10 General Aspects of Head and Neck Brachytherapy

General Aspects of Head and Neck Brachytherapy

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THE GEC ESTROHANDBOOKOF BRACHYTHERAPY | Part II Clinical Practice Version 1 - 30/01/2019

and graphical methods [7] which is possible in stepping source systems. A cautionary measure is to keep the hyperdose sleeves (200% isodose volumes) as thin as possible and not confluent with other 200%hyperdose sleeves [3]. Dose Non-uniformity Ratio (DNR = V 150 /V 100 ) should be equal to or lower than 0.36 [22]. However, optimal DNR values may not always be obtained in HN BT due to the anatomical constraints of the HN area that reduce the number of catheters implanted compared with other disease sites such as breast, prostate or soft tissue sarcomas. All these limitations may lead to an inadequate geometry with DNR values in the 0.35 - 0.50 range that can be acceptable in small CTVs but not in large volumes. The University of Navarra group showed that Grade ≥3 and ≥4 complications increased with TV 150 values of 13cm 3 or greater (p = 0.032) in a series of patients treated with PHDRB. No specific constraints could be obtained for the different OARs analyzed. The Erlangen group showed a correlation between soft tissue necrosis and V85 (tissue volume encompassed by the 85% isodose line) [16]. Dose through the skin should be avoided if possible - except when using surface molds or tumors with skin involvement [2]. Although clear surrogates of OAR damage are lacking in HN BT, it is reasonable to keep the dose in the neighbouring dose-limiting organs as low as possible. Adoption of OAR constraints used for HN IMRT or V-MAT is a prudent alternative in the absence of HN BT-specific OAR constraints (Section 8.4). 8.6. Dose Calculation The dose calculation in brachytherapy is typically performed using the TG-43 formalism [24]. Nevertheless, this highly standardized dose calculation method does not take into account tissue inhomogeneities or finite patient dimensions. Since a few years model-based dose calculation algorithms became commercially available in brachytherapy [25]. So far it seems that for head and neck HDR techniques the impact of the dose calculation algorithm on dose distributions of the CTV is limited [26]. 8.7. Treatment Recording and Reporting The ICRU report No. 58 [27] describes general recommendations for reporting interstitial therapy. This report substitutes the basal dose from the Paris system by the mean central dose (MCD) while the reference dose of the Paris system (85% of the basal dose) is substituted by the minimal target dose (MTD) that does not necessarily matches with the reference isodose from the Paris System. In the absence of more-specific recommendations, these general concepts could be followed in HN BT: a. To describe the clinical volumes, such as the size of the GTV and CTV and the criteria used in their definition. b. To describe the technique used for implantation. c. To specify the source (or sources used) in HN BT including the RAKR (Reference Air Kerma Rate) and TRAK (Total Reference Air Kerma). d. To describe the time-dose pattern. e. To describe the clinically meaningful dose coverage indices such as the CTV D 100 , D 98 , D 90 ,… (Minimal dose received by at least 100%, 98%,90% of the CTV, etc.). The CTVD 90 has become of particular relevance in different brachytherapy sites such as prostate and cervical cancer and should be used as the reference standardwhen reporting brachytherapy doses.The reporter should give preference to those parameters that have been consistently related with clinical outcomes.

f. To describe the OARs, including size and criteria used in their delineation (contrast enhancement, etc.) g. To describe clinically meaningful OAR constraints such as D2cc, etc. (minimal dose received the most exposed 2cm 3 of that particular OAR). h. The absorbed dose for target volumes and OARs should be given per fraction, and EQD2 should be calculated for the dose per fraction as well as for the total dose. i. To describe quality indices such as: Dose Homogeneity Index (DHI) = (V 100 – V 150 ) / V 100 Dose Non-uniformity Ratio (DNR) = V 150 / V 100 Where V 100 is the volume of CTV encompassed by the prescription 150 is the volume of CTV encompassed by 150% isodose and V

of the prescription isodose.

Conformity Index (CI) = CTV/TV

Where TV= Tissue Volume encompassed by the prescription isodose and CTV = Target volume.

9. BRACHYTHERAPY MODALITIES.

9.1. LDR LDR is defined as a dose rate lower than 1Gy/h. LDR was usually delivered by temporary sources (i.e, the sources were placed and removed after a specified time) in the formof continuous irradiation (no pulses). The majority of the temporary LDR implants used 192 Ir, although 125 I implants have also been occasionally used. The typical dose rate was 0.5Gy/h. Permanent seeds are a special type of LDR brachytherapy in which there is not implant removal and will not be discussed here. The largest series of HN BT were reported with LDR. 9.2. PDR The shortage of iridium wires as well as radiation safety concerns led to the use of other dose rates such as PDR brachytherapy. In PDR brachytherapy the continuous delivery of LDR irradiation is replaced by hourly pulses of irradiation delivered at a higher dose rate lasting a fewminutes. PDR has the advantage to be equivalent to LDR under the conditions of one hourly pulse, treatment 24 pulses per day and a dose-rate of 0.5 Gy/hour. [28] The total recommended dose for PDR is the same as for LDR brachytherapy. PDR brachytherapy doses of 25-30Gy are customarily used in combination with 45-50Gy of EBRT. If PDR brachytherapy is used alone, a total dose of 65-70Gy is recommended. Because of local logistics some authors have chosen a PDR schedule with less than 24 pulses per day (i.e, 12 daily pulses). With this schedule, a 15% reduction in the total dose is usually recommended i.e. 60Gy when brachytherapy alone is performed. These recommended doses are subject tomodifications, depending on tumor size, location and dose constraints. For instance, a tumor located in the lip can be treated with higher doses than a tumor of the floor of mouth, where the proximity of the mandible limits the total dose that can be safely reached.