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

8.5. Dose Prescription and Dose Evaluation The different steps involved in dose prescription and dose evaluation should be always carried out in a Dose Volume Histogram scenario with careful attention to small details; this includes a triple assessment of the dose to the CTV, doses at OARs and dose inhomogeneity. The prescription dose is usually the minimumdose received by the CTV or a CTV surrogate (i.e., the D 90 ≥ 100%, V 100 ≥ 90%) and the total dose administered will depend on the clinical indication of HN BT (Section 6). The former LDR 192 Ir-wire schedules remain the reference standard because most clinical data on HN BT was produced with that methodology and technology. Therefore, the current fractionation PDR and HDR schedules should aim to be to be at least biologically equivalent to the reference LDR programs. The LQ (Linear-Quadratic) model with an α/β ratio of 10 Gy and a repair half-time of 1.5 h can be used to calculate equi-effective doses of different fractionation schedules [15]. The EQD2 concept that is used to normalize any given fractionation to 2-Gy equi-effective doses should always be used with caution in brachytherapy because the calculation does not take into account the time effect on repopulation and the implications that this may have in extremely short treatment schedules. As a result, EQD2 calculations for HNBT schedules result in rather low values (i.e, 32 Gy in bifractionated 8 x 4 Gy in 4 days yields an EQD2 of 37.3Gy) as compared to classical fractionation where 66Gy results in the same EQD2 of 66Gy. However, the clinical results produced are much better than the predicted by such low EQD2 values and are comparable with data obtained from accelerated external beam radiation schedules. For instance, the CHART schedule of 54Gy in 36 fractions of 1.5Gy delivered over 10 days is clinically equivalent to a normofractionated regimen of 66 Gy in 6.5 weeks [17] in spite of a calculated EQD2 of only 51.7 Gy. Similarly, the EORTC acceleration study showed a 13% local control advantage (p=0.02) with accelerated 72 Gy in 5 weeks (EQD2 of 69.6Gy) compared to 70 Gy delivered with standard fractionation (EQD2 of 70Gy) [18]. Also the data from Dobrowski [19] of 55.3Gy in 33 fractions of 1.67Gy (EQD2 =53.7Gy) in are in line with the above data showing that accelerated radiotherapy produces equivalent results with lower EQD2 values. In addition to the time-effect limitation, the EQD2 formalismdoes not incorporate inhomogeneity indices, and therefore, the calculated dose values underestimate the clinical biological effect [20]. The impact of inhomogeneities can be calculated by complex dose integrating formulas such as EUD calculations [21] or by simpler dose inhomogeneity surrogate indices as DNR (=V 150 /V 100 ) or other relevant indices. Finally the higher RBE attributed to lower energy irradiation (1,3-fold for 192 Ir) should also be taken into account to explain the low EQD2 values related to short BT regimens. Efforts should be made to develop an EQD2 adapted formalism that incorporate these crucial parameters into the model to improve comparability with external beam schedules. As long as such comprehensive model is not available, we recommend despite the well recognized shortcomings nevertheless to record and report physical doses as well as EQD2 values to allow for comparison between the effects of different dose rates and fraction sizes since this is the currently major accepted model. During dose evaluation, the inhomogeneities need to be minimized following general rules such as those derived from the Paris system [3] with additional optimization if needed, mainly by geometrical

the lips, the mobile tongue, and the floor of mouth, is planned, to reduce the dose to the mandible and prevent osteoradionecrosis [15].This shielding system is made of 2mm thick lead encompassed by plastic protection (figure 10). It must be taken into account that dose absorption in the bone is proportional to the atomic number (Z 3 ) which is until now included in the TG 43 formalism. The Erlangen group has shown an increased risk for osteoradionecrosis using PDR brachytherapy for a Prescription Dose greater than 64.2 cGy/pulse (p = 0.028) and a High Dose greater than 80.3 cGy/ pulse (p = 0.037)[16]. The carotid vessels should be contoured if the implant has been placed very close to the vessel wall (i.e, neck implant) as well as in patients who have been previously irradiated, due to the risk of vessel rupture.The preimplant CT/MRI as well as the atherosclerotic plaques (if present) are helpful in defining the location of the carotid artery and its main branches. The use of intravenous contrast is obviously preferable for vessel definition but may not be feasible in the immediate post-implant or postoperative period. The definition of other organs such as the parotid glands, hard palate, lens, swallowing muscles, eyes, etc. depends on the location of the implant and the special characteristics of the patient. Due to the paucity of data relating dosimetric parameters and toxicity it is important that the leading Brachytherapy societies issue guidelines to create a panel of normal tissues to be contoured in all head and neck cases. This information can be shared among treatment groups to generate dose volume constraints. For instance, the Rotterdam group1 described doses to the 5 main swallowing muscular groups (SCM, MCM, ICM, CPM, OI) and found that the mean dose to the SCM and MCM correlated with dysphagia in a group of patients treated with HN Boost or IMRT boost. The HN BT subset had a lower rate of dysphagia derived from a lower dose (that was a consequence of smaller volumes). Figures 11 and 12 describe the 3D view and DVH obtained with modern TPS. 8.4. Catheter Reconstruction Catheter reconstruction is an important step in the planning process of HN BT. Catheters must be well visible in the image set and need to be distinguished from other foreign objects such as clips, seeds, drains, etc. Depending on the catheter manufacturer, these may appear as low or high-signal linear structures. The use of thin metal wires to enhance the visibility of plastic catheters is rarely required nowadays buymay be a practical solution in difficult cases. A CT slice thickness of 0.2–0.3 cm (in small tumors 0.1 cm) should be adequate to accurately reconstruct each individual catheter. Numbering of catheters in the treatment planning system must always be done following a diagram or a picture of the numbering of the actually implanted catheters. Numbered buttons at the catheter entry site are the most practical and reliable way to avoid confusion. Institutional standards in the numbering of the catheters (i.e, numbering left to right referenced to the hands of the operator manipulating the implant, lower row first, upper row last, etc. ) may help to minimize treatment misadministration.

- 1In lovingmemoryofProfessorPeterLevendagwhopassedawayduring thewritingof thischapter.