First system for fully-automated multi-criterial treatment planning for a high-magnetic field MR-Linac applied to rectal cancer

Figures & data

Table 1. Wish-list for automated rectal cancer treatment planning for an MRL.

Constraints
	Structure	Constraint function	Limit
	PTV	gEUD(20)	< 103% of PD
	Back*	Maximum dose	< 38 Gy
	PTV shell at 5 mm	Maximum dose	< 95% of PD
	PTV shell at 1 cm	Maximum dose	< 90% of PD
	PTV shell at 3 cm	Maximum dose	< 70% of PD
	PTV shell at 5 cm	Maximum dose	< 65% of PD
	PTV shell at 7 cm	Maximum dose	< 50% of PD
	1 cm entrance dose shell*	Maximum dose	< 50% of PD
Objectives
Priority	Structure	Aim & objective function	Goal value (Sufficient)
1	PTV	$\downarrow$ LTCP(95% of PD,$\alpha$=0.8)	0.06 (0.06)
2	OAR	$\downarrow$ Mean dose	0
3	PTV shell at 7cm	$\downarrow$ Maximum dose	0
4	PTV shell at 2cm	$\downarrow$ Maximum dose	60% of PD
5	PTV	$\uparrow$ Mean dose	PD (PD)
6	OAR	$\downarrow$ gEUD(10)	0

PD: Prescribed Dose (50 Gy); gEUD(k): generalized Equivalent Uniform Dose; k: volume parameter, LTCP(PD,$\alpha$): Logarithmic Tumor Control Probability [22], with $\alpha :$ cell sensitivity; OAR: composite organ at risk; $\downarrow :$ minimization; $\uparrow :$ maximization.

*Posterior to the PTV, the entrance dose shell was replaced by the back structure for separate steering on high dose in the back.

Table 2. Comparison of dosimetric plan parameters for MANplans and AUTOplans.

	MANplans		MANplans – AUTOplans
	Mean	SE	Mean	Range	p-Value
PTV
V95% [%]	99*	–	0*	–	–
V107% [%]	0.4	0.1	0.2	[–0.7 – 1.4]	<.01
Dmean [%]	50.2	0.3	–0.2	[–1.2 – 0.8]	.04
CI [–]	1.1	0.0	–0.1	[–0.1 – 0.0]	<.01
HI [%]	9.7	0.1	0.8	[–1.0 – 2.5]	.02
OAR
Dmean [Gy]	18.9	0.5	1.3	[–1.0 – 4.5]	<.01
Patient
V40Gy [%]	10.9	0.7	–0.1	[–0.7 – 0.6]	.04

SE: standard error; PTV: planning target volume; OAR: organ at risk; CI: conformity index; HI: homogeneity index.

* All plans normalized to 99%.

Figure 1. Absolute differences in dosimetric plan parameters between the MANplans and AUTOplans for all 23 patients. Positive values indicate a better AUTOplan. The first 5 patients were used to train the automated treatment planning workflow.

Figure 2. Dose distributions for the MANplan (left) and AUTOplan (right) for patient 14 (See Figure 1). The AUTOplan had clearly reduced dose in the OAR. Top: axial view, bottom sagittal view. Magenta contour = PTV, Red contour = OAR.

Figure 3. Parts of axial dose distribution for patient 14 (in PTV and posterior to PTV) (a) clinically acceptable dose distribution without magnetic field (6 MV conventional linac plan). (b) clinically unacceptable dose distribution with magnetic field (MRL plan) but no dose control by back structure; too high dose in back structure (arrow) related to ERE and enhanced build-up due to the magnetic field. (c) clinically acceptable dose distribution with magnetic field (MRL plan) and dose control using the back structure. Pink contour = PTV, dashed white contour = inactive back structure, solid white contour = active back structure. The treatment couch matrass is located right below the back structure.

Breedveld S, Storchi PR, Keijzer M, et al. A novel approach to multi-criteria inverse planning for IMRT. Phys Med Biol. 2007;52(20):6339–6353.

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