Intensity modulated radiation therapy with electrons using algorithm based energy/range selection methods

[Dose Distribution Combinations of Different Electron Beam Energy for Treatment Region Expansion in High-energy Electron Beam Radiation Therapy: A Feasibility Study]

INTRODUCTION

External electron beams have excellent distributions in treatment for superficial tumors while suppressing influence deeper normal tissue. However, the skin surface cannot be given a sufficient dose due to the build-up effect. In this study, we have investigated the combination of electron beams to expand the treatment region by keeping the dose gradient beyond d_max.

MATERIALS AND METHODS

The percentage depth doses of different electron beams were superimposed on a spreadsheet to determine the combinations of electron beams so that the treatment range was maximized. Based on the obtained weight for electron beams, dose distributions were calculated using a treatment planning system and examined for potential clinical application.

RESULTS

With the combination of 4 MeV and 9 MeV electron beams, the 90% treatment range in the depth direction increased by 8.0 mm, and with 4 MeV and 12 MeV beams, it increased by 4.0 mm, with the same maximum dose depth and halfdose depth of the absorbed dose. The dose calculations were performed using the treatment planning system yielded similar results with a matching degree of ±1.5%.

CONCLUSIONS

Although the influences of low monitor unit values and daily output differences remain to be considered, the results suggest that the proposed approach can be clinically applied to expand treatment regions easily.

Part 2: Dynamic mixed beam radiotherapy (DYMBER): Photon dynamic trajectories combined with modulated electron beams

PURPOSE

The purpose of this study was to develop a treatment technique for dynamic mixed beam radiotherapy (DYMBER) utilizing increased degrees of freedom (DoF) of a conventional treatment unit including different particle types (photons and electrons), intensity and energy modulation and dynamic gantry, table, and collimator rotations.

METHODS

A treatment planning process has been developed to create DYMBER plans combining photon dynamic trajectories (DTs) and step and shoot electron apertures collimated with the photon multileaf collimator (pMLC). A gantry-table path is determined for the photon DTs with minimized overlap of the organs at risk (OARs) with the target. In addition, an associated dynamic collimator rotation is established with minimized area between the pMLC leaves and the target contour. pMLC sequences of photon DTs and electron pMLC apertures are then simultaneously optimized using direct aperture optimization (DAO). Subsequently, the final dose distribution of the electron pMLC apertures is calculated using the Swiss Monte Carlo Plan (SMCP). The pMLC sequences of the photon DTs are then re-optimized with a finer control point resolution and with the final electron dose distribution taken into account. Afterwards, the final photon dose distribution is calculated also using the SMCP and summed together with the one of the electrons. This process is applied for a brain and two head and neck cases. The resulting DYMBER dose distributions are compared to those of dynamic trajectory radiotherapy (DTRT) plans consisting only of photon DTs and clinically applied VMAT plans. Furthermore, the deliverability of the DYMBER plans is verified in terms of dosimetric accuracy, delivery time and collision avoidance. For this purpose, The DYMBER plans are delivered to Gafchromic EBT3 films placed in an anthropomorphic head phantom on a Varian TrueBeam linear accelerator.

RESULTS

For each case, the dose homogeneity in the target is similar or better for DYMBER compared to DTRT and VMAT. Averaged over all three cases, the mean dose to the parallel OARs is 16% and 28% lower, D2% to the serial OARs is 17% and 37% lower and V10% to normal tissue is 12% and 4% lower for the DYMBER plans compared to the DTRT and VMAT plans, respectively. The DYMBER plans are delivered without collision and with a 4-5 min longer delivery time than the VMAT plans. The absolute dose measurements are compared to calculation by gamma analysis using 2% (global)/2 mm criteria with passing rates of at least 99%.

CONCLUSIONS

A treatment technique for DYMBER has been successfully developed and verified for its deliverability. The dosimetric superiority of DYMBER over DTRT and VMAT indicates utilizing increased DoF to be the key to improve brain and head and neck radiation treatments in future.

