Comparison of integral doses to normal tissue and organs at risk between interstitial high-dose-rate brachytherapy and modern external-beam radiotherapy techniques in breast and head and neck cancer patients

Patients and treatments

We selected treatment plans of patients who had previously been treated with irradiation to two tumor sites (breast and head and neck) at our institution. Thirty-four women with early-stage breast cancer were treated with multicatheter interstitial HDR BT using an Ir-192 source. In a previous study, the BT treatment plans were compared with the IMRT plans for organs at risk dosimetry [14]. In the comparative treatment plans, the same CT data and contours were used, with the same dose prescription of 7 × 4.3 Gy, in line with our clinical practice. In the BT plans, the planning target volume (PTV) was equal to the clinical target volume (CTV), while in the IMRT plans, the PTV was generated from the CTV with an isotropic margin of 5 mm.

The head and neck (H&N) patients included two groups. In the first group, 38 patients with mobile tongue, floor of the mouth, and base of the tongue cancer were treated definitively with interstitial HDR BT. In addition to the BT plans, VMAT treatment plans were also prepared using the same CT data and organ contours [15]. The second group included 20 patients with tongue and floor of the mouth tumors who received postoperative interstitial HDR BT [16]. VMAT and stereotactic CyberKnife (CK, Accuray Inc., Sunnyvale, CA, USA) plans were created for these patients in addition to BT plans. In clinical practice, different fractionation schemes are used for BT and EBRT. In order to make the comparison more realistic, the biologically effective dose (BED) was calculated for each treatment method using an α/β value of 3 Gy. A dose prescription of 15 × 3 Gy (BED3 = 90 Gy) was used in BT plans, 35 × 2 Gy (BED3 = 116.67 Gy) in 38 H&N definitive plans, and 30 × 2 Gy (BED3 = 100 Gy) in 20 postoperative H&N plans. With SBRT on CyberKnife, the schedule was 5 × 7 Gy (BED3 = 116.67 Gy). The subscript “3” in BED3 refers to α/β = 3 Gy. In the VMAT plans a 3-mm margin and in the CK plans a 2-mm margin was added to the CTV to create the PTV.

For BT the Oncentra Brachy v4.3 (Elekta, Brachytherapy, Veenendaal, the Netherlands), for IMRT the Eclipse v11 (Varian Medical Systems, Palo Alto, CA, USA), and for CK the Precision 2.0.0.1 (Accuray Inc., Sunnyvale, CA, USA) planning systems were used.

Integral doses

The NTIDs for three normal tissue volumes and organs at risk were calculated in the current study. We first created normal tissue structures from the volumes surrounded by the 10%, 5%, and 2% isodose surfaces, and then subtracted the volume of the PTV from them to create the volumes of NT_V10, NT_V5, and NT_V2, respectively. Figure 1 shows the three volumes on a transverse CT slice of a breast cancer patient. The same volumes of a H&N cancer patient are presented in Fig. 2. In the planning systems, the average dose for each generated volume was calculated and then multiplied by the volume to obtain the NTID value. In the literature, Gy · liter is the most commonly used unit of measurement for NTID, but joule (J) is also applied [6, 7, 12, 17,18,19,20]. The volumes can be measured in cubic centimeters, doses can be expressed as a percentage of the PD (relative dose), and the NTID can be calculated by multiplying these, allowing for a direct comparison when the dose prescription varies between different treatment modalities [9]. Another way to compare the NTID between treatment schedules with a different dose per fraction is to use the BED instead of the physical dose [5]. In the current study, BED3 was calculated for each treatment method given in Gy2, where the subscript “2” represents the reference dose of 2 Gy per fraction, and the unit of the NTID is Gy2 · liter [5]. In addition to normal tissues, the integral doses to organs at risk were also calculated and compared. For breast cases, the ipsilateral lung and the ipsilateral non-target breast were evaluated; for H&N cases, the salivary glands (bilateral parotid and submandibular glands) were evaluated.

Fig. 1figure 1

Volumes of normal tissue irradiated by 10% (NT_V10), 5% (NT_V5) and 2% (NT_V2) of the prescribed dose for a brachytherapy and b intensity modulated radiotherapy for a breast cancer patient

Fig. 2figure 2

Volumes of normal tissue irradiated by 10% (NT_V10), 5% (NT_V5), and 2 % (NT_V2) of the prescribed dose for a brachytherapy, b volume modulated radiotherapy and c stereotactic CyberKnife radiotherapy for a head and neck cancer patient

Statistical analysis

Descriptive statistics were used to characterize the volumes, mean doses, and integral doses. The Shapiro–Wilk test was used to check the normality of the data distribution. As most of the parameters were not normally distributed, the Wilcoxon matched-pairs signed-rank test was used for all comparisons. The difference was considered significant if the p-value was <0.05. Statistical evaluation was performed using Prism 8.0.1 software (GraphPad Software Inc., Boston, MA, USA).

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