The potential of intensity-modulated proton radiotherapy to reduce swallowing dysfunction in the treatment of head and neck cancer: A planning comparative study

Figures & data

Table 1. Volumes of interest and corresponding planning criteria.

Parameter	Criteria
General plan objectives
PTVs	≥ 98% of volume ≥ 95% of the prescribed dose
Maximum plan dose	≤ 77 Gy
Volume receiving 74.9 Gy	≤ 2 cm^3
Spinal cord Dmax	≤ 54 Gy*
Brain stem Dmax	≤ 60 Gy
Eyes Dmax	≤ 25 Gy
Optic nerves/optic chiasm Dmax	≤ 54 Gy
Mean dose parotid glands	minimized^$
Mean dose submandibular glands	minimized
Unspecified tissue	minimized
Additional objectives swallowing sparing plans
Superior PCM mean dose	minimized^†
Supraglottic larynx mean dose	minimized^†
Middle PCM mean dose	minimized^†
EIM V60 Gy	minimized^†

*Although the accepted maximum spinal cord dose was 54 Gy, it was attempted to keep it below 50 Gy.

^$Sparing of swallowing OARs was not allowed to increase the mean parotid gland dose.

^†The dose to swallowing OARs was minimized in the specified order of priority [7].

Dmax, maximum dose; EIM V_{60 Gy}, esophageal inlet muscle proportion receiving ≥ 60 Gy; OAR, organ at risk; PCM, pharyngeal constrictor muscle; PTV, planning target volume.

Table 2. Dose-volume results of relevant structures and corresponding NTCP values for swallowing dysfunction.

		Standard treatment		Swallowing-sparing treatment
		ST-IMRT	ST-IMPT	SW-IMRT	3B-SW-IMPT	7B-SW-IMPT
Coverage with ≥ 95% prescribed dose
PTV54 V95% ≥ 98%	Patients (%)	100	100	100	100	100
PTV70 V95% ≥ 98%	Patients (%)	100	100	100	100	100
Critical structures
Eyes < 25 Gy	Patients (%)	100	100	100	100	100
Optic chiasm and nerves < 54 Gy	Patients (%)	100	100	100	100	100
Brainstem < 60 Gy	Patients (%)	100	100	100	100	100
Spinal cord < 51 Gy	Patients (%)	100	100	100	100	100
PTV volumes
PTV54	(cm^3)	728 (498–1109)
PTV70	(cm^3)	237 (112–390)
Total volume
V5 Gy	(cm^3*10^3)	5.16 (1.75–7.75)	3.23 (1.37–4.41)	5.08 (1.80–7.79)	3.21 (1.33–4.40)	3.39 (1.32–4.53)
V10 Gy	(cm^3*10^3)	4.40 (3.08–6.31)	2.97 (2.15–3.93)	4.41 (3.08–6.35)	2.92 (2.08–3.92)	2.96 (2.11 3.96)
V20 Gy	(cm^3*10^3)	3.46 (2.33–5.15)	2.03 (1.49–2.81)	3.49 (2.36–5.24)	2.01 (1.45–2.79)	2.09 (1.44–2.95)
V30 Gy	(cm^3*10^3)	2.79 (1.73–3.97)	1.62 (1.14–2.22)	2.86 (1.76–4.08)	1.59 (1.11–2.21)	1.63 (1.09–2.30)
V40 Gy	(cm^3*10^3)	2.16 (1.23–2.98)	1.33 (0.90–1.88)	2.18 (1.28–3.05)	1.30 (0.87–1.86)	1.30 (0.87–1.85)
V51.5 Gy	(cm^3)	1216 (576 1859)	975 (621–1444)	1259 (685–1971)	943 (619–1427)	944 (621–1406)
V58 Gy	(cm^3)	507 (227–821)	638 (321–1053)	539 (260–967)	602 (346 1013)	622 (352–996)
V66.5 Gy	(cm^3)	308 (139–502)	295 (151 477)	319 (146–523)	286 (147 477)	285 (146–460)
V74.9 Gy	(cm^3)	0.0 (0.0–0.4)	0.0 (0.0–0.1)	0.2 (0.0–1.2)	0.0 (0.0–0.1)	0.0 (0.0–0.2)
Integral dose	(Gy*cm^3*10^4)	18.4 (12.0 26.8)	11.5 (8.2–15.9)	18.6 (12.3–27.4)	11.3 (8.0–15.7)	11.8 (8.3–16.1)
maximum dose	(Gy)	74.6 (72.4–75.6)	74.6 (73.3–75.4)	75.3 (73.9–76.2)	74.8 (73.1 76.0)	74.9 (73.7–75.8)
Salivary glands dose/volume
Parotid ipsilateral mean dose	(Gy)	45.5 (25.6 63.4)	28.6 (15.1–48.8)	45.4 (25.6–65.0)	28.4 (15.1–48.7)	28.5 (15.2–48.6)
Parotid contralateral mean dose	(Gy)	35.2 (24.1–49.5)	21.2 (6.2–30.6)	35.0 (24.3–48.3)	21.2 (6.5–30.5)	21.2 (6.5–30.7)
Submandibular ipsilateral mean dose	(Gy)	67.8 (58.4–71.1)	68.4 (63.0–71.3)	68.4 (58.7 72.1)	68.0 (60.3–70.9)	68.1 (60.7–71.4)
Submandibular contralateral mean dose	(Gy)	61.4 (48.0–70.6)	56.9 (24.7–70.9)	61.9 (49.7–71.0)	56.9 (28.3–70.7)	57.2 (28.6 71.5)
SWOARS dose/volume
Superior PCM mean dose	(Gy)	65 (55–72)	65 (56–72)	63 (50–71)	63 (47–72)	62 (45–70)
Middle PCM mean dose	(Gy)	63 (49–72)	64 (53–72)	59 (30–70)	59 (31–71)	57 (27 70)
Supraglottic larynx mean dose	(Gy)	60 (48–71)	59 (45 72)	53 (25–70)	46 (14 72)	40 (10–70)
EIM V60 Gy	(%)	8 (0–100)	8 (0–100)	7 (0 100)	8 (0 100)	8 (0–100)
NTCP swallowing dysfunction
Physician rated
RTOG Grade 2–4	(%)	46 (30–64)	45 (25 63)	37 (17–62)	34 (6–62)	29 (4–62)
Patient rated moderate to severe problems
Solid food
Age 18–65 years	(%)	32 (17–50)	31 (14–49)	24 (7 47)	22 (2 48)	18 (2 47)
Age > 65 years	(%)	50 (32 69)	49 (26–68)	40 (15–66)	35 (5 67)	30 (4 66)
Soft food
Age 18–65 years	(%)	12 (3–23)	13 (3–23)	10 (1–21)	11 (2–22)	10 (1–20)
Age > 65 years	(%)	32 (11–52)	34 (12 52)	28 (5 49)	28 (5–50)	27 (4–47)
Liquid food	(%)	7 (3–12)	7 (2–13)	5 (1–12)	5 (0 13)	4 (0 12)
Choking when swallowing	(%)	7 (2–37)	6 (1–38)	5 (0 35)	5 (0 36)	5 (0–33)

