Spatio-temporal stability of pre-treatment 18F-Fludeoxyglucose uptake in head and neck squamous cell carcinomas sufficient for dose painting

Figures & data

Table I. Mean dose, dose limits for planning target volumes (PTV) and dose constraints for organs at risk (OAR).

Target/OAR	Mean dose	Dmax	Dose volume limits
PTVPET	79.7 Gy	–	–
PTVgross-PET	73.1 Gy (± 1 Gy)	–	–
PTV1-gross	69.7 Gy (± 1 Gy)	–	–
PTV2-1	60 Gy (± 1 Gy)	–	–
PTV3-2	52.8 Gy (± 1 Gy)	–	–
PTV1	–	–	D98% > 64.6 Gy
PTV3	–	–	D98% > 46 Gy
Spinal cord	–	45 Gy	–
Brain stem	–	54 Gy	–
Mandible	–	70 Gy (72.5 can be accepted in certain circumstances)	–
Anterior eye	–	30 Gy	–
Chiasm and optic nerves	–	54 Gy	–
Inner ear	–	–	Dmean ≤ 45 Gy, D95% ≤ 45 Gy
Parotid glands	–	–	Dmean ≤ 26 Gy (ipsilateral) ≤ 20 (contralateral)
Larynx	–	–	V50Gy ≤ 33%
Submandibular glands	–	–	Dmean ≤ 35 Gy; only if level I and II is not in the target

PTV_PET, GTV_PET with a 4 mm margin; PTV_gross, GTV with a 4 mm margin (GTV, macroscopic tumor in T and N position defined from clinical evaluation and CT); PTV₁, CTV₁ with a 4 mm margin (_{CTV1 = GTV} with a 1 cm margin); PTV₂, CTV₂ with a 4 mm margin (CTV₂ = CTV₁ and volume with a high risk of subclinical tumor spread); PTV₃, CTV₃ with a 4 mm margin (CTV₃ = CTV₂ and elective lymph node levels without a margin, elective lymph node levels depends on tumor subsite, T and N stage).

Figure 1. CT slices (sagittal and axial) of the patient with the largest mismatch of 59% (A and B) and of a patient with a mismatch of 18% (D and E). The contours are: GTV_PET1 (red), GTV_PET2 (blue) and the volume receiving 79 Gy (yellow), converted from the dose color wash. Panels C and F illustrate the DVHs from the same two patients with mismatch of 59% and 18% mismatch, respectively. Curve colors are: GTV_PET1 (red), GTV_PET2 (blue), PTV_PET1 (green), PTV_PET2 (purple).

Table II. Target volumes, mean dose, dose distribution and tumor control probability.

	GTVPET1 (range)	GTVPET2 (range)	PTVPET1 (range)	PTVPET2 (range)	PTVMTV1 (range)	PTVMTV2 (range)
Median Volume (cm^3)	10.0 (0.7–50.0)	8.5 (0.7–40.9)	33.5 (3.9–112.8)	29.8 (3.7–97.4)	12.8 (3.4–35.8)	12.6 (2.7–46.4)
Median mean dose (Gy)	80.9 (78.8–81.5)	80.6 (77.2–81.4)	79.7 (79.7–79.7)^a	79.4 (72.3–79.9)	79.7 (79.7–79.7)	79.3 (74.2–79.8)
MedianV95% (%)	100 (83.7–100)	99.5 (82.2–100)	99.6 (94.4–100)	92.4 (56.6–100)	100.0 (98.5–100)	97.0 (59.3–100)
Median TCP (%)	94.8 (93.7–95.1)	94.7 (92.6–95.1)	94.2 (94.2–94.2)^a	94.0 (88.2–94.3)	^b	^b

Median FDG PET positive tumor volume (GTV_PET), planning target volume (PTV_PET) and the planning target volume for the iso-contour with SUV threshold of 50% of SUVmax (PTV_MTV) in cm³ from both scans. Median mean dose to GTV_PET, PTV_PET and PTV_MTV; median percentage of target volume receiving more than 95% of prescribed dose and median tumor control probability (TCP) from the two scans.^a There is no range since the mean dose was normalized to PTV_PET1 in the planning; ^b TCP cannot be estimated for the metabolic tumor volumes as the previously published model is not applicable.

Figure 2. Variability in tumor control probability (TCP) as a result of the recorded variability in GTV_PET. The gray dot is the probability of tumor control using the current mean dose (68 Gy). The black dot depicts the probability of control from the dose painted plans using the mean dose to the GTV_PET from the first scan (GTV_PET1). The black lines show the median dose on scan 2 (x-axis) and the resulting median TCP (y-axis). The two box-and-whiskers plots along the axis show the distribution of doses and TCP estimates, with an outlying patient marked as a black cross. The outlying patient had lymph nodes delineated on scan 2 that was not delineated on scan 1 – on review this difference was due to the interpretation of the uptake, rather than variability in uptake. The median detriment in TCP on scan 2 relative to scan 1 is 0.2%.