Evaluation of various boluses in dose distribution for electron therapy of the chest wall with an inward defect

Delivering radiotherapy to the postmastectomy chest wall can be achieved using matched electron fields. Surgical defects of the chest wall change the dose distribution of electrons. In this study, the improvement of dose homogeneity using simple, nonconformal techniques of thermoplastic bolus application on a defect is evaluated. The proposed phantom design improves the capability of film dosimetry for obtaining dose profiles of a patient's anatomical condition. A modeled electron field of a patient with a postmastectomy inward surgical defect was planned. High energy electrons were delivered to the phantom in various settings, including no bolus, a bolus that filled the inward defect (PB0), a uniform thickness bolus of 5 mm (PB1), and two 5 mm boluses (PB2). A reduction of mean doses at the base of the defect was observed by any bolus application. PB0 increased the dose at central parts of the defect, reduced hot areas at the base of steep edges, and reduced dose to the lung and heart. Thermoplastic boluses that compensate a defect (PB0) increased the homogeneity of dose in a fixed depth from the surface; adversely, PB2 increased the dose heterogeneity. This study shows that it is practical to investigate dose homogeneity profiles inside a target volume for various techniques of electron therapy.

Dosimetric verification of gated delivery of electron beams using a 2D ion chamber array

The purpose of this study was to compare the dosimetric characteristics; such as beam output, symmetry and flatness between gated and non-gated electron beams. Dosimetric verification of gated delivery was carried for all electron beams available on Varian CL 2100CD medical linear accelerator. Measurements were conducted for three dose rates (100 MU/min, 300 MU/min and 600 MU/min) and two respiratory motions (breathing period of 4s and 8s). Real-time position management (RPM) system was used for the gated deliveries. Flatness and symmetry values were measured using Imatrixx 2D ion chamber array device and the beam output was measured using plane parallel ion chamber. These detector systems were placed over QUASAR motion platform which was programmed to simulate the respiratory motion of target. The dosimetric characteristics of gated deliveries were compared with non-gated deliveries. The flatness and symmetry of all the evaluated electron energies did not differ by more than 0.7 % with respect to corresponding non-gated deliveries. The beam output variation of gated electron beam was less than 0.6 % for all electron energies except for 16 MeV (1.4 %). Based on the results of this study, it can be concluded that Varian CL2100 CD is well suitable for gated delivery of non-dynamic electron beams.

The use of intensity-modulated radiation therapy photon beams for improving the dose uniformity of electron beams shaped with MLC

Electrons are ideal for treating shallow tumors and sparing adjacent normal tissue. Conventionally, electron beams are collimated by cut-outs that are time-consuming to make and difficult to adapt to tumor shape throughout the course of treatment. We propose that electron cut-outs can be replaced using photon multileaf collimator (MLC). Two major problems of this approach are that the scattering of electrons causes penumbra widening because of a large air gap, and available commercial treatment planning systems (TPSs) do not support MLC-collimated electron beams. In this study, these difficulties were overcome by (1) modeling electron beams collimated by photon MLC for a commercial TPS, and (2) developing a technique to reduce electron beam penumbra by adding low-energy intensity-modulated radiation therapy (IMRT) photons (4 MV). We used blocks to simulate MLC shielding in the TPS. Inverse planning was used to optimize boost photon beams. This technique was applied to a parotid and a central nervous system (CNS) clinical case. Combined photon and electron plans were compared with conventional plans and verified using ion chamber, film, and a 2D diode array. Our studies showed that the beam penumbra for mixed beams with 90 cm source to surface distance (SSD) is comparable with electron applicators and cut-outs at 100 cm SSD. Our mixed-beam technique yielded more uniform dose to the planning target volume and lower doses to various organs at risk for both parotid and CNS clinical cases. The plans were verified with measurements, with more than 95% points passing the gamma criteria of 5% in dose difference and 5 mm for distance to agreement. In conclusion, the study has demonstrated the feasibility and potential advantage of using photon MLC to collimate electron beams with boost photon IMRT fields.

Comparison of modulated electron radiotherapy to conventional electron boost irradiation and volumetric modulated photon arc therapy for treatment of tumour bed boost in breast cancer

BACKGROUND AND PURPOSE

To compare few leaf electron collimator (FLEC)-based modulated electron radiotherapy (MERT) to conventional direct electron (DE) and volumetric modulated photon arc therapy (VMAT) for the treatment of tumour bed boost in breast cancer.

MATERIALS AND METHODS

Fourteen patients with breast cancer treated by lumpectomy and requiring post-operative whole breast radiotherapy with tumour bed boost were planned retrospectively using conventional DE, VMAT and FLEC-based MERT. The planning goal was to deliver 10Gy to at least 95% of the tumour bed volume. Dosimetry parameters for all techniques were compared.

