16. Cervix cancer - The GEC-ESTRO Handbook of Brachytherapy

Cervix cancer

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THE GEC ESTRO HANDBOOK OF BRACHYTHERAPY | Part II Clinical Practice Version 1 - 01/09/2023

to facilitate communication between Xray-based and volume image-based BT approaches. The TRAK is the integral of the reference air kerma rate from all sources at a distance of 1 m from the source over the treatment duration and is linked to the integral absorbed dose delivered to the patient. The concept of a product of source strength and treatment time is equivalent to prescription and reporting of radium treatments in units of mgh. Nowadays, TRAK is not used for prescription, but mainly for recording and reporting. Regarding HDR and PDR brachytherapy treatments, TRAK is defined per fraction and is equal to the product of the reference air kerma rate of the stepping source (source strength) and the sum of dwell times at all source dwell positions (total treatment time). TRAK is a purely physical parameter which is useful for radiation protection purposes but cannot be directly associated with a given biological effect because it does not take into account the dose distribution, fraction size, and dose rate. Within a given dose rate (e.g. HDR) and fractionation schedule, the TRAK should be used for comparison between treatments. Moreover, TRAK can be used to estimate ISVs (e.g. V60Gy, V75Gy, V85Gy) using a simple formula [44, 45] which can then be used to compare treatment intensity across different fractionation schedules and to link current practice with IGABT with historical experience. The dose to Point A (Figure 17) is the most commonly reported dose parameter in the published literature. Historically, it has been shown to correlate with tumour control and it has therefore been used for direct comparison of the dose delivered to different patients in different departments with different fractionation schedules and dose rates and different imaging modalities. The Point A dose is not dependent on target-volume contouring and is a useful safety guide to overcome underdosage due to contouring error. Point A dose and ISV (cm 3 ) can be regarded as indicators of treatment intensity. However, for combined IC/IS BT, Point A is less useful as it is located within a region of very heterogeneous dose distribution and can be highly influenced by a single dwell position within a needle. If the IS component is limited to one lateral part of the implant, the absorbed dose to Point A on the contralateral side can still be used for dose evaluation during treatment planning. For OAR, reference points for the rectum and bladder were proposed in ICRU Report 38 [26] to estimate the absorbed doses to these critical organs; these points were subsequently integrated into ICRU Report 89 [12] (Figure 18). In ICRU Report 89 [12] , the rectal reference point was renamed the ICRU recto-vaginal point as the dose at this point, which is defined as 5 mm from the posterior vaginal wall, was shown to correlate with vaginal stenosis/shortening [39]. In addition, new vaginal reference points were proposed (Figure 19) relative to the vaginal applicator and also the posterior-inferior border of the pubic symphysis (PIBS) which indicates the dose in the lower vagina [46]. 10.1.2 DVH parameters The cumulative DVH provides more comprehensive information about volume irradiated as a function of dose in an individual patient compared to point doses. However, recording and reporting the entire DVH for each patient is not practical for day-to-day clinical practice or in summarizing a series of patients. Single parameters derived from the cumulative DVH have therefore been selected to correlate with biological effects in the target and OAR (Figure 20). For the CTV-T_HR, the Gyn GEC ESTRO group originally recommended the reporting of D100% and D90%, defining the

minimum doses delivered to 100% and 90% of the target volume, respectively. These DVH parameters reflect the dose in the outer region of the target. D90% is more stable with respect to random uncertainties when compared with the absolute minimum target dose, D100%. However, due to the significant dose gradients, D90% might look favourable even though 10% of the target volume receives a much lower dose. D100% is very dependent on volume reconstruction and dose sampling in the treatment–planning system [47]. A more robust metric is the near-minimum dose D98%, in which 2% of the target volume receives less than this dose (D98% is also proposed for IMRT treatments in ICRU Report 83). D98% has therefore replaced D100% as a recommended parameter for reporting the dose to the CTV-T_HR in ICRU Report 89. D98% and D90% are also recommended for reporting dose to the CTV-T_IR if used for prescription. High-dose volumes around the IC applicator are regarded as important because they probably contribute to the excellent local control observed, even for large-volume disease [38]. The BT dose heterogeneity is substantial in the target region, with typical dose gradients of 5-25% per mm, which means that a considerable part of the tumour will be irradiated to more than 200% of the absorbed dose to Point A [41]. For the evaluation of these high-dose volumes, the DVH parameter D50% is recommended. For OAR, biological harm can be related to small normal-tissue sub-volumes (located mainly in the mucosa and sub-mucosa of the organ walls) irradiated to high doses (e.g. >65 Gy) as well as larger volumes of organs irradiated to intermediate dose levels (e.g. 50-60 Gy). The DVH parameters D0.1 cm 3 and D2 cm 3 represent hotspots in the organs, while body DVH parameters such as V60Gy3 are useful for assessment of larger volumes irradiated to intermediate doses. These OAR parameters were originally recommended by the GEC ESTRO GYN group in 2006 and have since been incorporated into ICRU Report 89. 10.2 Dose rate Traditionally, cervical BT was delivered at a continuous low-dose rate (LDR), typically 0.5 Gy/ h. Currently, it is more commonly delivered in 3-4 large (e.g. 7 Gy) fractions at high-dose rate (HDR) over a few days or weeks. Alternatively, it can be delivered in 1-2 fractions at pulsed-dose rate (PDR) with each fraction comprising a large number (40–80) of small pulses of 0.5-1 Gy (with a duration of minutes) separated by short time intervals of 1-1.5 h to simulate the biological effects of continuous LDR irradiation (mean PDR dose rate of an entire series of pulses is typically less than 0.6 Gy/h). 10.3 Fractionation Many different HDR and PDR time–dose schedules are used in clinical practice and the choice of schedule is often determined by the available resources in each department. For PDR BT, delivering the same total absorbed dose over a longer treatment time or with a larger number of pulses will reduce the dose rate. This in turn widens the therapeutic window as the EQD2 for OAR late effects is influenced by dose rate more strongly than the tumour EQD2. Increasing the number of pulses may be useful in cases where target coverage is compromised due to OAR dose constraints. Resource optimisation might involve fewer, but larger, fractions delivered by increasing the number of hourly pulses while keeping the average dose rate sufficiently low [48-50]. However, caution is needed