A literature survey on indicators for characterisation and optimisation of SAR distributions in deep hyperthermia, a plea for standardisation

Figures & data

Table

Symbol	Unit	Description
	W/kg	Volume averaged SAR in the total patient volume (the part of the patient inside the applicator)
	W/kg	Volume averaged SAR in the target volume
	W/kg	Maximum SAR in the total patient volume
	W/kg	Maximum SAR inside the target volume
	W/kg	Volume averaged SAR in a complaint region
	W/kg	SAR exceeded in 5% of the total patient volume
	W/kg	SAR exceeded in 5% of the target volume
	W/kg	SAR exceeded in 5% of a region volume
	W/kg	25% of the maximum SAR
coverage	–	Part of the target enclosed by 25%SARmax isosurface
	W	Deposited power in the total patient volume
Ptarg	W	Deposited power in the target volume
	m	A location inside the patient
	ml/kg/ min	Blood perfusion
	m^3	Total volume
	m^3	Target volume
	m^3	Volume where SAR > SAR1, i.e. the one percent of the patient volume with the highest power absorption.
	m^3	Volume enclosed by x%SARmax
	m^3	Volume enclosed by x%SARtarg max
	°C	Temperature exceeded in 50% of the target volume
	°C	Temperature exceeded in 50% of the target volume, at maximum patient tolerance

Figure 1. Outline of the study.

Table I.  Characteristics of the 36 patient models that were used to verify the found indicators. Averages and standard deviations are given.

	Average	SD
Tetrahedra	248,011	49,747
Patient volume inside applicator (cm^3)	31,715	7,820
Tumour volume (cm^3)	129	98
Fat (%)	47	13
Muscle (%)	32	7
Bone (%)	7	2

Figure 2. Axial view of the BSD2000 Sigma 60 applicator, with the four channels, each connected to two antennas and a combined patient-applicator model.

Table II.  Tissues, defined in the patient model and their electromagnetic and thermal properties at 77 MHz, 37 °C and under thermal stress [34],[44],[45].

Tissue	Relative permittivity	Effective electrical conductivity (S/m)	Blood perfusion (mL/kg/min)	Mass density (kg/m^3)	Heat capacity (J kg^−1 K^−1)	Thermal conductivity (W m^−1 K^−1)
Fat	13	0.07	200	900	3500	0.21
Muscle	69	0.7	300	1000	3500	0.642
Bone	16	0.06	100	1600	1000	0.436
Target (=cervix)	69	0.7	800	1000	3500	0.642
Bladder	24	0.29	150	1000	3500	0.6
Heart	99	0.7	600	1000	3500	0.642
Intestine	108	1.62	500	1000	3500	0.55
Kidney	109	0.77	4000	1000	3500	0.577
Liver	75	0.46	1000	1000	3500	0.64
Lung	35	0.71	200	500	1000	0.2
Myelum	6	0.04	100	1000	3500	0.642
Spleen	101	0.77	600	1000	3500	0.577
Stomach	82	0.89	600	1000	3500	0.577
Uterus	69	0.7	300	1000	3500	0.642
Vagina	69	0.7	300	1000	3500	0.642

Table III.  Quality indicators found in literature with a short explanation of their functionality.

Indicator	Formula	Unit
1		W
2		–
3		kg
4		kg m^−1.5
5		kg m^−4.5 s^−1
6		m^3s^−1
7		W
8	with STH the square target hotspot ratio	W/kg
9		W/kg
10		–
11	, SAR IEEE-1529 peak spatial SAR	W/kg
12		–
13	with Vx% SARmax the volume enclosed by the x% SAR isosurface	–
14	with HC the homogeneity coefficient	–
15	with SARmax,i the ith highest SAR value	W/kg
16	with SARxregion the SAR exceeded in x% of a region	–

Figure 3. Illustration of SAR₅ definition: SAR(tot)₅ covers a substantial part of the target (A), and of 25%SAR _{targ max} and 75%SAR _{targ max} definition versus SAR25_targ and SAR75_targ definition (B). Both histograms are averages of 10 patient models.

Table IV.  Heuristic evaluation of the quality indicators from Table III.

	Criterion A	Criterion B			Criterion C		Criterion D
Indicator	Independence of SARmax	SAR in target	SAR in target relative to normal tissue	Homogeneity in target	Hotspot SAR level	Hotspot SAR level relative to target SAR	Maximisation of target SAR or power	Minimisation of hotspot SAR or power
1	Y	N	N	N	N	N	Y	N
2	Y	N	N	N	N	N	Y	N
3	N	N	N	N	N	N	Y	Y
4	Y	N	N	N	N	N	Y	N
5	Y	N	N	N	N	N	Y	N
6	N	N	N	N	N	N	Y	Y
7	Y	N	N	N	N	N	Y	N
8	N	N	N	N	N	Y	Y	Y
9	Y	Y	N	N	N	N	Y	N
10	Y	N	Y	N	N	N	Y	N
11	Y	N	N	N	N	N	N	Y
12	Y	N	N	N	N	N	Y	Y
13	N	N	N	N	N	N	Y	N
14	N	N	N	Y	N	N	Y	N
15	N	N	N	N	Y	N	N	N
16	Y	N	N	N	N	Y	Y	Y

Table V.  Suggested modified and new quality indicators for characterisation and SAR optimisation.

Indicator	Formula	Unit
13-m	with Vtarg the target volume, and SAR5tot the SAR exceeded in 5% of the total volume	–
14-m	with SARntarg the SAR exceeded in n% of the target volume	–
16-m	with SAR(V1) the average SAR in V1. V1 is defined as first volume percentile	–
17a	with Vhs = V1	–
17b	with SARhs the SAR values inside V1	W/kg
17c	with V(n) the volume of an element n inside a region	W/kg
17		–
18		–

Figure 4. Correlation of all indicators with target temperatures. Each indicator is individually plotted in a separate subfigure. Above each subfigure, the indicator number is depicted. R² correlation coefficients in each subfigure indicate the amount of correlation between the respective indicator and the corresponding T50_targ value.

Figure 5. T50_targ at the maximum patient tolerance, after optimisation with each indicator. The average T50_targ over 36 patients is shown, together with SE of means as error bars.

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