The brachytherapy part of GYN treatment: an image-guided boost for cervical cancer after pelvic external beam, and vaginal cuff brachytherapy after hysterectomy. The planning concepts here — the high-risk CTV, D90, D2cc and adding doses in EQD2 — are what make GYN brachytherapy different from external beam planning. Cervix — Insertion & Imaging After pelvic external beam, applicators are placed in the uterus and vagina and the patient is imaged with them in place. Each insertion is planned on its own images. Applicators An intracavitary applicator — a tandem in the uterus with ovoids or a ring against the cervix — or a hybrid or template-based interstitial implant that adds needles into the tumor and parametria. The choice depends on the anatomy and how much tumor is left after external beam. When the high-risk CTV can't be covered with a tandem and ovoids or ring without exceeding organ limits, adding interstitial needles can improve coverage and reduce organ dose. Insertion Usually done under general or spinal anesthesia in lithotomy, after an examination of the residual tumor. A Foley catheter goes into the bladder, the cervix is dilated (often with ultrasound guidance), and the vagina is packed to hold the applicator; a rectal retractor or spatula can add distance from the rectum. Packing and fixation keep the applicator where it was imaged — the plan is only valid if the applicator doesn't move between imaging and treatment. Protocols ask for an applicator check before treatment and re-planning if it has moved. Image fusion — MRI at brachytherapy Planning images with the applicator in place — CT or MRI, slices 3 mm or less. MRI is suggested for at least the first or second insertion, using T2-weighted images in planes aligned with the applicator; later insertions may be planned on CT with the earlier MRI used as a guide. MRI shows the residual tumor and its relation to the cervix and parametria, which is why the GEC-ESTRO recommendations base the target volumes on it; an NRG consensus atlas compares how the high-risk CTV is drawn on CT and on MRI. Moving the patient between MRI and CT adds uncertainty about applicator and organ position, and one template discourages deformable registration of MRI to CT because it can distort the target. Applicator reconstruction The channels the source travels through are defined in the planning system, using dummy source markers or the applicator as seen on the images. An error of a few millimeters in where the source dwells moves the high-dose region, and with it the dose to the bladder, rectum and sigmoid. On MRI the applicator can be clearer on some sequences — one template notes T1 images may show less titanium artifact. Cervix — Target Volumes Image-guided brachytherapy uses volumes defined at the time of each insertion, not at diagnosis. No planning margin is added. GTV-res Residual tumor at brachytherapy The macroscopic tumor still present at the time of brachytherapy, from examination and MRI. Residual tumor carries the highest risk, and EMBRACE II gives it its own dose aim (D98). HR-CTV High-risk CTV The residual GTV plus the whole cervix and any presumed tumor extension beyond the cervix at the time of brachytherapy. The HR-CTV is the volume the brachytherapy dose is prescribed to, and its D90 — the minimum dose to the hottest 90% — is the main measure of the plan. IR-CTV Intermediate-risk CTV The HR-CTV with a safety margin, defined at the time of brachytherapy. It covers tissue that may hold microscopic disease, at a lower dose aim than the HR-CTV. Point A Traditional dose point A point defined relative to the applicator — 2 cm up from the cervical os along the tandem and 2 cm to the side — from the era of film-based planning. Point A is still reported, and some centers still prescribe to it. It doesn't follow the tumor, so with volume-based planning it is a check, not the target. Adding External Beam and Brachytherapy External beam and HDR brachytherapy are given at very different doses per fraction, so their physical doses can't simply be added. Each is converted to EQD2 — the equivalent dose in 2 Gy fractions — and then summed: EQD2 = D × (d + α/β) / (2 + α/β) where D is the total dose and d the dose per fraction. Targets use α/β = 10 and organs α/β = 3 . The external beam dose is usually counted as its prescription dose, and for organs each brachytherapy fraction's D2cc (the minimum dose to the most exposed 2 cc) is converted and added. Spreadsheets such as the American Brachytherapy Society's dose-equivalence worksheets are commonly used. As an example from the NRG template, 45 Gy in 25 fractions followed by 4 × 7 Gy HDR gives 83.9 Gy EQD2 at point A, and 50 Gy in 25 followed by 5 × 6 Gy gives 90.0 Gy. Cervix — Planning Aims & Limits Totals in EQD2 including 45 Gy in 25 fractions of external beam, from EMBRACE II. Planning aims are what the plan tries to reach; limits are the levels that shouldn't be crossed. | Structure | Parameter | Planning aim | Limit | |---|---|---|---| | HR-CTV | D90 | 90 Gy (and <95 Gy) | 85 Gy | | HR-CTV | D98 | 75 Gy | — | | GTV-res | D98 | 95 Gy | 90 Gy | | IR-CTV | D98 | 60 Gy | — | | Point A | Dose | 65 Gy | — | | Bladder | D2cc | <80 Gy | <90 Gy | | Rectum | D2cc | <65 Gy | <75 Gy | | Recto-vaginal point | Dose | <65 Gy | <75 Gy | | Sigmoid | D2cc | <70 Gy | <75 Gy | | Bowel | D2cc | <70 Gy | <75 Gy | - The NRG cervix template uses the same bladder, rectum and sigmoid D2cc levels as its per-protocol and variation-acceptable criteria, and asks for HR-CTV D90 ≥85 Gy, D98 ≥75 Gy and IR-CTV D98 ≥60 Gy. - EMBRACE II notes that sigmoid and bowel D2cc are meaningful only when the loops don't move between fractions. - For the vagina, EMBRACE II keeps the 140% isodose out of the vaginal tissue as far as possible. Source: EMBRACE II, Pötter 2018, Table 4. Organ limits are planning goals; target coverage and organ trade-offs are the physician's decision. Vaginal Cuff Brachytherapy After hysterectomy for endometrial cancer, the top of the vagina (the cuff) can be treated with a vaginal cylinder — alone, or as a boost after pelvic external beam. - Applicator and imaging. A cylinder (or ovoids) in contact with the vaginal wall. Planning is done on CT, or a standard library plan is used; markers or clips at the apex show whether the applicator sits against the cuff. - Prescription point. Either the cylinder surface or 5 mm depth — the same number means a very different dose depending on which is used, so the point is part of the prescription. - Length. One template treats 3–5 cm of vagina; another, the upper half or upper 4 cm, whichever is greater. - Example schedules (NRG endometrial template): alone, 7 Gy × 3 at 5 mm , 6 Gy × 5 at the surface or 4 Gy × 6 at the surface ; as a boost after 45 Gy, 5–6 Gy × 2–3 at 5 mm . The ABS fractionation compendium lists others. - Plan check. Dose points along both sides of the cylinder every 2 cm should be within about 10% of the prescription. The NRG template asks for no volumetric organ limits here, given the low toxicity and lack of evidence for cuff constraints, but highly recommends checking the dome position. Your protocol sets the schedule, the prescription point and the length treated. Protocols & Literature

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