared with distributions calculated on the machine originally used for treatment using gradient compensation (GC) with 2%/2‐mm criteria.

Results

Interchanging the two machines for pelvic 3DCRT and VMAT (prostate, brain, and lung) plans resulted in an average change in target mean dose of 0.9%, −0.5%, 0.6%, 0.5%, respectively. Based on the differences in dose to the prescription point when changing machines, statistically, nearly one‐fourth of the prescribed fractions could be transferred between linacs for 3DCRT plans. While all of the prescribed fractions could typically be transferred among prostate VMAT plans, a rather large number of treatment fractions, 31% and 38%, could be transferred among brain and lung VMAT plans, respectively, without exceeding a ±5% change in the prescribed dose for two Elekta machines. Additionally, the OAR dosage was not affected within the given criterion with change of machine.

Conclusions

Despite small differences in calculated dose, transferring patients between two unmatched Elekta machines with similar multileaf collimator (MLC)‐head for target coverage and minimum changes in OAR dose is possible for a limited number of fractions (≤3) to improve clinical flexibility and institutional throughput along with patient satisfaction. A similar study could be carried out for other machines for operational throughput.

Automatically gated image-guided breath-hold IMRT is a fast, precise, and dosimetrically robust treatment for lung cancer patients

BACKGROUND

High-dose radiotherapy of lung cancer is challenging. Tumors may move by up to 2 cm in craniocaudal and anteroposterior directions as a function of breathing cycle. Tumor displacement increases with treatment time, which consequentially increases the treatment uncertainty.

OBJECTIVE

This study analyzed whether automatically gated cone-beam-CT (CBCT)-controlled intensity modulated fast deep inspiration breath hold (DIBH) stereotactic body radiation therapy (SBRT) in flattening filter free (FFF) technique and normofractionated lung DIBH intensity-modulated radiotherapy (IMRT)/volumetric-modulated arc therapy (VMAT) treatments delivered with a flattening filter can be applied with sufficient accuracy within a clinically acceptable timeslot.

MATERIALS AND METHODS

Plans of 34 patients with lung tumors were analyzed. Of these patients, 17 received computer-controlled fast DIBH SBRT with a dose of 60 Gy (5 fractions of 12 Gy or 12 fractions of 5 Gy) in an FFF VMAT technique (FFF-SBRT) every other day and 17 received conventional VMAT with a flattening filter (conv-VMAT) and 2-Gy daily fractional doses (cumulative dose 50-70 Gy).

RESULTS

FFF-SBRT plans required more monitor units (MU) than conv-VMAT plans (2956.6 ± 885.3 MU for 12 Gy/fraction and 1148.7 ± 289.2 MU for 5 Gy/fraction vs. 608.4 ± 157.5 MU for 2 Gy/fraction). Total treatment and net beam-on times were shorter for FFF-SBRT plans than conv-VMAT plans (268.0 ± 74.4 s vs. 330.2 ± 93.6 s and 85.8 ± 25.3 s vs. 117.2 ± 29.6 s, respectively). Total slot time was 13.0 min for FFF-SBRT and 14.0 min for conv-VMAT. All modalities could be delivered accurately despite multiple beam-on/-off cycles and were robust against multiple interruptions.

CONCLUSION

Automatically gated CBCT-controlled fast DIBH SBRT in VMAT FFF technique and normofractionated lung DIBH VMAT can be applied with a low number of breath-holds in a short timeslot, with excellent dosimetric accuracy. In clinical routine, these approaches combine optimally reduced lung tissue irradiation with maximal delivery precision for patients with small and larger lung tumors.

Dosimetric evaluation of radiation dose rate effect in respiratory gated intensity modulated radiation therapy

Purpose:

To investigate the dosimetric accuracy of the sliding window gated IMRT compared with the static treatment, using varying dose rates.

Materials and methods:

This study measured changes in output and diode array response with changing dose rate, verified the precision of the motion table, and measured changes in dose distribution accuracy with film and diodes at two depths with changing dose rate. During 4DCT (4 Dimensional Computed Tomography), the patient’s respiratory signals and target motion were recorded and imported to the XY4D simulation table of SUN NUCLEAR Corporation to simulate the patient’s respiration and tumour motion. A single field of each sliding window IMRT plan with 30º wedge and one for lung cancer were used in this study. Three irradiating conditions, static and moving target with and without gating, were applied to both plans.

Results:

The standard deviations of output, with the dose rates changing from 300–600 MU/min, were 0.065 cGy and 0.169 cGy for the ionisation chamber and diode, respectively. The verification of the motion table shows very good precision with 9.98 ± 0.02 cm (true value = 10.0 cm). The measurements by MapCheck show the gamma index of the planned absolute dose distribution in static and moving targets with gating, resulting in more than 96% passing for all dose rates. The absolute dose distribution measured by film for the static target was agreeable with the value of moving target with gating.

