ESTRO 2024 - Abstract Book

S3623

Physics - Dose prediction, optimisation and applications of photon and electron planning

ESTRO 2024

Material/Methods:

From October 2019 to October 2023, 100 patients were treated with VMAT TBI at our institution. Organ sparing depended on the regimen: myeloablative (lungs, kidneys, and lenses); non-myeloablative (lungs, kidneys, lenses, gonads, brain, and thyroid). Treatment planning was performed using Eclipse Scripting Application Programming Interface (ESAPI) auto-planning scripts. Data on patient, treatment details, and dosimetric indices were collected. Robustness was evaluated for the first ten patients by performing local and global isocenter shifts of 5 mm. Treatment was delivered using IGRT for every isocenter and every fraction. In-vivo measurements were performed on the matchline between the VMAT and AP/PA fields and on the testes for the first fraction.

Results:

Median patient age was 13 years (range, 1– 64). The patient height and width range were 83.6-197.3 cm and 24.9 60.3 cm, respectively. Forty-eight percent of patients were treated using myeloablative and 52% nonmyeloablative regimens. For all patients the lungs, lungs-1cm, and kidneys Dmean were consistently spared to 59.8±4.6%, 43.1±6.4%, and 70.7±7.5% of the prescription dose, respectively. Gonadal sparing (Dmean=31.1±6.0%) was achieved for all patients with benign disease. In addition, brain and thyroid was spared for these patients. For nine patients, VMAT TBI offered additional organ sparing (liver, heart, and previously irradiated areas) and possibility to perform the simultaneously integrated boost. The average PTV D1cc was 120.3±6.4% for all patients. PTV D1cc correlated with patient height and width (R2 = 0.62 and 0.53). Seventeen patients (17%) with height <116cm were treated with 3-isocenter VMAT only plans, 83 patients (83%) were treated with 4-7 isocenters depending on the height and width: 3-4 isocenter VMAT plans in head-first-supine (HFS) position and 1-3 AP/PA plan in feet-first supine (FFS) position. Custom-made rotational platform, Spinning Manny, was developed to change the patient orientation from HFS to FFS. For the first 10 patients, robustness evaluation showed that the PTV Dmax and lungs Dmean are insensitive to small positioning deviations between the VMAT isocenters (1.1±2.4% and 1.2±1.0%, respectively). The average matchline dose measurement indicated patient setup was reproducible (96.1±4.5% relative to planned dose). For the first 35 patients, treatment time, including patient setup and beam-on, was 47.5±9.5 min.

Table 1: Plan quality metrics achieved for 100 patients treated with VMAT TBI

Constraint

Average (% of Rx)

σ

Lungs

Dmean < 55%

59.8%

4.6%

Lungs-1cm

Dmean < 40%

43.1%

6.4%

Kidneys

Dmean < 75%

70.7%

7.5%

Testes/Ovaries

Dmax < 30%

31.1%

6.0%

Brain

Dmean < 75%

74.9%

7.4%

Thyroid

Dmean < 75%

74.1%

10.7%

PTV

D1cc < 120%

120.3%

6.4%