ESTRO 2024 - Abstract Book

S364

Beachytherapy - Physics

ESTRO 2024

Purpose/Objective:

We have previously published our experience, relating to survival 1 and functional outcome 2 , as a national referral centre for brachytherapy in children. Here we detail the development of the planning process of brachytherapy in children with localised rhabdomyosarcoma of genito-urinary and other pelvic sites over the last 14 years.

Material/Methods:

Following review of prior imaging, transrectal and abdominal ultrasound is used to guide the placement of flexible catheters in the tumour. These are held in place by a plastic template affixed to the perineum. A thermoplastic skirt-like shell covering the pelvic area and hold the legs apart aims to reduce movement and the risk of catheter displacement. A planning CT scan is then performed, and the CTV is delineated along with nearby OAR. The brachytherapy physicists reconstruct the catheters using the Oncentra Brachytherapy (OCB) Planning System (Elekta) aiming to produce a robust plan to be treated for 5 fractions. To account for systematic changes due to oedema, the clinician will typically grow the sup slices of the CTV by 5mm (if not close to an OAR). Typically, the goal is to achieve a 90% isodose to maximal target coverage, and to avoid getting hotspots above 150% close to the urethra. The dwell positions are activated every 3-5mm depending on the volume of the target. The dwell times are then altered manually for each dwell position to create a robust plan that achieves target coverage and keep the OARs within tolerance levels. Following qualitative verification of the catheter placement using a C-arm planar image, the plan is then delivered with an HDR 192-iridium source using the Flexitron afterloader unit (Elekta). Prior to fractions on day 2 and day 3, the catheter placements are checked by a verification CT scan where the position of each needle is compared to the planning CT. The displacement of each catheter in the cranial-caudal direction is evaluated visually and assessed quantitatively. The displacement values are entered into the plan to offset each catheter to visualise the effect of movement on the dose distribution. The two plans are then compared side-by-side. If the dose coverage and OAR tolerances are not compromised, the plan is delivered. This is followed by a retrospective reconstruction of the plan which accurately shows the distribution for a better informed decision to be made for subsequent fractions. If major movement of the catheters has occurred, then these may be repositioned, and re-outlining of the volumes and re-planning is performed for more accurate treatment delivery of the subsequent fractions. The catheters are removed following the final fraction. The prescription dose is 27.5Gy in five fractions over 3 days, with one fraction on the first day and two fractions on the subsequent two days.

Results:

35 patients, 21 male and 14 female, median age 2 years 11 months (range 8 months to 17.6 years) have been treated. The sites were bladder/prostate 10, prostate 9, bladder 4, cervix 4, vagina 2, perineum 2, urethra/bladder 2, vagina/urethra 1 and vulva 1. The CTV volume varies in size with an average of 20.5cc.

Site

Number catheters

of

CTV Volume (cc)

CTV V 100% (%)

COIN

Bladder

7

10.9 24.2

NR

82.3 92.8

10

0.71 0.44 0.74

5 9

8.1

95.52 97.39

15.6