Feasibility of a simple method of hybrid collimation for megavoltage grid therapy

Dosimetrical and geometrical parameters in single-fraction lattice radiotherapy for the treatment of bulky tumors: Insights from initial clinical experience

PURPOSE

This study aims to investigate lattice radiotherapy (LRT) for bulky tumor in 10 patients, analyzing geometrical and dosimetrical parameters and correlations among variables.

METHODS

Patients were prescribed a single-fraction of 18 Gy to 50 % of each spherical vertex (1.5 cm diameter). Vertices were arranged in equidistant planes forming a triangular pattern. Center-to-center distance (Dc-c) between vertices was varied from 4 to 5 cm. A new method for calculating the valley-to-peak dose ratio (VPDR) was proposed and compared to other two from existing literature. GTV volumes (VGTV), vertex number (Nvert), low-dose related parameters and vertex D99%, D50%, and D1% were recorded. Beam-on time and Monitor Units (MU) were also evaluated. Correlations were assessed using Spearman's coefficient, with significant differences analyzed using Mann-Whitney U test.

RESULTS

Tumor volumes ranged from 417 to 3615 cm3. Median vertex number was 14.5 (IQR:11.3-17.8). VPDR ranged from 0.16 to 0.28. Median D99% spanned from 10.0 to 13.7 Gy, median D50% exceeded 18.0 Gy, and median D1% surpassed 23.3 Gy. Periphery dose remained under 4.0 Gy. Plans exhibited high modulation, with median beam-on time and MU of 8.8 min (IQR:8.2-10.1) and 13,069 MU (IQR:11574-13639). Significant correlations were found between Nvert and VGTV (p < 0.01), MU (p < 0.02) and beam-on time (p < 0.01) and between Dc-c and two VPDR definitions (p < 0.02) and periphery dose (p < 0.01). Significant differences were observed among the three valley dose definitions (p < 0.01) and the three peak dose definitions (p < 0.01).

CONCLUSIONS

Reporting geometrical and dosimetrical parameters in LRT is crucial, alongside the need for unified definitions of valley and peak doses.

Spatially fractionated radiation therapy: a critical review on current status of clinical and preclinical studies and knowledge gaps

Spatially fractionated radiation therapy (SFRT) is a therapeutic approach with the potential to disrupt the classical paradigms of conventional radiation therapy. The high spatial dose modulation in SFRT activates distinct radiobiological mechanisms which lead to a remarkable increase in normal tissue tolerances. Several decades of clinical use and numerous preclinical experiments suggest that SFRT has the potential to increase the therapeutic index, especially in bulky and radioresistant tumors. To unleash the full potential of SFRT a deeper understanding of the underlying biology and its relationship with the complex dosimetry of SFRT is needed. This review provides a critical analysis of the field, discussing not only the main clinical and preclinical findings but also analyzing the main knowledge gaps in a holistic way.

Assessing dosimetric advancements in spatially fractionated radiotherapy: From grids to lattices

Spatially fractionated radiotherapy (SFRT) techniques have undergone transformative evolution, encompassing physical GRID therapy, MLC-based grids, virtual TOMO GRIDs, and 3-dimensional high-dose lattices. Historical roots trace back to Alban Köhler's pioneering Spatially fractionated grid therapy (SFGRT), utilizing physical grids for dose modulation. Technological innovations introduced multi-leaf collimators (MLCs), enabling adaptable spatial fractionation and a shift to the broader term "SFRT." Physics and dosimetry-based studies have demonstrated the feasibility of computerized treatment planning and identified the potential to minimize the peripheral dose while using such high-dose therapy. Meanwhile, 3-dimensional high-dose lattices showed enhanced precision. The meticulous placement of high-dose volumetric spheres enables a reduction in the volume of high-dose spills. Advancements in 3-dimensional lattices through intensity-modulated radiotherapy and volumetric modulated arc therapy (VMAT) techniques offer enhanced therapeutic options. A database of SFRT studies identified 723 articles. This review shows the trajectory of SFRT from traditional grids to MLC-based approaches, virtual TOMO GRIDs, and innovative 3-dimensional lattices. Technological innovations, dosimetric advancements, and clinical feasibility have underscored the continual progress in refining spatially fractionated radiotherapy. The integration of MLCs and lattice techniques has demonstrated improved therapeutic outcomes, solidifying their relevance in modern radiation therapy protocols. Research has yet to reveal a clear correlation between treatment outcomes and dosimetric parameters. Additional investigations are necessary to assess the impact of various dosimetric parameters, such as EUD, peak-to-valley ratio (PVDR), D5%, D10%, D20%, D90%, etc., on the effectiveness of treatments.

