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.