KEY POINTS
- The paired planning study used CT datasets from 30 consecutive bladder cancer patients, generating both VMAT and 11-field step-and-shoot IMRT plans on identical anatomy. The bladder received 66 Gy in 33 fractions, with a uniform 1.5-cm PTV margin and daily CBCT verification.
- Target coverage was essentially identical. PTV V95 was 98.65% with VMAT versus 98.39% with IMRT, D98 was 62.14 versus 62.22 Gy, and no significant differences were seen in target dose, conformity or homogeneity.
- Incidental nodal irradiation followed a striking anatomical hierarchy: obturator, external iliac, internal iliac and common iliac regions received approximately 75%, 45%, 25% and <5% of prescription dose, respectively. The pattern was highly consistent between VMAT and step-and-shoot IMRT.
- Obturator exposure was particularly substantial. Median mean dose was approximately 51 Gy bilaterally, with roughly 70–75% of the obturator volume receiving ≥40 Gy and about 53–57% receiving ≥50 Gy depending on technique and side.
- External iliac nodes received an intermediate dose, with median mean doses around 29–31 Gy, whereas internal iliac mean doses were approximately 17–19 Gy. Common iliac mean doses were only about 2 Gy, reflecting the rapid cranial fall-off from the bladder target.
- Several regional comparisons reached statistical significance, but absolute differences were generally <1 Gy, inconsistent across nodal basins and not considered clinically meaningful. The authors therefore found no systematic nodal-dose advantage for either delivery technique.
- VMAT was more delivery-efficient, requiring a median 914 versus 1,123 monitor units (P<.001); IMRT used more monitor units in 29/30 patients. Importantly, the relatively large 1.5-cm PTV margin may have increased incidental nodal exposure compared with contemporary adaptive bladder workflows.
CLINICAL TAKEAWAY
“Bladder-only” radiotherapy is not truly node-sparing: the obturator nodes receive a substantial fraction of therapeutic dose, while the external and internal iliac regions also receive meaningful exposure. This may matter when interpreting bladder-preservation studies and designing trials of elective nodal irradiation, but the study cannot show that incidental dose controls microscopic nodal disease.