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

Dosimetric impact of intrafraction patient motion on MLC-based 3D-conformal spatially fractionated radiation therapy treatment of large and bulky tumors

PURPOSE

To evaluate the dosimetric impact on spatially fractionated radiation therapy (SFRT) plan quality due to intrafraction patient motion via multi-field MLC-based method for treating large and bulky (≥8 cm) unresectable tumors.

METHODS

For large tumors, a cone beam CT-guided 3D conformal MLC-based SFRT method was utilized with 15 Gy prescription. An MLC GTV-fitting algorithm provided 1 cm diameter apertures with a 2 cm center-to-center distance at the isocenter. This generated a highly heterogeneous sieve-like dose distribution within an hour, enabling same-day SFRT treatment. Fifteen previously treated SFRT patients were analyzed (5 head & neck [H&N], 5 chest and lungs, and 5 abdominal and pelvis masses). For each plan, intrafraction motion errors were simulated by incrementally shifting original isocenters of each field in different x-, y-, and z-directions from 1 to 5 mm. The dosimetric metrics analyzed were: peak-to-valley-dose-ratio (PVDR), percentage of GTV receiving 7.5 Gy, GTV mean dose, and maximum dose to organs-at-risk (OARs).

RESULTS

For ±1, ±2, ±3, ±4, and ±5 mm isocenter shifts: PVDR dropped by 3.9%, 3.8%, 4.0%, 4.1%, and 5.5% on average respectively. The GTV(V7.5) remained within 0.2%, and the GTV mean dose remained within 3.3% on average, compared to the original plans. The average PVDR drop for 5 mm shifts was 4.2% for H&N cases, 10% for chest and lung, and 2.2% for abdominal and pelvis cases. OAR doses also increased. The maximum dose to the spinal cord increased by up to 17 cGy in H&N plans, mean lung dose (MLD) changed was small for chest/lung, but the bowel dose varied up to 100 cGy for abdominal and pelvis cases.

CONCLUSION

Due to tumor size, location, and characteristics of MLC-based SFRT, isocenter shifts of up to ±5 mm in different directions had moderate effects on PVDR for H&N and pelvic tumors and a larger effect on chest tumors. The dosimetric impact on OAR doses depended on the treatment site. Site-specific patient masks, Vac-Lok bags, and proper immobilization devices similar to SBRT/SRT setups should be used to minimize these effects.

Overview and Recommendations for Prospective Multi-institutional Spatially Fractionated Radiation Therapy Clinical Trials

PURPOSE

The highly heterogeneous dose delivery of spatially fractionated radiation therapy (SFRT) is a profound departure from standard radiation planning and reporting approaches. Early SFRT studies have shown excellent clinical outcomes. However, prospective multi-institutional clinical trials of SFRT are still lacking. This NRG Oncology/American Association of Physicists in Medicine working group consensus aimed to develop recommendations on dosimetric planning, delivery, and SFRT dose reporting to address this current obstacle toward the design of SFRT clinical trials.

METHODS AND MATERIALS

Working groups consisting of radiation oncologists, radiobiologists, and medical physicists with expertise in SFRT were formed in NRG Oncology and the American Association of Physicists in Medicine to investigate the needs and barriers in SFRT clinical trials.

RESULTS

Upon reviewing the SFRT technologies and methods, this group identified challenges in several areas, including the availability of SFRT, the lack of treatment planning system support for SFRT, the lack of guidance in the physics and dosimetry of SFRT, the approximated radiobiological modeling of SFRT, and the prescription and combination of SFRT with conventional radiation therapy.

CONCLUSIONS

Recognizing these challenges, the group further recommended several areas of improvement for the application of SFRT in cancer treatment, including the creation of clinical practice guidance documents, the improvement of treatment planning system support, the generation of treatment planning and dosimetric index reporting templates, and the development of better radiobiological models through preclinical studies and through conducting multi-institution clinical trials.

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.

Script-based implementation of automatic grid placement for lattice stereotactic body radiation therapy

Background and purpose

Spatially fractionated radiation therapy (SFRT) has demonstrated promising clinical response in treating large tumors with heterogeneous dose distributions. Lattice stereotactic body radiation therapy (SBRT) is an SFRT technique that leverages inverse optimization to precisely localize regions of high and lose dose within disease. The aim of this study was to evaluate an automated heuristic approach to sphere placement in lattice SBRT treatment planning.

Materials and methods

A script-based algorithm for sphere placement in lattice SBRT based on rules described by protocol was implemented within a treatment planning system. The script was applied to 22 treated cases and sphere distributions were compared with manually placed spheres in terms of number of spheres, number of protocol violations, and time required to place spheres. All cases were re-planned using script-generated spheres and plan quality was compared with clinical plans.

Results

The mean number of spheres placed excluding those that violate rules was greater using the script (13.8) than that obtained by either dosimetrist (10.8 and 12.0, p < 0.001 and p = 0.003) or physicist (12.7, p = 0.061). The mean time required to generate spheres was significantly less using the script (2.5 min) compared to manual placement by dosimetrists (25.0 and 29.9 min) and physicist (19.3 min). Plan quality indices were similar in all cases with no significant differences, and OAR constraints remained met on all plans except two.

Conclusion

A script placed spheres for lattice SBRT according to institutional protocol rules. The script-produced placement was superior to that of manually-specified spheres, as characterized by sphere number and rule violations.