RESULTS

Dose evaluation volume (DEV) coverage and homogeneity were best for MERT (D(98)=9.77Gy, D(2)=11.03Gy) followed by VMAT (D(98)=9.56Gy, D(2)=11.07Gy) and DE (D(98)=9.81Gy, D(2)=11.52Gy). Relative to the DE plans, the MERT plans predicted a reduction of 35% in mean breast dose (p<0.05), 54% in mean lung dose (p<0.05) and 46% in mean body dose (p<0.05). Relative to the VMAT plans, the MERT plans predicted a reduction of 24%, 36% and 39% in mean breast dose, heart dose and body dose, respectively (p<0.05).

CONCLUSIONS

MERT plans were a considerable improvement in dosimetry over DE boost plans. There was a dosimetric advantage in using MERT over VMAT for increased DEV conformity and low-dose sparing of healthy tissue including the integral dose; however, the cost is often an increase in the ipsilateral lung high-dose volume.

Comparison of electron IMRT to helical photon IMRT and conventional photon irradiation for treatment of breast and chest wall tumours

BACKGROUND AND PURPOSE

Conventional irradiation of breast and chest wall tumours may cause high doses in underlying organs. Intensity-modulated radiation therapy (IMRT) with photons achieves high conformity between treated and tumour volume but is associated with considerable low-dose effects which may induce secondary malignancies. We compare treatment plans of electron IMRT to helical photon IMRT and conventional irradiation.

MATERIAL AND METHODS

Treatment planning for three patients (breast, chest wall plus lymph nodes, sarcoma of medial chest wall/sternum) was performed using XiO 4.3.3 (CMS) for conventional photon irradiation, Hi-Art 2.2.2.05 (TomoTherapy) for helical photon IMRT, and a self-designed programme for electron IMRT.

RESULTS

The techniques resulted in similar mean and maximum target doses. Target coverage by the 95%-isodose was best with tomotherapy. Mean ipsilateral lung doses were similar with all techniques. Electron IMRT achieved best sparing of heart, and contralateral breast. Compared with photon IMRT, electron IMRT allowed better sparing of contralateral lung and total healthy tissue.

CONCLUSIONS

Electron IMRT is superior to conventional irradiation, as it allows satisfying target coverage and avoids high doses in underlying organs. Its advantage over photon IMRT is better sparing of most organs at risk (low-dose effects) which reduces the risk of radiation-induced malignancies.

Characterization of an add-on multileaf collimator for electron beam therapy

An add-on multileaf collimator for electrons (eMLC) has been developed that provides computer-controlled beam collimation and isocentric dose delivery. The design parameters result from the design study by Gauer et al (2006 Phys. Med. Biol. 51 5987-6003) and were configured such that a compact and light-weight eMLC with motorized leaves can be industrially manufactured and stably mounted on a conventional linear accelerator. In the present study, the efficiency of an initial computer-controlled prototype was examined according to the design goals and the performance of energy- and intensity-modulated treatment techniques. This study concentrates on the attachment and gantry stability as well as the dosimetric characteristics of central-axis and off-axis dose, field size dependence, collimator scatter, field abutment, radiation leakage and the setting of the accelerator jaws. To provide isocentric irradiation, the eMLC can be placed either 16 or 28 cm above the isocentre through interchangeable holders. The mechanical implementation of this feature results in a maximum field displacement of less than 0.6 mm at 90 degrees and 270 degrees gantry angles. Compared to a 10 x 10 cm applicator at 6-14 MeV, the beam penumbra of the eMLC at a 16 cm collimator-to-isocentre distance is 0.8-0.4 cm greater and the depth-dose curves show a larger build-up effect. Due to the loss in energy dependence of the therapeutic range and the much lower dose output at small beam sizes, a minimum beam size of 3 x 3 cm is necessary to avoid suboptimal dose delivery. Dose output and beam symmetry are not affected by collimator scatter when the central axis is blocked. As a consequence of the broader beam penumbra, uniform dose distributions were measured in the junction region of adjacent beams at perpendicular and oblique beam incidence. However, adjacent beams with a high difference in a beam energy of 6 to 14 MeV generate cold and hot spots of approximately 15% in the abutting region. In order to improve uniformity, the energy of adjacent beams must be limited to 6 to 10 MeV and 10 to 14 MeV respectively. At the maximum available beam energy of 14 MeV, radiation leakage results mainly from the intraleaf leakage of approximately 2.5% relative dose which could be effectively eliminated at off-axis distances remote from the field edge by adjusting the jaw field size to the respective opening of the eMLC. Additionally, the interleaf and leaf-end leakage could be reduced by using a tongue-and-groove leaf shape and adjoining the leaf-ends off-axis respectively.