Values are means with ranges in parenthesis.

EIM, esophageal inlet muscle; Integral dose, mean dose within patient outline × patient outline volume; NTCP, normal tissue complication probability; PCM, pharyngeal constrictor muscle; PTVx, planning target volume prescribed × Gy; RTOG, Radiation Therapy Oncology Group; ST-IMRT, standard intensity-modulated radiation therapy; SW-IMRT, swallowing-sparing IMRT; ST-IMPT, standard intensity-modulated proton therapy; SW-IMPT, swallowing-sparing IMPT with 3 beams (-3b) or 7 beams (-7b); Vx, volume receiving ≥ x Gy or ≥ x% of the prescribed dose.

Figure 1. Sagittal and axial representation of dose distributions with standard (ST)-intensity-modulated radiotherapy (IMRT), swallowing-sparing (SW)-IMRT and 7-beam SW-intensity-modulated proton therapy (IMPT) in a sample patient.

Figure 2. Mean dose delivered to the superior pharyngeal constrictor muscle (PCM) (A) and supraglottic larynx (B), and corresponding normal tissue complication probability (NTCP) of physician-rated RTOG grade 2–4 swallowing dysfunction (C) with standard (ST)-intensity-modulated radiotherapy (IMRT), swallowing-sparing (SW)-IMRT and SW-intensity-modulated proton therapy (IMPT). In each figure, patients were re-sorted according to the corresponding value with ST-IMRT.

Figure 3. Calculated normal tissue complication probability (NTCP) values of physician-rated RTOG grade 2–4 swallowing dysfunction in a patient with a considerable benefit. According to our models [7], the NTCP with standard (ST)-intensity-modulated radiotherapy (IMRT), swallowing-sparing (SW)-IMRT and SW-intensity-modulated proton therapy with 7 beams (7B-SW-IMPT) depends not only on the mean dose to the superior pharyngeal constrictor muscle (PCM), but also on the mean dose to the supraglottic larynx region.

Figure 4. Potential reductions in the normal tissue complication probability (NTCP) of physician-rated RTOG grade 2–4 swallowing dysfunction as achieved by swallowing-sparing intensity-modulated radiotherapy (SW-IMRT) or intensity-modulated proton therapy with 3 or 7 beams (3B-SW-IMPT and 7B-SW-IMPT, respectively) relative to standard (ST)-IMRT. Cumulative plots are shown.

Christianen MEMC, Schilstra C, Beetz I, Muijs CT, Chouvalova O, Burlage FR, . Predictive modelling for swallowing dysfunction after primary (chemo)radiation: Results of a prospective observational study. Radiother Oncol 2011 (in press) doi: 10.1016/j.radonc.2011.08.009.

 Google Scholar