Characteristics of 2.5 MV beam and imaging dose to patients

The effects of different photon beam energies in stereotactic radiosurgery with cones

BACKGROUND

Stereotactic radiosurgery (SRS) relies on small fields to ablate lesions. Currently, linac based treatment is delivered via circular cones using a 6 MV beam. There is interest in both lower energy photon beams, which can offer steeper dose fall off as well as higher energy photon beams, which have higher dose rates, thus reducing radiation delivery times. Of interest in this study is the 2.5 MV beam developed for imaging applications and both the 6 and 10 MV flattening-filter-free (FFF) beams, which can achieve dose rates up to 2400 cGy/min.

PURPOSE

This study aims to assess the benefit and feasibility among different energy beams ranging from 2.5 to 10 MV beams by evaluating the dosimetric effects of each beam and comparing the dose to organs-at-risk (OARs) for two separate patient plans. One based on a typical real patient tremor utilizing a 4 mm cone and the other a typical brain metastasis delivered with a 10 mm cone.

METHODS

The Monte Carlo codes BEAMnrc/DOSXYZnrc were used to generate beams of 2.5 MV, 6 MV-FFF, 6 MV-SRS, 6 MV, 10 MV-FFF, and 10 MV from a Varian TrueBeam except 6 MV-SRS, which is taken from a Varian TX model linear accelerator. Each beam's energy spectrum, mean energy, %dd curve, and dose profile were obtained by analyzing the simulated beams. Calculated patient dose distributions were compared among six different energy beam configurations based on a realistic treatment plan for thalamotomy and a conventional brain metastasis plan. Dose to OARs were evaluated using dose-volume histograms for the same target dose coverage.

RESULTS

The mean energies of photons within the primary beam projected area were insensitive to cone sizes and the values of percentage depth-dose curves (%dd) at d = 5 cm and SSD = 95 cm for a 4 mm (10 mm) cone ranges from 62.6 (64.4) to 82.2 (85.7) for beam energy ranging from 2.5 to 10 MV beams, respectively. Doses to OARs were evaluated among these beams based on real treatment plans delivering 15 000 and 2200 cGy to the target with a 4 and 10 mm cone, respectively. The maximum doses to the brainstem, which is 10 mm away from the isocenter, was found to be 434 (300), 632 (352), 691 (362), 733 (375), 822 (403), and 975 (441) cGy for 2.5 MV, 6 MV-FFF, 6 MV-SRS, 6 MV, 10 MV-FFF, and 10 MV beams delivering 15 000 (2200) cGy target dose, respectively.

CONCLUSION

Using the 6 MV-SRS as reference, changes of the maximum dose (691 cGy) to the brain stem are -37%, -9%, +6%, +19%, and 41% for 2.5 MV, 6 MV-FFF, 6 MV, 10 MV-FFF, and 10 MV beams, respectively, based on the thalamotomy plan, where the "-" or "+" signs indicate the percentage decrease or increase. Changes of the maximum dose (362 cGy) to brain stem, based on the brain metastasis plan are much less for respective beam energies. The sum of 21 arcs beam-on time was 39 min on our 6 MV-SRS beam with 1000 cGy/min for thalamotomy. The beam-on time can be reduced to 16 min with 10 MV-FFF.

Stability and reproducibility comparisons between deep inspiration breath-hold techniques for left-sided breast cancer patients: A prospective study

PURPOSE

Deep inspiration breath-hold (DIBH) is crucial in reducing the lung and cardiac dose for treatment of left-sided breast cancer. We compared the stability and reproducibility of two DIBH techniques: Active Breathing Coordinator (ABC) and VisionRT (VRT).

MATERIALS AND METHODS

We examined intra- and inter-fraction positional variation of the left lung. Eight left-sided breast cancer patients were monitored with electronic portal imaging during breath-hold (BH) at every fraction. For each patient, half of the fractions were treated using ABC and the other half with VRT, with an equal amount starting with either ABC or VRT. The lung in each portal image was delineated, and the variation of its area was evaluated. Intrafraction stability was evaluated as the mean coefficient of variation (CV) of the lung area for the supraclavicular (SCV) and left lateral (LLat) field over the course of treatment. Reproducibility was the CV for the first image of each fraction. Daily session time and total imaging monitor units (MU) used in patient positioning were recorded.

