18. Primary vaginal cancer and vaginal recurrences - The GEC-ESTRO Handbook of Brachytherapy

Primary vaginal cancer and vaginal recurrences

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

(HIPO) algorithm [20) would be the preferred choice, comparable to cervical cancer BT [21]. It allows locking of the loading of the intracavitary implant after initial optimization, and inverse optimisation of the loading of interstitial needles, based on total dose objectives for targets and OARs. For interstitial loading only, the inverse planning simulated annealing (IPSA) algorithm could be applied [22, 23).

TABLE 2 HDR PLANNING AIM DOSES DESCRIBED IN LITERATURE FOR THE TREATMENT FOR PRIMARY AND RECURRENT VAGINAL TUMOURS (27, 28, 30-32) Number of fractions Physical dose per fraction (Gy)

2

5.5 – 8.5

3

6.0 – 7.0

10. DOSE, DOSE RATE AND FRACTIONATION

4

6.0 – 7.0

Traditionally, vaginal tumours were treated with continuous low dose rate (LDR) brachytherapy [24]. However, due to, among others, protection issues this technique has been abandoned and replaced by PDR and HDR. Among the large European centres there is a tendency to treat these tumours with PDR [24]; it resembles the former LDR technique and is thought to better spare the organs at risk and vaginal mucosa leading to less toxicity. In the treatment of large tumours especially, when an intracavitary and interstitial (IC/IS) approach is applied, the preference is to treat with PDR. However, the availability of PDR is declining and an increasing number of centres are using HDR. In general, for superficial tumours treated with intracavitary only techniques HDR can be safely used with brachytherapy planning based on volumetric imaging with CT or MRI, and preferably with the use of a multichannel vaginal cylinder (MCVC) to spare the parts of the uninvolved vaginal wall [25, 26]. In recent years, some small studies have been published about the use of HDR in large tumours treated with an intracavitary and interstitial approach with encouraging results [27-29]. Typically, brachytherapy is given as a boost at the end or after EBRT (typical schedule: 45-50 Gy in 1.8-2.0 Gy fraction). Depending on the logistics in the centre one to three applications are used, and one to five fractions are given per application in the case of HDR. In the case of PDR, in general one or two applications are performed with an hourly pulse dose schedule with total of 30-60 pulses of 0.5-1.0 Gy per pulse. HDR schedules are listed in Table 2 [27, 28, 30-32]. An overall treatment time of less than 50 days should be aimed for, especially in case of primary vaginal cancer or recurrent cervical cancer. According to the GEC-ESTRO/ACROP recommendations for image-guided adaptive brachytherapy (IGABT) in primary vaginal cancer, the dose should be prescribed to a clinical target volume instead of to a dose point [12]. In general, dose is prescribed as the D90 (the minimum dose received by 90% of the volume) of the high-risk CTV (CTV-T HR ), and to the D98 of the residual gross tumour volume (GTV-T res ) and the intermediate risk target volume (CTV-T IR ) which are important parameters to be taken into consideration. Moreover, the D0.1 cm 3 and the D2cm 3 doses for the adjacent organs (rectum, bladder, sigmoid, bowel, urethra and anal canal) should be documented [12]. In case of a complete response, and no visible abnormalities on the MRI that can be contoured as high-risk CTV, alternatively the dose could be prescribed to 3-5 mm tissue depth (depending on the thickness of the vaginal wall) perpendicular to the vaginal wall at the location where the tumour at diagnosis was originated (corresponding with the CTV-T IR ) Due to the rarity of vaginal cancer, no strong recommendations can be given regarding the prescription dose. In principle, dose prescription follows that for cervical cancer and is prescribed to

5

4.0 – 5.0

into account the planning aim dose for the overall vagina, and sparing of nearby organs at risk. Multichannel cylinders again have an advantage to shape the dose distribution at the edge of challenging OARs. Loading of the interstitial implant component should follow the principles of the Paris system (ICRU report 58), in order to keep control of the size of hyper dose sleeves around the active needles. According to a survey among MR-IGABT treating centres in the GEC-ESTRO GYN network, typically up to 150-200% of the CTV-T HR planning aim dose (see paragraph 10) are considered. The volumes of these isodoses should not be confluent with hyperdose sleeves of neighbouring needles. The same survey showed that special attention is given to the surface dose at the IC applicator, taking into account the additional contribution arising from the active needles. Typically, a surface dose planning aim to stay below 150-200% of the planning aim dose for CTV T HR , can be achieved. However, there is no clear clinical evidence to recommend a maximum surface dose yet. The planner should consider that a constraint for the surface dose and high dose islands around the interstitial needles which are based on a percentage of the planning aim dose will be highly dependent on the choice of planning aim and fractionation. Individual assessment of this constraint is therefore essential. For OAR dose reporting, recommendations from ICRU89 can be analogously applied. These include the reporting of volumes, D0.1cm³ and D2cm³ doses for bladder, rectum, sigmoid, bowel and anal canal; and D0.1cm³ and D10% for urethra. In addition, the ICRU-bladder point, and the PIBS and PIBS-2cm points in case of a vaginal tumour located in the proximal part of the vagina, might have additional value to report. D98% should be reported for all target volumes and represents a near minimum dose. D90% is additionally reported for larger target volumes, such as CTV-T HR and CTV-T IR . Total reference air kerma (TRAK) should be reported. All dose metrics should be reported for individually planned fractions and for the total treatment, taking into account an EBRT treatment component. For calculating the equi-effective dose in 2 Gy fractions (EQD2), using the linear quadratic model, the use of α/β =10Gy is recommended for target, and 3 Gy for organs at risk. Inverse planning may be applied, taking into account differences in available algorithms. In order to allow for unequal loading of interstitial and intracavitary applicators for hybrid intracavitary and-interstitial implants, the hybrid inverse planning optimization