Page 28 - ARNM-2-1

Advances in Radiotherapy
& Nuclear Medicine Is 5-mm PTV margin 4D-CT-based radiotherapy

lung V20 (volume of lung minus GTV receiving 20 Gy) of the recommended dosage. PTV margins is also a
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and mean lung dose can be decreased with 4D-CT-based combination of internal and setup margins. Hence, if
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planning. These results (Table 2) are consistent with our the internal margin or internal motion is taken care of by
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results; the combined GTV based on 4D-CT (IGTV-4D) motion management techniques such as 4D-CT, only the
exceeded the GTV based on FBCT 187.2 cc vs. 171.5 cc, as setup margin needs to be added to the PTV margin. In
expected. principle, this should reduce the PTV margin compared
with the general population margin without the use of
Ahmed et al. compared conventional three-
dimensional (3D) conformal radiotherapy (3DCRT) plans motion management techniques. However, the extent to
which the reduced margin is safe remains questionable. The
made with regular helical FBCT with plans developed PTV margin calculation for head and neck cancer and lung
using 4D-CT contoured volumes to ascertain whether cancer using the Van Herk formula was slightly different.
target volume coverage is adequate. They discovered that In head and neck cancer, the internal margin is almost
the combined GTV produced on 4D-CT (IGTV_4D) zero, and the entire PTV margin is set up by error only.
for both main and nodal disease, together or separately, However, the internal margin is significant in lung cancer.
was considerably greater than the GTV generated on a There are certain other inherent problems with using Van
helical FBCT (GTV_3D) scan. Although not statistically Herk’s formula in lung radiotherapy; for example, the
significant, the average PTV produced on helical FBCT construction of this dose model does not consider the
(PTV_3D) was greater than the average PTV generated target size, tissue density, or plan conformity and assumes
on 4D-CT (PTV_4D). They extrapolated the radiotherapy that the dosimetric effects of motion can be modeled with
plan based on 3D-CT on the target volume generated on a convolution. Ample evidence suggests that a PTV
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4D-CT and revealed that PTV_4D coverage with a 95% margin of ≤5 mm is adequate for treating early-stage lung
isodose line was inferior by approximately 10%. The spinal cancer or small lung metastases with stereotactic ablative
cord and esophagus doses were significantly lower with radiotherapy using motion management techniques,
4D-CT-based planning. In this study, Ju et al. examined including 4D-CT. 30-33 Hence, it should not be different in
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the dosimetric benefits of 4D-CT-based PTV generation locally advanced lung cancers with larger tumors and/or
over the traditional PTV definition with a population-based nodes. In our study, PTV margins calculated from set-up
margin for lung cancer radiation. They concluded that errors using Van Herk’s formula were >5 mm in the X and
radiation planning and PTV definition based on 4D-CT can Y axes, but these might not be true representations of the
reduce the dosage to OARs (lung, heart, etc.), enhance target PTV margin, as we have already added the ITV margin
coverage, and decrease the PTV in patients with minor based on 4D-CT assessment. However, the margin of
respiratory motion. This technique can prevent geographic the translational vector is within 5 mm. Offline imaging
target misses in patients with considerable respiratory verification confirmed that for uncorrected setup errors,
motion, particularly in those whose motion exceeds 1.5–2 the primary tumor was within the PTV margin of 5 mm
cm, without appreciably increasing the dose of OARs. 23 in 95% of cases, and with imaging correction, the primary

In a comparative study, Cole et al. examined the possible tumor was within the PTV in 100% of cases.
dosimetric and clinical advantages of employing 4D-CT However, this study had limitations. First, set-up errors
rather than 3D-CT when planning radical radiotherapy are the main, but not the only, contributor to interfraction
for non-small cell lung cancer. They demonstrated that uncertainties. Physiological and anatomical changes can
plans based on 4DCT had reduced PTV volumes, doses make a difference in some patients, and this analysis does
to organs at risk, and expected rates of normal tissue not address this issue. Second, the analysis of the setup
complication probability. Therefore, 4D-CT has the error data was a retrospective assessment of errors based
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potential to increase the therapeutic ratio and pave the way on daily imaging. Third, the lack of 4D-cone beam CT
for dose escalation in radiotherapy for lung cancer. Hence, (CBCT) in our center might have added more precision in
most current dose escalation study protocols emphasize ascertaining the safe PTV margin. Finally, 4D-CT itself has
the incorporation of motion management techniques, limitations, including motion artifacts, irregular breathing
including 4D-CT, in radical radiotherapy treatment patterns, and rigorous quality assurance. 34-36
planning for locally advanced lung cancer. 1,25,26
Marcel Van Herk developed the PTV margin recipe 5. Conclusion
(PTV margin = 2.5∑ + 0.7δ) based on the analysis of 4D-CT-based IGRT planning for locally advanced lung
the systemic and random geometric uncertainties, cancer with a reduced PTV margin of 5 mm can significantly
aiming to determine the minimal margin required to reduce PTV and OAR doses without compromising
reach 90% of the population with a full coverage of 95% PTV coverage compared to FBCT-based planning with a

Volume 2 Issue 1 (2024) 7 https://doi.org/10.36922/arnm.2784