The implementation of low instantaneous dose rate total body irradiation with linear accelerator in small-size treatment rooms

Purpose

This paper describes the implementation of an instantaneous low-dose-rate total body irradiation (TBI) technique using block-filtered 6 MV X-rays with a linear accelerator (LINAC) to reduce pulmonary toxicity.

Methods

In the absence of dedicated TBI-specific meter-set dose rates in LINAC and sufficient treatment room size, a 2-cm-thick transmission block was used together with a 200-cm source-to-surface distance (SSD) to reduce the instantaneous dose rates of 6 MV x-rays down to 10 cGy/min, thus alteration to the beam properties. A TBI-specific dose calculation model was built with data acquired at the treatment planning system (TPS)-permitted maximum 140-cm SSD and was validated in phantoms at a 180-cm SSD. As for planning strategies, we adopted large anterior-to-posterior/posterior-to-anterior (AP/PA) open fields with multi-leaf collimator shielding for lungs to achieve target coverage, lung protection, and efficient dose delivery. A custom-designed sliding couch (Patent No. ZL202123085880.1) was manufactured to support patients during treatment. Measures to control the quality and safety of TBI treatment include machine interlocks, pretreatment checklists, and in-vivo dose monitoring.

Results

The instantaneous dose rate of block-filtered 6MV X-ray was reduced to approximately 7.0 cGy/min at 12.5–7.5 cm depth with a 185–200 cm SSD. The dose calculated by TPS differs from the measurements by 0.15%–1.55% in the homogeneous phantom and 1.2%–4.85% in the CIRS thorax phantom. The open-field TBI technique achieved V_90% (PTV) ≈ 96.8% and MLD = 6.6 Gy with 1-h planning and 50-min beam delivery in a single fraction. From February 2021 to July 2023, 30 patients received TBI treatments in our center, and in-vivo monitoring results differed from TPS calculations by −1.49%–2.10%. After 6–12 months of follow-ups, all the patients treated in our center showed no pulmonary toxicities of grade 2 or higher.

Conclusion

A low instantaneous dose rate TBI technique can be implemented in the clinic.