Measuring dose in lung identifies peripheral tumour dose inaccuracy in SBRT audit

Improved commissioning of lung stereotactic body radiotherapy using a customized respiratory motion Phantom: a single- institutional study

Stereotactic body radiation therapy (SBRT) involves delivering high doses of radiation with geometric precision in a few hypofractionated schedules. In lung SBRT, respiratory motion is an additional concern as it could cause the delivered dose distribution to deviate from the treatment plan. Therefore, it is crucial to conduct accurate commissioning tests on a dynamic phantom. In this study, the QUASAR™ Respiratory Motion Phantom was customized using 3D-printed parts to minimize motion-induced errors in measurements. The customisations included a specialized ion chamber insert designed to move with the tumour and measure the average dose at its centre. A film insert was also developed for secure fixation, enabling precise dose verification on a static plane while minimizing the risk of friction-related damage. The quality assurance (QA) tests were performed on the plans created for phantom studies indicated that ion chamber measurements were within 1.9% of the planned dose, and film gamma analysis demonstrated pass rates over 95% using the 3%/1 mm criteria. A set of SBRT volumetric modulated arc therapy (VMAT) plans were created for a suite of test patients using both flattened and flattening filter free (FFF) 6 MV beams and utilising robust optimization. A standardized patient-specific QA protocol was used to evaluate the treatment plans of 20 test patients, yielding film gamma pass rates above 98.8%. The suggested approach, using the 3D-printed inserts, effectively mitigated dose-blurring, providing a robust tool for lung SBRT commissioning and ensuring the reliability of lung cancer treatment with SBRT.

Design and validation of a novel dosimetry phantom for motion management audits

BACKGROUND

We present a novel phantom design for conducting end-to-end dosimetry audits for respiratory motion management of two anatomical treatment sites. The design enables radiochromic film measurements of the dose administered to the target throughout the respiratory cycle (motion-included) and the dose delivered to the time-averaged motion of the phantom (motion-excluded) to be conducted simultaneously.

PURPOSE

To demonstrate the phantom's utility in a dosimetry audit and capacity to detect errors by quantifying spatial and dosimetric reproducibility.

METHODS

Spatial and dosimetric reproducibility was quantified by repeat exposures using a simple lateral beam. Five exposures per measurement configuration were used. In each series of five measurements, the median film was used as the series reference to quantify the reproducibility of the remaining "test films." Spatial reproducibility was quantified by comparing the position of isodose lines in two axes on the test films back to the series reference. Dosimetric reproducibility was quantified using gamma comparison between each test film and the series reference. Proof-of-concept of the motion-excluded measurement capability was also established by comparing all films to treatment planning system (TPS) calculated dose distributions.

RESULTS

Spatial reproducibility was better than 1 mm on all assessed metrics across all measurements. Film-to-film local gamma passing rates at 3%/0.6 mm were above 90% for all measurements. Film-to-TPS global gamma passing rates at 3%/1 mm were >95% in the motion-excluded measurement series', but <80% in the motion-included series, highlighting the utility of the motion-excluded measurements.

CONCLUSIONS

Measurements were highly reproducible and of sufficient accuracy/reproducibility to facilitate multiple avenues of analysis in a prospective dosimetry audit. Motion-excluded measurements were directly comparable to the TPS dose distribution. Motion-included measurements may yield more clinically-relevant information about the actual dose administered to the target. This promises greater sensitivity to motion management-related errors, detectable in the setting of a dosimetry audit for motion management.

Commissioning small fields in lung using Monte Carlo corrected film measurements

PURPOSE

Current commissioning of radiotherapy treatment planning systems with heterogeneous phantoms generally measures the dose delivered in solid water downstream of the lung material. The dose delivered directly in lung material is important to characterise the uncertainty of lung stereotactic body radiotherapy (SBRT) treatments, but film measurements require a correction to accurately measure dose to lung.

METHODS

Monte Carlo (MC) modelled corrections were applied to film measurements used for commissioning of lung SBRT. Medium dependent correction factors, kmed, were established using 6 and 10 MV simulations to account for film being calibrated in water but measuring in regions of lung material. The correction factors are dependent on energy, field size, and position. To avoid the onerous requirement of modelling each individual beam to correctly match penumbra an alternative approach is presented where the correction is applied as a function of isodose level for a nominal field size.

RESULTS

Improvement in central axis dose agreements was seen for all field sizes, with the largest improvement of 7 % observed for 6MV 1 × 1 cm2 field. Application of position dependent corrections improved the percentage of points passing a 5 %/1 mm or 3 %/1 mm gamma assessment in all cases, whilst a uniform central axis correction did not improve the passing rate in most cases.