RESULTS

The mean intrafraction stability across all patients for the LLat field was 1.3 ± 0.7% and 1.5 ± 0.9% for VRT and ABC, respectively. Similarly, this was 1.5 ± 0.7% and 1.6 ± 0.8% for VRT and ABC, respectively, for the SCV field. The mean interfraction reproducibility for the LLat field was 11.0 ± 3.4% and 14.9 ± 6.0% for VRT and ABC, respectively. Similarly, this was 13.0 ± 2.5% and 14.8 ± 9% for VRT and ABC, respectively, for the SCV. No difference was observed in the number of verification images required for either technique.

CONCLUSIONS

The stability and reproducibility were found to be comparable between ABC and VRT. ABC can have larger interfractional variation with less feedback to the treating therapist compared to VRT as shown in the increase in geometric misses at the matchline.

Treatment planning with a 2.5 MV photon beam for radiation therapy

PURPOSE

The shallow depth of maximum dose and higher dose fall-off gradient of a 2.5 MV beam along the central axis that is available for imaging on linear accelerators is investigated for treatment of shallow tumors and sparing the organs at risk (OARs) beyond it. In addition, the 2.5 MV beam has an energy bridging the gap between kilo-voltage (kV) and mega-voltage (MV) beams for applications of dose enhancement with high atomic number (Z) nanoparticles.

METHODS

We have commissioned and utilized a MATLAB-based, open-source treatment planning software (TPS), matRad, for intensity-modulated radiation therapy (IMRT) dose calculations. Treatment plans for prostate, liver, and head and neck (H&N), nasal cavity, two orbit cases, and glioblastoma multiforme (GBM) were performed and compared to a conventional 6 MV beam. Additional Monte Carlo calculations were also used for benchmarking the central axis dose.

RESULTS

Both beams had similar planning target volume (PTV) dose coverage for all cases. However, the 2.5 MV beam deposited 6%-19% less integral doses to the nasal cavity, orbit, and GBM cases than 6 MV photons. The mean dose to the heart in the liver plan was 10.5% lower for 2.5 MV beam. The difference between the doses to OARs of H&N for two beams was under 3%. Brain mean dose, brainstem, and optic chiasm max doses were, respectively, 7.5%-14.9%, 2.2%-8.1%, and 2.5%-19.0% lower for the 2.5 MV beam in the nasal cavity, orbit, and GBM plans.

CONCLUSIONS

This study demonstrates that the 2.5 MV beam can produce clinically relevant treatment plans, motivating future efforts for design of single-energy LINACs. Such a machine will be capable of producing beams at this energy beneficial for low- and middle-income countries, and investigations on dose enhancement from high-Z nanoparticles.

Region-of-interest intra-arc MV imaging to facilitate sub-mm positional accuracy with minimal imaging dose during treatment deliveries of small cranial lesions

Purpose

To automate the generation of region‐of‐interest (ROI) apertures for use with megavoltage imaging for online positional corrections during cranial stereotactic radiosurgery.

Materials and methods

Digitally reconstructed radiographs (DRRs) were created for a 3D‐printed skull phantom at 5 degree gantry angle increments for a three‐arc beam arrangement. At each angle, 3000 random rectangular apertures were generated, and 100 shifts on a grid were applied to the anatomy within the frame. For all shifts, the mutual information (MI) between the shifted and unshifted DRR was calculated to derive an average MI gradient. The top 10% of apertures that minimized registration errors were overlaid and discretely thresholded to generate imaging plans. Imaging was acquired with the skull while implementing simulated patient motion on a linac. Control point‐specific couch motions were derived to align the skull to its planned positioning.

Results

Apertures with a range of repositioning errors less than 0.1 mm possessed a 42% larger average MI gradient when compared with apertures with a range greater than 1 mm. Dose calculations with Monte Carlo exhibited an 84% reduction in the dose received by 50% of the skull with the 50% thresholded plan when compared to a constant 22 × 22 cm² imaging plan. For all different imaging plans (with and without motion), the calculated median 3D‐errors with respect to the tracking of a metal‐BB fiducial positioned at isocenter in the skull were sub‐mm except for the 80% thresholded plan.

Conclusions

Sub‐mm positional errors are achievable with couch motions derived from control point–specific ROI imaging. Smaller apertures that conform to an anatomical ROI can be utilized to minimize the imaging dose incurred at the expense of larger errors.

Doses delivered by portal imaging quality assurance in routine practice of adjuvant breast radiotherapy worth to by monitored and compensated in some cases

Background

Imaging, in radiotherapy, has become a routine tool for repositioning of the target volume at each session. The repositioning precision, currently infracentimetric, evolves along with the irradiation techniques. This retrospective study aimed to identify practices and doses resulting from the use of high energy planar imaging (portal imaging) in daily practice.