Conclusion:

The sliding window gated IMRT technique is able to deliver an accurate dose to a moving target with the dose rate of 300–600 MU/min that is suitable for clinical treatment.

Define baseline levels of segments per beam for intensity-modulated radiation therapy delivery for brain, head and neck, thoracic, abdominal, and prostate applications

The purpose of this study was to evaluate the number of segments per beam for intensity-modulated radiation therapy (IMRT) treatments and its effects on the plan quality, treatment delivery time, machine quality assurance, and machine maintenance. We have retrospectively analyzed 24 patients treated with IMRT. Five were selected within each of the following regions: head and neck, thoracic, abdomen, and prostate. Four patients were optimized within the brain region. The clinically treated plans were re-optimized using Philips Pinnacle3 v. 8 with the direct machine parameter optimization algorithm. The number of segments per beam from the treated plan was systematically reduced by 80%, 60%, 40%, and 30%, and the following statistics have been analyzed for plan quality: target min, mean, and max doses; critical structure doses; and integral dose. We have attempted to define the smallest number of segments per beam for IMRT treatment plans. Results indicate that IMRT plans can be delivered with acceptable quality with approximately 3-6 segments per beam for the anatomical regions analyzed. A reduction in the number of segments decreases treatment delivery time, reduces machine wear and tear, and minimizes the amount of time the patient is on the treatment table, which in turn reduces the chances of intrafractional uncertainties.

The inter- and intrafraction reproducibilities of three common IMRT delivery techniques

PURPOSE

Intensity modulated radiation therapy (IMRT) treatment delivery requires higher precision than conventional 3D treatment delivery because of the sensitivity of the resulting dose to small geometric misalignment of the modulated beamlets. The chosen treatment delivery technique will affect the treatment precision in different ways, based on the characteristics of the delivery method. Delivery using a multileaf collimator (MLC) can reduce treatment time and therapist workload, but typically requires a greater number of monitor units and the fields are prone to both systematic and random leaf positioning errors. An alternative to MLC-based fields, patient specific brass compensators, do not suffer from these leaf positioning errors. In our study, we set out to investigate which delivery method will provide the highest levels of dosimetric reproducibility and the minimum amount of interfraction variability.

METHODS

In our study, a seven field IMRT plan for a head and neck treatment was created using the Pinnacle3 treatment planning system and the intensity maps for each field were obtained. The intensity maps of the fields were delivered with a Varian 2100C/D linear accelerator, using solid compensators and sliding window (SW) and step-and-shoot (SS) MLC segments. Three fields were selected from the seven-beam IMRT plan for comparison. Analysis was carried out using the MatriXX ion chamber array, radiochromic film, and Varian dynalog files.

RESULTS

Our results show that the error in MLC leaf positioning has no gantry angle dependence. The compensator and SW deliveries showed excellent agreement, even when stricter than usual gamma criteria were applied. However, we noted that under these strict conditions, the SS field had at least ten times more pixels out of range than did the compensators. When using step-and-shoot MLC fields, it was observed that the increase in dose rate or the increase of MU/segment degrades the quality of the plan. Analysis of the dynalog files showed that while each individual field had its own propensity for error, all fields showed the same trend: a greater percentage of time the leaves are out of position as dose rate increases, MUs decrease, or both.

CONCLUSIONS

The compensator-based field and both types of MLC-based fields have MatriXX results that are within the clinically acceptable tolerance of 3% dose difference and 2 mm DTA. However, when the criteria are tightened, it becomes evident that the compensators have a definite advantage over their comparable MLC-based competitors in terms of interfraction reproducibility. Fewer monitor units are required to deliver each portal, potentially improving patient outcomes and reducing unwanted side effects to both patients and therapists. In centers without MLC, compensators represent a simple and cost effective way to offer patients state of the art treatment. Based on the results of this study, compensator-based IMRT is a reliable, viable option for use in clinics both with and without MLC-equipped linacs.

Evaluation of delivered monitor unit accuracy of gated step-and-shoot IMRT using a two-dimensional detector array

PURPOSE

To overcome the problem of organ motion in intensity-modulated radiation therapy (IMRT), gated IMRT is often used for the treatment of lung cancer. In this study, the authors investigated the accuracy of the delivered monitor units (MUs) from each segment during gated IMRT using a two-dimensional detector array for user-specific verification purpose.