CONCLUSIONS

MC simulations provide a method for correcting dose measured in lung materials allowing more accurate comparison with treatment planning system doses. In this work a generic approach to correct small field film lung measurements as a function of isodose levels is presented.

ACPSEM position paper: pre-treatment patient specific plan checks and quality assurance in radiation oncology

The Australasian College of Physical Scientists and Engineers in Medicine (ACPSEM) has not previously made recommendations outlining the requirements for physics plan checks in Australia and New Zealand. A recent workforce modelling exercise, undertaken by the ACPSEM, revealed that the workload of a clinical radiation oncology medical physicist can comprise of up to 50% patient specific quality assurance activities. Therefore, in 2022 the ACPSEM Radiation Oncology Specialty Group (ROSG) set up a working group to address this issue. This position paper authored by ROSG endorses the recommendations of the American Association of Physicists in Medicine (AAPM) Task Group 218, 219 and 275 reports with some contextualisation for the Australia and New Zealand settings. A few recommendations from other sources are also endorsed to complete the position.

Experimental validation of absorbed dose-to-medium calculation algorithms in heterogeneous media

Objective.The aim of this work was to determine heterogeneous correction factorshQclin,Qreffclin,frefdetm,wto validate absorbed dose-to-mediumDm,Qclinm,fclincalculation algorithms from detector readings. The impact of detector orientation perpendicular and parallel to the beam central axis on the correction factors was also investigated.Approach.ThehQclin,Qreffclin,frefdetm,wfactors were calculated for four types of detectors (PTW PinPoint T31016, PTW microDiamond T60019, PTW microSilicon T60023 and EBT3 film) placed in different media (cortical bone, lung, adipose tissue, Teflon and RW3) for the 6 MV energy beam with a 10 × 10 cm2field size. These corrections were then applied to the detector measurements performed at different depths in heterogeneous phantoms.Main results.ThehQclin,Qreffclin,frefdetm,wfactors mainly depended on the media and slightly on the type of detector. Considering all detectors, the largest corrections were found in high-density media with values ranging from 0.911 to 0.934 in cortical bone. For comparison, the corrections in other media were closer to unity with values from 0.966 (lung and RW3) to 0.991 (adipose tissue). Except for the PinPoint T31016, detector orientation-dependence was observed especially in high-density media. A good agreement (≤1.5%) was found betweenDm,Qclinm,fclincalculations and the detector readings corrected with thehQclin,Qreffclin,frefdetm,wfactor for all studied heterogeneous phantoms.Significance.This paper could serve as an initial guideline for medical physicists involved in the validation of the advanced type-b dose calculation algorithms reportingDm,Qclinm,fclin. To our knowledge, this is the first study to assess the impact of the orientation of different detectors in heterogeneous media. The orientation dependence of the detector response observed in water may not reflect what is observed in heterogeneous media, especially in high-density media. The knowledge of thehQclin,Qreffclin,frefdetm,wfactors becomes mandatory for accurate interpretation of detector readings and comparisons withDm,Qclinm,fclincalculations.

Early results of a remote dosimetry audit program for lung stereotactic body radiation therapy

Background and purpose

A dosimetry audit program based on alanine electron paramagnetic resonance (EPR) and radiochromic film dosimetry, may be a valuable tool for monitoring and improving the quality of lung stereotactic body radiotherapy (SBRT). The aim of this study was to report the initial, independent assessment of the dosimetric accuracy for lung SBRT practice using these dosimeters in combination with a novel phantom design.

Materials and Methods

The audit service was a remote audit program performed on a commercial lung phantom preloaded with film and alanine detectors. An alanine pellet was placed in the centre of the target simulated using silicone in a 3D-printed mould. Large film detectors were placed coronally through the target and the lung/tissue interface and analysed using gamma analysis. The beam output was always checked on the same day with alanine dosimetry in water. We audited 29 plans from 14 centres up to now.

Results

For the alanine results 28/29 plans were within 5 % with 19/29 plans being within 3 %. The passing rates were > 95 % for the film through the target for 27/29 plans and 17/29 plans for the film at the lung/tissue interface. For three plans the passing rate was < 90 % for the film on top of the lungs.

Conclusions

The preliminary results were very satisfactory for both detectors. The high passing rates for the film in the interface region indicate good performance of the treatment planning systems. The phantom design was robust and performed well on several treatment systems.