Conebeam CT-guided 3D MLC-based spatially fractionated radiation therapy for bulky masses

For fast, safe, and effective management of large and bulky (≥8 cm) non‐resectable tumors, we have developed a conebeam CT‐guided three‐dimensional (3D)‐conformal MLC‐based spatially fractionated radiation therapy (SFRT) treatment. Using an in‐house MLC‐fitting algorithm, Millennium 120 leaves were fitted to the gross tumor volume (GTV) generating 1‐cm diameter holes at 2‐cm center‐to‐center distance at isocenter. SFRT plans of 15 Gy were generated using four to six coplanar crossfire gantry angles 60° apart with a 90° collimator, differentially weighted with 6‐ or 10‐MV beams. A dose was calculated using AcurosXB algorithm, generating sieve‐like dose channels without post‐processing the physician‐drawn GTV contour within an hour of CT simulation allowing for the same day treatment. In total, 50 extracranial patients have been planned and treated using this method, comprising multiple treatment sites. This novel MLC‐fitting algorithm provided excellent dose parameters with mean GTV (V7.5 Gy) and mean GTV doses of 53.2% and 7.9 Gy, respectively, for 15 Gy plans. Average peak‐to‐valley dose ratio was 3.2. Mean beam‐on time was 3.32 min, and treatment time, including patient setup and CBCT to beam‐off, was within 15 min. Average 3D couch correction from original skin‐markers was <1.0 cm. 3D MLC‐based SFRT plans enhanced target dose for bulky masses, including deep‐seated large tumors while protecting skin and adjacent critical organs. Additionally, it provides the same day, safe, effective, and convenient treatment by eliminating the risk to therapists and patients from heavy gantry‐mounted physical GRID‐block—we recommend other centers to use this simple and clinically useful method. This rapid SFRT planning technique is easily adoptable in any radiation oncology clinic by eliminating the need for plan optimization and patient‐specific quality assurance times while providing dosimetry information in the treatment planning system. This potentially allows for dose‐escalation to deep‐seated masses to debulk unresectable large tumors providing an option for neoadjuvant treatment. An outcome study of clinical trial is underway.

Radiobiological and Treatment-Related Aspects of Spatially Fractionated Radiotherapy

The continuously evolving field of radiotherapy aims to devise and implement techniques that allow for greater tumour control and better sparing of critical organs. Investigations into the complexity of tumour radiobiology confirmed the high heterogeneity of tumours as being responsible for the often poor treatment outcome. Hypoxic subvolumes, a subpopulation of cancer stem cells, as well as the inherent or acquired radioresistance define tumour aggressiveness and metastatic potential, which remain a therapeutic challenge. Non-conventional irradiation techniques, such as spatially fractionated radiotherapy, have been developed to tackle some of these challenges and to offer a high therapeutic index when treating radioresistant tumours. The goal of this article was to highlight the current knowledge on the molecular and radiobiological mechanisms behind spatially fractionated radiotherapy and to present the up-to-date preclinical and clinical evidence towards the therapeutic potential of this technique involving both photon and proton beams.

The role of the spatially fractionated radiation therapy in the management of advanced bulky tumors

Spatially fractionated radiation therapy (SFRT) refers to the delivery of a single large dose of radiation within the target volume in a heterogeneous pattern using either a custom GRID block, multileaf collimators, and virtual methods such as helical tomotherapy or synchrotron-based microbeams. The potential impact of this technique on the regression of bulky deep-seated tumors that do not respond well to conventional radiotherapy has been remarkable. To date, a large number of patients have been treated using the SFRT techniques. However, there are yet many technical and medical challenges that have limited their routine use to a handful of clinics, most commonly for palliative intent. There is also a poor understanding of the biological mechanisms underlying the clinical efficacy of this approach. In this article, the methods of SFRT delivery together with its potential biological mechanisms are presented. Furthermore, technical challenges and clinical achievements along with the radiobiological models used to evaluate the efficacy and safety of SFRT are highlighted.

A novel, yet simple MLC-based 3D-crossfire technique for spatially fractionated GRID therapy treatment of deep-seated bulky tumors

Purpose

Treating deep‐seated bulky tumors with traditional single‐field Cerrobend GRID‐blocks has many limitations such as suboptimal target coverage and excessive skin toxicity. Heavy traditional GRID‐blocks are a concern for patient safety at various gantry‐angles and dosimetric detail is not always available without a GRID template in user’s treatment planning system. Herein, we propose a simple, yet clinically useful multileaf collimator (MLC)‐based three‐dimensional (3D)‐crossfire technique to provide sufficient target coverage, reduce skin dose, and potentially escalate tumor dose to deep‐seated bulky tumors.

