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

Figure 16. Comparison of relative TRAK in IC and combined IC/IS implants (Reprinted from Int. J. Radiat. Oncol. Biol. Phys., 106/5, Serban M, Kirisits C, de Leeuw A, et al, Ring Versus Ovoids and Intracavitary Versus Intracavitary-Interstitial Applicators in Cervical Cancer Brachytherapy: Results from the EMBRACE I Study, 1052-1062, 2020, with permission from Elsevier).

approaches, tumours less than 2 cm in radius tend to receive doses significantly higher than the Point A dose while those greater than 2 cm receive a lower dose. Also, the OARs tend to be closer to the implant in small volume tumours, which means that OAR constraints are more often violated for small tumours when standardised approaches are applied. An IC implant produces a high dose region located around the applicators which usually encompasses the GTV-T and the central parts of the CTV-T_HR. Combination treatments with IS needles should aim to maintain the high-dose region around the IU tandem as high doses in the centre of the CTV-T_HR have been shown to be more effective for local control compared with homogenous dose profiles [38]. At the level of the lower cervix and upper vagina, the lateral width of the ISV is larger than the antero-posterior thickness which spares the bladder and the rectum somewhat. Standard dose distributions also deliver very high doses to the endometrium and inner myometrium with more moderate doses to the outer myometrium, parametrium, ureters, nerves, and vessels. The dose to the vaginal mucosa is also high due to the proximity of the sources. In recent years, there has been focus on reducing the vaginal dose to reduce the risk of vaginal stenosis [39, 40]. Loading and high dose regions around the tandem and IS needles in the vaginal dome and ectocervix should be avoided unless required for target coverage of vaginal disease [41]. If the CTV-T_HR does not extend superiorly into the uterine corpus significantly, it seems unnecessary to load the full length of the IU tandem. This is particularly advantageous if there are loops of sigmoid and small bowel close to the tip of the tandem. Reducing the superior dwells in the tandem must be done with caution if only CT is used for imaging as the superior extent of the GTV-T is not easily visualised on CT. As previously mentioned, target configurations extending beyond a certain lateral distance from the tandem or the vaginal sources

will require placement of additional IS sources to improve dose coverage in these regions. The ratio of the loading in the IU, vaginal, and IS applicators influences the spatial dose distribution substantially. Transfer of total reference air kerma (TRAK) from the IC component to IS needles should be done carefully and only as much as is required to achieve target coverage. The size and location of high dose volumes should be monitored. It has been shown that the addition of IS needles can decrease total TRAK in small to intermediate target volumes and can increase total TRAK in large target volumes (>80 cm 3 ) compared to standard loading patterns [41]. Clinical experience of combined IC/ IS implants has largely delivered 75-90% of the absorbed dose via the IC component while limiting the contribution from IS needles to 10-25% [29-31]. The median TRAK per needle is typically around 4% of total TRAK and normally less than 15% of total TRAK (Figure 16). A mono centric study on advanced IC/IS implants using parallel/oblique needles [41] reported an average linear needle TRAK of 1.3% per cm of loaded needle length, relative to total TRAK. However, large variations in loading pattern can occur for advanced implants involving many needles where the total needle loading can reach 50% of total loading in some clinical scenarios. 9.7 Dose optimisation methods At present, iterative forward planning remains the gold standard for dose planning. An initial standard loading pattern is applied, and the dose distribution is evaluated using the constraints for dose points, DVH parameters, and careful anatomical inspection of the isodose distribution. If the resulting dose distribution does not meet the planning aims, modifications can be made manually including adding dwell positions in the tandem, vaginal applicator or IS needles to improve areas of under-dose, adjusting individual dwell times to improve target coverage or decrease OAR doses or scaling the dose distribution by overall multiplication of all dwell times by a factor. After each iteration, the dose distribution is evaluated until either the planning aims are fulfilled, or an acceptable compromise is reached. The final TRAK should be compared with a standard-plan TRAK to measure the scope of the changes relative to an initial loading pattern.