Total body irradiation (TBI) combining volumetric modulated arc therapy (VMAT) for thorax at standard source axis distance (SAD) with intensity modulated radiation therapy (IMRT) at extended SAD.

PURPOSE: Total body irradiation (TBI) is an important part of conditioning regimens prior to hematopoietic stem cell transplantation (HSCT). At our institution, a conventional TBI technique employing bilateral fields at extended source-to-axis distance (eSAD) and a solid three-dimensional (3D) compensator has been standard practice since the 80s. However, its ability to minimize lung dose remains limited.

AIMS: This study aims to modernize the existing TBI technique using the capabilities of our treatment planning system (TPS). The proposed hybrid method replaces solid compensators with virtual compensation by combining bilateral intensity-modulated radiotherapy (IMRT) fields at eSAD of 500 cm with volumetric modulated arc therapy (VMAT) fields at standard SAD 100 cm for the thoracic region.

METHODS: At first, the Anisotropic Analytical Algorithm (AAA) V16 and AcurosXB (AXB) V16 dose calculation algorithms of the Eclipse TPS (Varian) were commissioned for use at eSAD. Dosimetric measurements were performed in water, solid water, and inhomogeneous phantoms. Subsequently, a treatment planning strategy was developed to minimize the influence of positioning uncertainties on lung dose. Anonymized computed tomography (CT) datasets representing diverse patient anatomies were utilized for treatment planning evaluation. Final validation was conducted through an end-to-end test using an anthropomorphic phantom. SAD VMAT and eSAD IMRT fields were verified using a two-dimensional (2D) dosimetry chamber array positioned at both caudal and cranial thoracic levels under SAD and eSAD setups. The resulting cumulative dose was then compared with the AXB-calculated dose.

RESULTS: At eSAD, mean measured-calculated dose deviations were < 2% for AXB and < 4% for AAA, with confidence limits of 3.5% and 5%, respectively. These results affirmed the clinical viability of both algorithms, with AXB providing superior dose calculation accuracy. Planning studies showed consistent PTV coverage with lung dose reduction across diverse anatomies, and end-to-end phantom validation confirmed the workflow practicability. Verification of summed dose using the 2D array, with 95% of evaluated points meeting a gamma criterion of 5 mm and 5%, provided additional confidence in the dosimetric robustness of the treatment concept.

CONCLUSION: This study showed the feasibility of a novel hybrid eSAD IMRT/VMAT TBI technique providing a clinically viable approach enabling effective lung dose sparing while maintaining the robustness of large-field irradiation.