METHODS

The authors planned a 6 MV photon, seven-port step-and-shoot lung IMRT delivery. The respiration signals for gated IMRT delivery were obtained from the one-dimensional moving phantom using the real-time position management (RPM) system (Varian Medical Systems, Palo Alto, CA). The beams were delivered using a Clinac iX (Varian Medical Systems, Palo Alto, CA) with the Millennium 120 MLC. The MatriXX (IBA Dosimetry GmbH, Germany) was validated through consistency and reproducibility tests as well as comparison with measurements from a Farmer-type ion chamber. The authors delivered beams with varying dose rates and duty cycles and analyzed the MatriXX data to evaluate MU delivery accuracy.

RESULTS

There was quite good agreement between the planned segment MUs and the MUs computed from the MatriXX within +/- 2% error. The beam-on times computed from the MatriXX data were almost identical for all cases, and they matched well with the RPM beam-on and beam-off signals. A slight difference was observed between them, but it was less than 40 ms. The gated IMRT delivery demonstrated an MU delivery accuracy that was equivalent to ungated IMRT, and the delivered MUs with a gating signal agreed with the planned MUs within +/- 0.5 MU regardless of dose rate and duty cycle.

CONCLUSIONS

The authors can conclude that gated IMRT is able to deliver an accurate dose to a patient during a procedure. The authors believe that the methodology and results can be transferred to other vendors' devices, particularly those that do not provide MLC log data for a verification purpose.

Time delays in gated radiotherapy

In gated radiotherapy, the accuracy of treatment delivery is determined by the accuracy with which both the imaging and treatment beams are gated. Time delays are of four types: (1) beam on imaging time delay is the time between the target entering the gated region and the first gated image acquisition; (2) beam off imaging time delay is the time between the target exiting a gated region and the last image acquisition; (3) beam on treatment time delay is the time between the target entering the gated region and the treatment beam on; and (4) beam off treatment time delay is the time between the target exiting the gated region and treatment beam off. Asynchronous time delays for the imaging and treatment systems may increase the required internal target volume (ITV) margin. We measured time delay on three fluoroscopy systems, and three linear accelerator treatment beams, varying gating type (amplitude vs. phase), beam energy, dose rate, and period. The average beam on imaging time delays were −0.04±0.05sec (amplitude, 1 SD), −0.11±0.04sec (phase); while the average beam off imaging time delays were −0.18±0.08sec (amplitude) and −0.15±0.04sec (phase). The average beam on treatment time delays were +0.09±0.02sec (amplitude, 1 SD), +0.10±0.03sec (phase); while the average beam off time delays for treatment beams were +0.08±0.02sec (amplitude) and +0.07±0.02sec (phase). The negative value indicates the images were acquired early, and the positive values show the treatment beam was triggered late. We present a technique for calculating the margin necessary to account for time delays. We found that the difference between these imaging and treatment time delays required a significant increase in the ITV margin in the direction of tumor motion at the gated level.

PACS number: 87.53.Dq

Examination of geometric and dosimetric accuracies of gated step-and-shoot intensity modulated radiation therapy

Due to the complicated technical nature of gated radiation therapy, electronic and mechanical limitations may affect the precision of delivery. The purpose of this study is to investigate the geometric and dosimetric accuracies of gated step-and-shoot intensity modulated radiation treatments (SS-IMRT). Unique segmental MLC plans are designed, which allow quantitative testing of the gating process. Both ungated and gated deliveries are investigated for different dose sizes, dose rates, and gating window times using a commercial treatment system (Varian Trilogy) together with a respiratory gating system [Varian Real-Time Position Management system]. Radiographic film measurements are used to study the geometric accuracy, where it is found that with both ungated and gated SS-IMRT deliveries the MLC leaf divergence away from planned is less than or equal to the MLC specified leaf tolerance value for all leafs (leaf tolerance being settable from 0.5-5 mm). Nevertheless, due to the MLC controller design, failure to define a specific leaf tolerance value suitable to the SS-IMRT plan can lead to undesired geometric effects, such as leaf motion of up to the maximum 5 mm leaf tolerance value occurring after the beam is turned on. In this case, gating may be advantageous over the ungated case, as it allows more time for the MLC to reach the intended leaf configuration. The dosimetric precision of gated SS-IMRT is investigated using ionization chamber methods. Compared with the ungated case, it is found that gating generally leads to increased dosimetric errors due to the interruption of the "overshoot phenomena." With gating the average timing deviation for intermediate segments is found to be 27 ms, compared to 18 ms for the ungated case. For a plan delivered at 600 MU/min this would correspond to an average segment dose error of approximately 0.27 MU and approximately 0.18 MU for gated and ungated deliveries, respectively. The maximum dosimetric errors for individual intermediate segments are found to deviate by up to approximately 0.64 MU from their planned value when delivered at 600 MU/min using gating, this compares to only approximately 0.32 MU for the ungated case.