Design of a computer-controlled multileaf collimator for advanced electron radiotherapy

A multileaf collimator for electrons (eMLC) has been designed that fulfils the technical requirements for providing advanced irradiation techniques with electrons. In the present work, the basic design parameters of leaf material, leaf height, leaf width and number of leaves as well as leaf overtravel and leaf shape were determined such that an eMLC with motorized leaves can be manufactured by a company specialized in MLC technology. For this purpose, a manually driven eMLC with variable source-to-collimator distance (SCD) was used to evaluate the chosen leaf specification and investigate the impact of the SCD on the off-axis dose distribution. In order to select the final SCD of the eMLC, a compromise had to be found between maximum field size, minimum beam penumbra and necessary distance between eMLC and isocentre to eliminate patient realignments during gantry rotation. As a result, the eMLC is placed according to the target position at 72 and 84 cm SCD, respectively. This feature will be achieved by interchangeable distance holders. At these SCDs, the corresponding maximum field sizes at 100 cm source-to-isocentre distance are 20 x 20 cm and 17 x 17 cm, respectively. Finally, the off-axis dose distribution at the maximum opening of the eMLC was improved by fine-tuning the settings of the accelerator jaws and introducing trimmer bars above the eMLC. Following this optimization, a prototype eMLC consisting of 2 x 24 computer-controlled brass leaves is manufactured by 3D Line Medical Systems.

Characterization of megavoltage electron beams delivered through a photon multi-leaf collimator (pMLC)

A study is presented that characterizes megavoltage electron beams delivered through an existing double-focused photon multi-leaf collimator (pMLC) using film measurements in a solid water phantom. Machine output stability and linearity were evaluated as well as the effect of source-to-surface distance (SSD) and field size on the penumbra for electron energies between 6 and 18 MeV over an SSD range of 60-100 cm. Penumbra variations as a function of field size, depth of measurement and the influence of the jaws were also studied. Field abutment, field flatness and target coverage for segmented beams were also addressed. The measured field size for electrons transported through the pMLC was the same as that for an x-ray beam up to SSDs of 70 cm. At larger SSD, the lower energy electron fields deviated from the projected field. Penumbra data indicated that 60 cm SSD was the most favourable treatment distance. Backprojection of P(20-80) penumbra data yielded a virtual source position located at 98.9 cm from the surface for 18 MeV electrons. For 6 MeV electrons, the virtual source position was at a distance of 82.6 cm. Penumbra values were smaller for small beam slits and reached a near-constant value for field widths larger than 5 cm. The influence of the jaws had a small effect on the penumbra. The R90 values ranged from 1.4 to 4.8 cm between 6 and 21 MeV as measured at 60 cm SSD for a 9 x 9 cm2 field. Uniformity and penumbra improvement could be demonstrated using weighted abutted fields especially useful for small segments. No detectable electron leakage through the pMLC was observed. Bremsstrahlung measurements taken at 60 cm SSD for a 9 x 9 cm2 field as shaped by the pMLC compared within 1% to bremsstrahlung measurements taken at 100 cm SSD for a 10 x 10 cm2 electron applicator field at 100 cm SSD.

Radiotherapy with laser-plasma accelerators: Monte Carlo simulation of dose deposited by an experimental quasimonoenergetic electron beam

The most recent experimental results obtained with laser-plasma accelerators are applied to radio-therapy simulations. The narrow electron beam, produced during the interaction of the laser with the gas jet, has a high charge (0.5 nC) and is quasimonoenergetic (170 +/- 20 MeV). The dose deposition is calculated in a water phantom placed at different distances from the diverging electron source. We show that, using magnetic fields to refocus the electron beam inside the water phantom, the transverse penumbra is improved. This electron beam is well suited for delivering a high dose peaked on the propagation axis, a sharp and narrow tranverse penumbra combined with a deep penetration.