Materials/methods

Thirteen patients (multiple sites) who underwent conventional single‐field cerrobend GRID‐block therapy (maximum, 15 Gy in 1 fraction) were re‐planned using an MLC‐based 3D‐crossfire method. Gross tumor volume (GTV) was used to generate a lattice pattern of 10 mm diameter and 20 mm center‐to‐center mimicking conventional GRID‐block using an in‐house MATLAB program. For the same prescription, MLC‐based 3D‐crossfire grid plans were generated using 6‐gantry positions (clockwise) at 60° spacing (210°, 270°, 330°, 30°, 90°, 150°, therefore, each gantry angle associated with a complement angle at 180° apart) with differentially‐weighted 6 or 18 MV beams in Eclipse. For each gantry, standard Millenium120 (Varian) 5 mm MLC leaves were fit to the grid‐pattern with 90° collimator rotation, so that the tunneling dose distribution was achieved. Acuros‐based dose was calculated for heterogeneity corrections. Dosimetric parameters evaluated include: mean GTV dose, GTV dose heterogeneities (peak‐to‐valley dose ratio, PVDR), skin dose and dose to other adjacent critical structures. Additionally, planning time and delivery efficiency was recorded. With 3D‐MLC, dose escalation up to 23 Gy was simulated for all patient's plans.

Results

All 3D‐MLC crossfire GRID plans exhibited excellent target coverage with mean GTV dose of 13.4 ± 0.5 Gy (range: 12.43–14.24 Gy) and mean PVDR of 2.0 ± 0.3 (range: 1.7–2.4). Maximal and dose to 5 cc of skin were 9.7 ± 2.7 Gy (range: 5.4–14.0 Gy) and 6.3 ± 1.8 Gy (range: 4.1–11.1 Gy), on average respectively. Three‐dimensional‐MLC treatment planning time was about an hour or less. Compared to traditional GRID‐block, average beam on time was 20% less, while providing similar overall treatment time. With 3D‐MLC plans, tumor dose can be escalated up to 23 Gy while respecting skin dose tolerances.

Conclusion

The simple MLC‐based 3D‐crossfire GRID‐therapy technique resulted in enhanced target coverage for de‐bulking deep‐seated bulky tumors, reduced skin toxicity and spare adjacent critical structures. This simple MLC‐based approach can be easily adopted by any radiotherapy center. It provides detailed dosimetry and a safe and effective treatment by eliminating the heavy physical GRID‐block and could potentially provide same day treatment. Prospective clinical trial with higher tumor‐dose to bulky deep‐seated tumors is anticipated.

A Current Review of Spatial Fractionation: Back to the Future?

Spatially fractionated radiation therapy represents a significant departure from canonical thinking in radiation oncology despite having origins in the early 1900s. The original and most common implementation of spatially fractionated radiation therapy uses commercially available blocks or multileaf collimators to deliver a nonconfluent, sieve-like pattern of radiation to the target volume in a nonuniform dose distribution. Dosimetrically, this is parameterized by the ratio of the valley dose in cold spots to the peak dose in hot spots, or the valley-to-peak dose ratio. The radiobiologic mechanisms are postulated to involve radiation-induced bystander effects, microvascular alterations, and/or immunomodulation. Current indications include bulky or locally advanced disease that would not be amenable to conventional radiation or that has proved refractory to chemoradiation. Early-phase clinical trials have shown remarkable success, with some response rates >90% and minimal toxicity. This has promoted technological developments in 3-dimensional formats (LATTICE), micron-size beams (microbeam), and proton arrays. Nevertheless, more clinical and biological data are needed to specify ideal dosimetry parameters and to formulate robust clinical indications and guidelines for optimal standardized care.

Dose evaluation of Grid Therapy using a 6 MV flattening filter-free (FFF) photon beam: A Monte Carlo study

PURPOSE

Spatially fractionated radiotherapy is a strategy to overcome the main limitation of radiotherapy, i.e., the restrained normal tissue tolerances. A well-known example is Grid Therapy, which is currently performed at some hospitals using megavoltage photon beams delivered by Linacs. Grid Therapy has been successfully used in the management of bulky abdominal tumors with low toxicity. The aim of this work was to evaluate whether an improvement in therapeutic index in Grid Therapy can be obtained by implementing it in a flattening filter-free (FFF) Linac. The rationale behind is that the removal of the flattening filter shifts the beam energy spectrum towards lower energies and increase the photon fluence. Lower energies result in a reduction of lateral scattering and thus, to higher peak-to-valley dose ratios (PVDR) in normal tissues. In addition, the gain in fluence might allow using smaller beams leading a more efficient exploitation of dose-volume effects, and consequently, a better normal tissue sparing.

METHODS

Monte Carlo simulations were used to evaluate realistic dose distributions considering a 6 MV FFF photon beam from a standard medical Linac and a cerrobend mechanical collimator in different configurations: grid sizes of 0.3 × 0.3 cm² , 0.5 × 0.5 cm² , and 1 × 1 cm² and a corresponding center-to-center (ctc) distance of 0.6, 1, and 2 cm, respectively (total field size of 10 × 10 cm² ). As figure of merit, peak doses in depth, PVDR, output factors (OF), and penumbra values were assessed.

