Hydrogel rectum-prostate spacers mitigate the uncertainties in proton relative biological effectiveness associated with anterior-oblique beams

Figures & data

Table 1. Clinical dose requirements (rigidly enforced for all plans).

Clinical target volume	At least 90% volume at 79.2 Gy
	At least 99% volume at 77.6 Gy
Planning target volume [strategy (1): only applied for a  subset of plans]	At least 95% volume at 77.6 Gy
Bladder	At most 80 Gy at 10% volume
	At most 75 Gy at 15% volume
	At most 70 Gy at 25% volume
	At most 65 Gy at 30% volume
	At most 55 Gy at 45% volume
	At most 45 Gy at 50% volume
Rectum	At most 1 cm^3 at 83.16 Gy
	At most 75 Gy at 10% volume
	At most 70 Gy at 15% volume
	At most 60 Gy at 30% volume
	At most 50 Gy at 45% volume
	At most 40 Gy at 50% volume
Femoral heads	At most 45 Gy at 5% volume

Figure 1. Dose-volume histogram (DVH) comparison between treatment planning strategies (A)–(C) for all 10 rectum spacer patients. Each solid/dashed line corresponds to a DVH plot for an individual patient; the shaded regions indicate the inter-patient range for each plan type. (a) Plans with uniform 5mm CTV to PTV expansion; (b) plans with no CTV to PTV expansion.

Figure 2. Demonstrations of variable RBE weighted (vRBEw) dose distributions for anterior-oblique spot-scanned proton therapy plans with a uniform 5 mm CTV to PTV margin. The planning target volume is contoured in black; the hydrogel, contoured using additional magnetic resonance data, is shown in blue; and the rectum is contoured in pink. The dose distributions overlaid here are for vRBEw dose calculated according to the McNamara model with an α/β of 3 Gy. (a) Sagittal view of case where spacer was least effective; (b) axial view of case where spacer was least effective; (c) sagittal view of more typical case; (d) axial view of more typical case.

Figure 3. Investigating the McNamara model’s [11] sensitivity to α/β value and fractionation regimen. (a) Plans with uniform 5 mm CTV to PTV expansion: boxplots of the maximum dose to 1cc of the rectum for all 10 patients. The horizontal line shows the constraint that no 1cc of the rectum should receive more than 103% of the prescription dose; (b) plans with uniform 5 mm CTV to PTV expansion: boxplots of the CTV equivalent uniform dose (EUD) for all 10 patients, assuming an EUD a-value of -10 [20]; (c) example application of the McNamara model for various fractionation schemes and α/β values for a simple proton SOBP in water, range = 20 cm, modulation = 10 cm; (d) considering the ratio (vRBEw dose with an α/β of 1.5 Gy)/(vRBEw dose with an α/β of 3 Gy) for the fractionation schemes shown in Figure 3(c).

McNamara AL, Schuemann J, Paganetti H. A phenomenological relative biological effectiveness (RBE) model for proton therapy based on all published in vitro cell survival data. Phys Med Biol. 2015;60:8399–8416.

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