28 Primary and secondary liver malignancies

Primary and secondary liver malignancies

10

THE GEC ESTROHANDBOOKOF BRACHYTHERAPY | Part II Clinical Practice Version 1 - 15/07/2022

catheter is known and can be checked against the manual catheter reconstruction, namely the distance of the end of the angiography sheath to the (radiopaque) tip of the brachytherapy catheter on the CT.The angiography sheath itself can be seen with great reliability, as it is fully radio-opaque (see Figure 6). The catheter reference point is commonly defined by radio- opaque markers within the tip of the brachytherapy catheter or alternatively CT/MR markers are used, which are inserted into the brachytherapy catheters prior to the simulation imaging. After catheter reconstruction, dwell position activation is performed. All dwell positions inside any target volume should be automatically activated if the TPS offers this functionality. Additionally, all dwell positions should be activated, which are for example outside of all PTVs, but which might be helpful to reach distant parts of the PTV not efficiently coverable by other dwell positions or which might help improve OAR sparing. For this purpose the 3D view of catheters and regions of interest can be very helpful. A 2 mmdwell position spacing is commonly used for liver HDR brachytherapy. Dose optimization is either performedmanually or via semiautomatic inverse dose optimization or as a combination of both. It is feasible to generate a first version of the plan via automated dose planning and then proceed with manual fine adjustment of dwell times, especially in the case of large PTVs with several catheters. When optimizing the dose, as a first step it should be aimed to achieve full coverage of the PTVwith the prescribed dose. In the following steps, the reduction of overall dwell time and sparing of OARs will be achieved. Compared to other entities, where a template-based needle placement is possible (e.g. interstitial multi-catheter HDR brachytherapy of the breast [52]), the dose distribution in liver brachytherapy plans is usually more inhomogeneous with very high central doses. This is the reason why homogeneity indices are not a useful metric in HDR brachytherapy of the liver and should not be used to judge the quality of a treatment plan. It is more appropriate to aim for complete dose coverage of the PTV (important DVH parameters are the minimum dose D100%, or near minimum dose D98% [as recommended in ICRU reports 83 [53]] and 89 [54], in addition to D95% and D90%), while meeting all OAR constraints (see table 2). In other words, dose heterogeneity is accepted to maintain optimal conformity. In further plan optimisation steps, attempts can be made to reduce the overall dwell time. Neighbouring dwell positions should not exhibit extreme variations of dwell time in order to increase the plan robustness against spatial uncertainties but the dwell time modulation restrictions should also not be too strict [55]. To increase homogeneity, the insertion of numerous catheters would be necessary, depending on the volume of the treated lesion. However, due to the potential risk of procedural complications during catheter placement, usually only a limited number of catheters are inserted as compared to other anatomic districts (e.g. breast, prostate). The trade-off between the loss of full dose coverage and efficacy of the treatment or the acceptance of possible higher risks for side effects must be evaluated in close collaboration with the physician in each specific case. However, usually, the sparing of OAR has priority over full dose coverage. The puncture tract is frequently irradiated with ~5 Gy to the catheter surface up to the skin of the patient to avoid seeding along the puncture tract (see Figure 7) [56]. During dose optimisation the planning physicist should be aware of the underlying dose calculation algorithm and its limitations and uncertainties. In case of the TG43 formalism, larger dose calculation uncertainties are present in the lower dose region and tissue heterogeneity effects can cause large errors at tissue-lung or tissue-air interfaces (patient surface) due to different

scatter conditions. This will result in an overestimation of dose inside the tissue close to a tissue-air interface for example. Larger TG43 dose calculation errors also occur in high-Zmaterial. In the mid- and high-dose region inside the liver, the TG43 formalism yields accurate results. After plan approval, the technical plan data is transferred to the brachytherapy control computer. The integrity of this data transfer must be validated for each plan and plausibility checks should be performed. All relevant parameters of the treatment planmust be cross checked with the ones of the brachytherapy device, e.g. the current source activity or overall irradiation time. The duration of the irradiation is determined by the size of the target volume, the number of catheters, the planned dose and the activity of the source (diminishing with time due to radioactive decay); typically, it is between 5 and 40 minutes for a source with an activity of 370 GBq, but can even exceed 90 minutes in cases of very large target volumes. The dose is usually applied in a single fraction. The prescribed dose depends on the tumour histology. An overview is given in table 2. The dose prescription aims to achieve a full target coverage with the aim that the prescribed dose should encompass 100% of the PTV volume. Therefore, the D100%, D98%, D95% and D90% dose values of the PTV are usually of primary interest when evaluating the target coverage and should be reported in addition to the median PTV dose. Generally, the dose reporting principle is based on the recommendations for IMRT treatments of ICRU report 83 [53] and on the recommendations for brachytherapy of the cervix of ICRU report 89 [54]. Most metrics can be directly used in HDR brachytherapy of the liver, but for example the near maximum dose D2% of the PTV loses its validity, since extremely high dose values can occur close to the source inside the target volume. Since small regions of very high dose can considerably influence the PTVmean dose, the PTVmedian dose is favourable, although the PTV mean dose might also be of prognostic value and can be reported additionally [57]. For the same reason, the relative hyperdose volumes inside the PTV (e.g. V150%, V200% or V300%) can be reported optionally. Regarding constraints for organs at risk, most constraints are D0.1cm 3 or D1.0cm 3 dose constraints (e.g. esophagus, stomach, duodenum, bowel, heart, vessels, bile ducts, spinal canal), while for parallel organs (e.g. liver, kidney) usually volumetric constraints are applied. Level 2 dose reporting (DVH parameters [see table 2], treatment technical data and additional information about the TPS) is the current standard of care in modern HDR brachytherapy. The typical dose rate of a newly delivered iridium-192 source with a source strength of 40 kU (ca. 370GBq) is > 7Gy/minute at 1 cm from the source and thus, radiobiological effects are comparable to that of strongly hypofractionated or even single fraction external beam radiotherapy treatments, such as stereotactic body radiotherapy or stereotactic radiosurgery. It should be noted that the average gamma energy emitted by an iridium-192 brachytherapy source is approximately by a factor 15 lower than the average photon energy of a 6 MV beam from a linear accelerator. 10. DOSE, DOSE RATE, FRACTIONATION