Investigating ion recombination effects in a liquid-filled ionization chamber array used for IMRT QA measurements

MR-OCTAVIUS 4D with 1500 MR and 1600 MR arrays is suitable for plan QA in a 1.5 T MRI-linac

To ensure the accuracy of radiation delivery to patients in a 1.5 T MRI-linac, the implementation of quality assurance (QA) devices compatible with MR technology is essential. The OCTAVIUS 4D MR, made by PTW (Freiburg, Germany) is designed to ensure consistent and ideal alignment of its detectors with the direction of each beam segment. This study focuses on investigating the fundamental characteristics of the detector response for the OCTAVIUS Detector (OD) 1500 MR and OCTAVIUS 1600 MR when used in the MR-compatible OCTAVIUS 4D. Characteristics examined included short-term reproducibility, dose linearity, field size dependency, monitor unit (MU) rate dependency, dose-per-pulse dependency, and angular dependency. The evaluation of OD 1500 MR also involved measuring 25 clinical treatment plans across diverse target sizes and anatomical sites, including the liver/pancreas, rectum, prostate, lungs, and lymph nodes. One plan was measured with the standard setup and with a 5 cm left offset. The OD 1600 MR was not available for these measurements. The capability of the OD 1500 MR to identify potential errors was assessed by introducing a MU and positional shift within the software. The results demonstrated no significant differences in short-term reproducibility (<0.2%), dose linearity (<1%), field size dependency (<0.7%for field sizes larger than 5 cm × 5 cm), MU rate dependency (<0.8%), dose-per-pulse dependency (<0.4%) and angular dependency (standard deviation<0.5%). All tests of clinical plans were successfully completed. The OD 1500 MR demonstrated compatibility with the standard 95% pass rate when employing a global 3%/3 mm gamma criterion, and a 90% pass rate using a global 2%/2 mm gamma criterion. The detector demonstrated the capacity to measure treatment plans with a 5 cm left offset. With the standard parameters, the gamma test was sensitive to position errors but required an addition tests of mean/median dose or point dose in order to detect small dose difference.

Operation and calibration of the novel PTW 1600SRS detector for the verification of single isocenter stereotactic radiosurgery treatments of multiple small brain metastases

Objectives:

The aim of this work was to evaluate the operation of the 1600SRS detector and to develop a calibration procedure for verifying the dose delivered by a single isocenter stereotactic radiosurgery (SRS) treatment of small multiple brain metastases (BM).

Methods:

14 clinical treatment cases were selected with the number of BM ranging from 2 to 11. The dosimetric agreement was investigated between the calculated and the measured dose by an OCTAVIUS 1600SRS array detector in an OCTAVIUS 4D phantom equipped with dedicated SRS top. The cross-calibration procedure deviated from the manufacturer’s as it applied field sizes and dose rates corresponding to the volumetric modulated arc therapy segments in each plan.

Results:

Measurements with a plan specific cross-calibration showed mean ± standard deviation (SD) agreement scores for cut-off values 50%, 80%, 95%, of 98.6 ± 1.7%, 96.5 ± 4.6%, 97.3 ± 4.4% for the 6 MV plans respectively, and 98.6 ± 1.5%, 96.6 ± 4.0% 96.4 ± 6.3%, for the 6 MV flattening filter free (FFF) plans respectively. Using the default calibration procedure instead of the plan specific calibration could lead to a combined systematic dose offset of 4.1% for our treatment plans.

Conclusion:

The 1600SRS detector array with the 4D phantom offers an accurate solution to perform routine quality assurance measurements of single isocenter SRS treatments of multiple BM. This work points out the necessity of an adapted cross-calibration procedure.

Advances in knowledge:

A dedicated calibration procedure enables accurate dosimetry with the 1600SRS detector for small field single isocenter SRS treatment of multiple brain metastases for a large amount of BM.

Pretreatment Dose Verification in Volumetric Modulated Arc Therapy Using Liquid Ionization Chamber

Purpose:

The purpose of the present study was to evaluate the practicability of liquid ionization chamber (LIC) for pretreatment dose verification of the advanced radiotherapy techniques such as volumetric modulated arc therapy (VMAT).

Materials and Methods:

The dosimetric characteristics of LIC such as repeatability, sensitivity, monitor unit linearity, dose rate dependence, angular dependence, voltage-current response, and output factors were investigated in 6 MV therapeutic X-ray beams. The LIC was cross-calibrated against 0.125-cc air-filled thimble ionization chamber. A dedicated dosimetry insert made up of Perspex to incorporate the LIC at proper location in the intensity-modulated radiation therapy thorax phantom was locally fabricated. The collection efficiency and ion recombination correction factor was determined using the two-dose rate method. Pretreatment dose verification measurement of VMAT treatment plans were carried out using the liquid ionization chamber as well as small volume (0.125 cc) air-filled thimble ionization chamber. The measured dose values by the two dosimeters and TPS calculated dose at a given point were compared.

Results:

The relative percentage differences between the TPS calculated and measured doses were within ± 1.57% for LIC and ± 2.21% for 0.125 cc ionization chamber, respectively.

Conclusions:

The measured dose values by the two dosimeters and TPS calculated dose at a given point were found comparable suggesting that the LIC could be a good choice of dosimeter for pretreatment dose verification in VMAT.

Development and clinical characterization of a novel 2041 liquid-filled ionization chambers array for high-resolution verification of radiotherapy treatments

PURPOSE

The aim of this study was to present a novel 2041 liquid-filled ionization chamber array for high-resolution verification of radiotherapy treatments.

MATERIALS AND METHODS

The prototype has 2041 ionization chambers of 2.5 × 2.5 mm² area filled with isooctane. The detection elements are arranged in a central square grid of 43 × 43, totally covering an area of 107.5 × 107.5 mm² . The central inline and cross-line are extended to 227 mm and the diagonals to 321 mm to be able to perform profile measurements of large fields. We have studied stability, pixel response uniformity, dose rate dependence, depth and field size dependence and anisotropy. We present results for output factors, tongue-and-groove, garden fence, small field profiles, irregular fields, and verification of dose planes of patient treatments.

RESULTS

Comparison with other detectors used for small field dosimetry (SFD, CC13, microDiamond) has shown good agreement. Output factors measured with the device for square fields ranging from 10 × 10 to 100 × 100 mm² showed relative differences within 1%. The response of the detector shows a strong dependence on the angle of incident radiation that needs to be corrected for. On the other hand, inter-pixel relative response variations in the 0.95-1.08 range have been found and corrected for. The application of the device for the verification of dose planes of several treatments has shown gamma passing rates above 97% for tolerances of 2% and 2 mm. The verification of other clinical fields, like small fields and irregular fields used in the commissioning of the TPS, also showed large passing rates. The verification of garden fence and tongue-and-groove fields was affected by volume-averaging effects.

CONCLUSIONS

The results show that the liquid filled ionization chamber prototype here presented is appropriate for the verification of radiotherapy treatments with high spatial resolution. Recombination effects do not affect very much the verification of relative dose distributions. However, verification of absolute dose distributions may require normalization to a radiation field which is representative of the dose rate of the treatment delivered.