Clinical experience of total body irradiation performed on Halcyon™ for a patient diagnosed with acute myeloid leukemia

INTRODUCTION

Total body irradiation (TBI) has been used extensively in bone marrow transplantation as it induces immunosuppression and helps avoid rejection of the donor’s marrow. It also helps eradicate cancer cells in not only the same sites where chemotherapy acts but also sanctuary sites such as the brain and testes which are not breached by chemotherapy.^[1] TBI is an important part of conditioning regimens for bone marrow transplantation for hematological malignancies such as acute myeloid leukemia, acute lymphoid leukemia, chronic myeloid leukemia, chronic lymphocytic leukemia, multiple myeloma, lymphoma, and other indications.

The traditional method of TBI employed conventional techniques involving larger radiation treatment fields using lung blocks with the patient positioned standing or lying on the side at large distances with an extended SSD. This required larger and more expensive LINAC rooms.^[2]

Recently, many centers have initiated the use of CT-based 3D planning with IMRT and inverse planning intending to reduce the dose of OARS and achieve better dose homogeneity. Conformal- avoidance TBI using either Volumetric Arc Modulated Therapy (VMAT) or Helical Tomotherapy is being used in the Western world albeit with limited clinical experience.^[3]

Uehara et al. presented the first feasibility study of VMAT with Halcyon™ LINAC for TBI.^[4]

We would like to report a case treated for TBI on Halcyon™ machine which is the first clinically reported treatment amidst available medical literature.

MATERIAL AND METHODS

Halcyon™ (from m/s Varian Medical Systems, USA) is a single energy 6MV flattening filter-free ring-based gantry linear accelerator which has 114 double-layered stacked and staggered MLC with 5 cm/s speed. The maximum field size of 28 × 28 cm² with a maximum gantry speed of 4 rotations/min. Our institution is equipped with a Halcyon™ Elite Class LINAC with KVCBCT, MV-MV, and MVCBCT imaging and it has a 5 mm virtual MLC thickness at isocenter. Conventionally, TBI was started using conventional planning by delivering simple AP and PA beams. With the advancement in systems and technology, presently TBI can be done in conventional LINAC using Volumetric Modulated Arc Therapy but in Halcyon™, it comes with its limitations which make it difficult to achieve desired TBI dose distribution.^[4]

Plan evaluation and treatment delivery

The planning aim was that 90% of the prescription dose is received by 95% of the planning target volume [Figures 1 and 2]. Lungs and kidneys received 80% of the prescribed dose, maximum dose of <130% of the prescribed dose is received by 2 cc of whole-body volume.

Close

Figure 1:

Coronal view.

Export to PPT

Close

Figure 2:

Sagittal view.

Export to PPT

Quality assurance of the approved plan was done using portal dosimetry and ArcCheck (m/s Sun Nuclear Corporation, USA) patient-specific QA, both tests were passed. During treatment, in vivo dosimetry was done using Gafchromic film, where an average of 7% variation between planned and delivered skin dose is measured.

Kv-CBCT imaging was done for each isocenter and matching was done as necessary before the treatment. The total time taken for delivery of a single fraction is 1 h 15 min.

DISCUSSION

Gruen et al. have used a vacuum bag with an adjustable board to provide a steady setup during all treatment processes. In companion, we used the Timo headrest and full body vacuum bag to immobilize the patient, extremities were kept close to the body to minimize the lateral separation and minimize air gaps to achieve homogeneous dose distribution.^[5]

Like the feasibility study by Uehara et al., which employed 13 isocenters, we used 15 isocenters.^[4] We inserted lateral isocenters for regions where the lateral separation between isocenters is 10 cm.^[6]

While studies by Springer et al. where isodose curves have been converted into structures to achieve homogeneous dose distributions; in our study, we have contoured gradient regions injunction of both image sets which provided a greater control while achieving a homogeneous dose distribution.^[6]

The planning parameters described in various dosimetric studies describing the feasibility of TBI with Halcyon™ and the first clinical experience of TBI with VMAT were used as benchmarks for plan evaluation. In our study, we were able to achieve the pre-specified parameters of 90% of the prescription dose that should be received by 95% of the planning target volume, and OARs such as the lung and kidney should receive <85% of the prescribed dose.^[4,6]

Mancosu et al. too employed Gafchromic™ films for in vivo dosimetry for multi-isocentric VMAT irradiation of Total Marrow Lymph Nodes and found them to be in good agreement with planned parameters. Using this study as a reference, we used small-sized Gafchromic™ films and placed them at various locations on the body. Recorded values were in good agreement with the calculated values.^[7]

CONCLUSION

This is the first known reported clinical treatment of TBI on Halcyon machine amidst available medical literature.