Methods

A retrospective survey of portal images (PIs) was carried out over 10 years for 2,403 patients and for three linacs (1 Elekta SLi, 2 Varian Clinac) for postoperative mammary irradiations. Images were taken using a standardized number of monitor units (MU) for all patients. Due to the variable sensitivities of the detectors and the possibility of adjustment of the detector-patient distance, the number of MU were 3; 2 and 1 respectively, for Elekta SLi^®, Clinac 600^® and Clinac 2100^®. Then, a representative cumulated dose was calculated in simplified reference conditions (5 cm depth, beam of 10 cm × 10 cm, 6 MV), considering the total number of images taken during the whole treatment course. The consistency between the representative doses and the actual absorbed doses received by the patients was verified by simulating a series of typical cases with the treatment plan dose calculation system.

Results

The delivered doses differ significantly between the three linacs. The mean representative dose values by complete treatment were 0.695; 0.241 and 0.216 Gy, respectively, for SLi, Clinac 600 and Clinac 2100. However, 15 patients were exposed to a dose >2 Gy with a maximum dose of 5.05 Gy. The simulated doses were very similar to the representative doses.

Conclusions

A significant dose delivery was highlighted by this study. These representative doses are presently communicated weekly to the radiation oncologist for the radiation protection of their patients. Moreover, they should be taken into account in a possible study of long-term stochastic risks.

Investigation of planar image quality for a novel 2.5 MV diamond target beam from a radiotherapy linear accelerator

Background and purpose

A commercial 2.5 MV beam has been clinically available for beam’s-eye-view imaging in radiotherapy, offering improved contrast-to-noise ratio (CNR) compared to therapeutic beams, due to the softer spectrum. Previous research suggested that imaging performance could be improved using a low-Z diamond target to reduce the self-absorption of diagnostic energy photons. The aim of this study was to 1) investigate the feasibility of two 2.5 MV diamond target beamline configurations and 2) characterize the dosimetry and planar image quality of these novel low-Z beams.

Materials and methods

The commercial 2.5 MV beam was modified by replacing the copper target with sintered diamond. Two beamlines were investigated: a carousel-mounted diamond target beamline and a ‘conventional’ beamline, with the diamond target in the target arm. Planar image quality was assessed in terms of spatial resolution and CNR.

Results

Due to image artifacts, image quality could not be assessed for the carousel-mounted low-Z target beam. The ‘conventional’ 2.5 MV low-Z beam quality was softer by 2.7% compared to the commercial imaging beam, resulting in improved CNR by factors of up to 1.3 and 1.7 in thin and thick phantoms, respectively. In regard to spatial resolution, the ‘conventional’ 2.5 MV low-Z beam slightly outperformed the commercial imaging beam.

Conclusion

With a simple modification to the 2.5 MV commercial beamline, we produced an improved energy spectrum for imaging. This 2.5 MV diamond target beam proved to be an advantageous alternative to the commercial target configuration, offering both superior resolution and CNR.

Image guidance doses delivered during radiotherapy: Quantification, management, and reduction: Report of the AAPM Therapy Physics Committee Task Group 180

BACKGROUND

With radiotherapy having entered the era of image guidance, or image-guided radiation therapy (IGRT), imaging procedures are routinely performed for patient positioning and target localization. The imaging dose delivered may result in excessive dose to sensitive organs and potentially increase the chance of secondary cancers and, therefore, needs to be managed.

AIMS

This task group was charged with: a) providing an overview on imaging dose, including megavoltage electronic portal imaging (MV EPI), kilovoltage digital radiography (kV DR), Tomotherapy MV-CT, megavoltage cone-beam CT (MV-CBCT) and kilovoltage cone-beam CT (kV-CBCT), and b) providing general guidelines for commissioning dose calculation methods and managing imaging dose to patients.

MATERIALS & METHODS

We briefly review the dose to radiotherapy (RT) patients resulting from different image guidance procedures and list typical organ doses resulting from MV and kV image acquisition procedures.

RESULTS

We provide recommendations for managing the imaging dose, including different methods for its calculation, and techniques for reducing it. The recommended threshold beyond which imaging dose should be considered in the treatment planning process is 5% of the therapeutic target dose.

DISCUSSION

Although the imaging dose resulting from current kV acquisition procedures is generally below this threshold, the ALARA principle should always be applied in practice. Medical physicists should make radiation oncologists aware of the imaging doses delivered to patients under their care.

CONCLUSION

Balancing ALARA with the requirement for effective target localization requires that imaging dose be managed based on the consideration of weighing risks and benefits to the patient.