DISCUSSION
VMAT-TBI is a promising modality with increasing clinical utilization. In this study, we analyzed organ sparing using the VMAT technique in comparison to the conventional 2D technique. 2D and VMAT plans were generated for the same patients and directly compared. The VMAT-TBI treatment was well tolerated and significant improvements in target coverage and lung sparing were achieved with the VMAT technique versus the 2D comparison plans. In addition, the VMAT technique provided the opportunity to spare other organs, such as kidneys, gonads, brain, thyroid, lenses when deemed appropriate. Other benefits of using VMAT-TBI technique are more comfortable patient positioning lying down, ability to treat TBI under anesthesia, more accurate dose calculation and image-guided treatment delivery, and the ability to treat TBI patients in a small vault.
The 2D conventional technique with lung blocks is the current standard of care. At our institution, lung blocks are created to allow, on average, 50% transmission at mid-lung point for an average-size patient. The size of the blocks are drawn to match the lungs–1cm contour and 1 cm below clavicle to enable dose coverage to the bones (clavicle and ribs). Therefore, a good portion of the lungs will still receive significant dose (>50% of the prescription dose). In addition, there is a dose contribution from chest wall boosts, adding up to a mean lung dose of 80.6% of the prescription.
Although the study improves upon the kidneys, brain, thyroid, lenses sparing, it is unclear that this benefit will be clinically meaningful, although the early outcomes and toxicity report is promising18. Despite this possibility, our institution generally abides by the ALARA (as low as reasonably achievable) principle, which would argue that lower organ doses are desirable in and of itself for patients without active disease; however, further studies will be necessary to elucidate the exact dose response of this lung toxicity at these lower observed doses.