RESULTS

Dose at the entrance is slightly higher than in conventional Grid Therapy. However, it is compensated by the large PVDR obtained at the entrance, reaching a maximum of 35 for a grid size of 1 × 1 cm² . Indeed, this grid size leads to very high PVDR values at all depths (≥ 10), which are much higher than in standard Grid Therapy. This may be beneficial for normal tissues but detrimental for tumor control, where a lower PVDR might be requested. In that case, higher valley doses in the tumor could be achieved by using an interlaced approach and/or adapting the ctc distance. The smallest grid size (0.3 × 0.3 cm² ) leads to low PVDR at all depths, comparable to standard Grid Therapy. However, the use of very thin beams might increase the normal tissue tolerances with respect to the grid size commonly used (1 × 1 cm² ). The gain in fluence provided by FFF implies that the important OF reduction (0.6) will not increase treatment time. Finally, the intermediate configuration (0.5 × 0.5 cm² ) provides high PVDR in the first 5 cm, and comparable PVDR to previous Grid Therapy works at depth. Therefore, this configuration might allow increasing the normal tissue tolerances with respect to Grid Therapy thanks to the higher PVDR and thinner beams, while a similar tumor control could be expected.

CONCLUSIONS

The implementation of Grid Therapy in an FFF photon beam from medical Linac might lead to an improvement of the therapeutic index. Among the cases evaluated, a grid size of 0.5 × 0.5 cm² (1-cm-ctc) is the most advantageous configuration from the physics point of view. Radiobiological experiments are needed to fully explore this new avenue and to confirm our results.

Grid Block Design Based on Monte Carlo Simulated Dosimetry, the Linear Quadratic and Hug-Kellerer Radiobiological Models

Purpose

The clinical efficacy of Grid therapy has been examined by several investigators. In this project, the hole diameter and hole spacing in Grid blocks were examined to determine the optimum parameters that give a therapeutic advantage.

Methods

The evaluations were performed using Monte Carlo (MC) simulation and commonly used radiobiological models. The Geant4 MC code was used to simulate the dose distributions for 25 different Grid blocks with different hole diameters and center-to-center spacing. The therapeutic parameters of these blocks, namely, the therapeutic ratio (TR) and geometrical sparing factor (GSF) were calculated using two different radiobiological models, including the linear quadratic and Hug-Kellerer models. In addition, the ratio of the open to blocked area (ROTBA) is also used as a geometrical parameter for each block design. Comparisons of the TR, GSF, and ROTBA for all of the blocks were used to derive the parameters for an optimum Grid block with the maximum TR, minimum GSF, and optimal ROTBA. A sample of the optimum Grid block was fabricated at our institution. Dosimetric characteristics of this Grid block were measured using an ionization chamber in water phantom, Gafchromic film, and thermoluminescent dosimeters in Solid Water™ phantom materials.

Results

The results of these investigations indicated that Grid blocks with hole diameters between 1.00 and 1.25 cm and spacing of 1.7 or 1.8 cm have optimal therapeutic parameters (TR > 1.3 and GSF~0.90). The measured dosimetric characteristics of the optimum Grid blocks including dose profiles, percentage depth dose, dose output factor (cGy/MU), and valley-to-peak ratio were in good agreement (±5%) with the simulated data.

Conclusion

In summary, using MC-based dosimetry, two radiobiological models, and previously published clinical data, we have introduced a method to design a Grid block with optimum therapeutic response. The simulated data were reproduced by experimental data.

Technical Note: Fabricating Cerrobend grids with 3D printing for spatially modulated radiation therapy: A feasibility study

PURPOSE

Grid therapy has promising applications in the radiation treatment of large tumors. However, research and applications of grid therapy are limited by the accessibility of the specialized blocks that produce the grid of pencil-like radiation beams. In this study, a Cerrobend grid block was fabricated using the 3D printing technique.

METHODS

A grid block mold was designed with flared tubes which follow the divergence of the beam. The mold was 3D printed using a resin with the working temperature below 230 °C. The melted Cerrobend liquid at 120 °C was cast into the resin mold to yield a block with a thickness of 7.4 cm. At the isocenter plane, the grid had a hexagonal pattern, with each pencil beam diameter of 1.4 cm; the distance between the beam centers was 2.1 cm.

RESULTS

The dosimetric properties of the grid block were studied using small field dosimeters: a pinpoint ionization chamber and a stereotactic diode. For a 6 MV photon beam, its valley-to-peak ratio was 20% at dmax and 30% at 10 cm depth; the output factor was 84.9% at dmax and 65.1% at 10 cm depth.

CONCLUSIONS

This study demonstrates that it is feasible to implement 3D printing technique in applying grid therapy in clinic.