Does electron and proton therapy reduce the risk of radiation induced cancer after spinal irradiation for childhood medulloblastoma? A comparative treatment planning study

The aim of this treatment planning comparison study was to explore different spinal irradiation techniques with respect to the risk of late side-effects, particularly radiation-induced cancer. The radiotherapy techniques compared were conventional photon therapy, intensity modulated x-ray therapy (IMXT), conventional electron therapy, intensity/energy modulated electron therapy (IMET) and proton therapy (IMPT).CT images for radiotherapy use from five children, median age 8 and diagnosed with medulloblastoma, were selected for this study. Target volumes and organs at risk were defined in 3-D. Treatment plans using conventional photon therapy, IMXT, conventional electron therapy, IMET and IMPT were set up. The probability of normal tissue complication (NTCP) and the risk of cancer induction were calculated using models with parameters-sets taken from published data for the general population; dose data were taken from dose volume histograms (DVH). Similar dose distributions in the targets were achieved with all techniques but the absorbed doses in the organs-at-risk varied significantly between the different techniques. The NTCP models based on available data predicted very low probabilities for side-effects in all cases. However, the effective mean doses outside the target volumes, and thus the predicted risk of cancer induction, varied significantly between the techniques. The highest lifetime risk of secondary cancers was estimated for IMXT (30%). The lowest risk was found with IMPT (4%). The risks associated with conventional photon therapy, electron therapy and IMET were 20%, 21% and 15%, respectively. This model study shows that spinal irradiation of young children with photon and electron techniques results in a substantial risk of radiation-induced secondary cancers. Multiple beam IMXT seems to be associated with a particularly high risk of secondary cancer induction. To minimise this risk, IMPT should be the treatment of choice. If proton therapy is not available, advanced electron therapy may provide a better alternative.

Photon and electron collimator effects on electron output and abutting segments in energy modulated electron therapy

In energy modulated electron therapy a large fraction of the segments will be arranged as abutting segments where inhomogeneities in segment matching regions must be kept as small as possible. Furthermore, the output variation between different segments should be minimized and must in all cases be well predicted. For electron therapy with add-on collimators, both the electron MLC (eMLC) and the photon MLC (xMLC) contribute to these effects when an xMLC tracking technique is utilized to reduce the x-ray induced leakage. Two add-on electron collimator geometries have been analyzed using Monte Carlo simulations: One isocentric eMLC geometry with an isocentric clearance of 35 cm and air or helium in the treatment head, and one conventional proximity geometry with a clearance of 5 cm and air in the treatment head. The electron fluence output for 22.5 MeV electrons is not significantly affected by the xMLC if the shielding margins are larger than 2-3 cm. For small field sizes and 9.6 MeV electrons, the isocentric design with helium in the treatment head or shielding margins larger than 3 cm is needed to avoid a reduced electron output. Dose inhomogeneity in the matching region of electron segments is, in general, small when collimator positions are adjusted to account for divergence in the field. The effect of xMLC tracking on the electron output can be made negligible while still obtaining a substantially reduced x-ray leakage contribution. Collimator scattering effects do not interfere significantly when abutting beam techniques are properly applied.

Effects on electron beam penumbra using the photon MLC to reduce bremsstrahlung leakage for an add-on electron MLC

Electron IMRT treatments have the potential to reduce the integral dose due to the limited range of the electrons. However, bremsstrahlung produced in the scattering foils could penetrate an added electron MLC (eMLC), thus producing an unmodulated dose contribution that could become unacceptable in electron IMRT treatments. To limit this bremsstrahlung contribution, the photon MLC (xMLC) was used to track the eMLC, but with a margin to avoid penumbra widening through partial screening of the effective electron source. The purpose of this work was to study the effect of the photon-electron MLC tracking on the electron beam penumbra for different treatment head designs. Both isocentric designs and designs where the eMLC is used close to the patient (proximity geometry) have been analysed using Monte Carlo simulations. At 22.5 MeV energy, a tracking margin of 1 cm was enough to avoid penumbra degradation for a helium-filled isocentric geometry, while air-filled geometries (including proximity geometries) require a 2-3 cm margin. Illustrated by an example of a chest wall treatment by electron IMRT, the use of 1 cm tracking margin will reduce the collimator leakage contribution by a factor of 36 as compared to using a static setting of the photon collimator.