ESTRO 37 Abstract book

ESTRO 37

S604

Gland Right

Dose

Gy

Gy

Mean Dose Mean Dose Mean Dose Mean Dose

46.6 Gy 21.4 Gy 47.6 Gy 31.2 Gy

45.1 Gy 21.1 Gy 47.1 Gy 27.3 Gy

9 Larynx

-1.4 Gy

-0.3 Gy

10 Oral Cavity

Pharynx Constrictor

-0.5 Gy

11

-3.9 Gy

12 Esophagus

PO-1073 The effect of PTV margin reduction on out- of-field doses in paediatric radiotherapy of brain tumours E. Lagoni Juhl 1 , A.M.J. Dam 2 , A.R. Beierholm 3 , D.E. Nygaard 4 , J. Hansen 5 1 Copenhagen University Hospital, Department of Oncology, Copenhagen, Denmark 2 Herlev Hospital, Department of Oncology, Herlev, Denmark 3 Danish Health Agency, Radiation Protection, Herlev, Denmark 4 Metropolitan University College, Bachelor’s Degree Programme in Radiography, Copenhagen, Denmark 5 Aarhus University Hospital, Department of Oncology, Aarhus, Denmark Purpose or Objective Radiotherapy techniques such as volumetric-modulated arc therapy (VMAT) and image-guided radiotherapy (IGRT) has increased treatment delivery precision, making it possible to apply smaller margins to reduce doses to healthy tissues. The purpose of this study was to evaluate the effect of PTV margin reduction on the dose to out-of-field organs, for radiotherapy of paediatric brain cancer. Radiotherapy of the brain is associated with a minimal extent of involuntary organ motion. Furthermore, paediatric brain cancer patients generally have a long expected lifetime after radiotherapy, therefore any risk of radiotherapy-induced secondary cancer needs to be carefully evaluated and minimized, if possible. Material and Methods Thermoluminescence dosimeters (TLD-700) were used to measure out-of-field doses to the thyroid, mamma and testes of two CIRS ATOM anthropomorphic phantoms resembling a 1-year and 10–year old child. Coplanar double-arc VMAT plans (6 MV x-rays) were made with the target situated in the superior fossa. For each phantom, plans were made for two different PTV margins (5 mm and 3 mm), for two spherical PTVs of different size (2 cm and 5 cm diameter). Each treatment plan should deliver a total dose of 54 Gy to the PTV in 30 fractions, using a Varian Truebeam linear accelerator. For each phantom and PTV size, the clinical equivalence of the treatment plans for PTV margins of 5 mm and 3 mm was validated by comparing the plan conformity index, ranging between 1,174 and 1,289. TLDs were calibrated using a Sr-90 source prior to the measurements. TLDs were placed on the surface of the phantoms, above the investigated organs, and the resulting measurements could therefore only be used to quantify relative comparisons between PTV margins. The out-of-field surface dose was measured for one treatment fraction repeated three times, each measurement using two TLDs at each investigated organ. Results A decrease in dose to the thyroid (3-12 %) and mamma (6- 12 %) was observed with reduced PTV margin (5 mm to 3

Table 2: Dose results of both techniques, and dose difference (A-P VMAT minus VMAT). N=10.

A-P VMAT

Objective VMAT

Difference

Structure

1 PTV 5030 cGy D95%

98.1% 97.2% - 0.9%

2 PTV 6800 cGy D95%

98.4% 98.7% +0.3%

41.2 Gy 28.8 Gy 19.2 Gy 23.7 Gy 39.5 Gy

36.7 Gy 21.5 Gy 17.7 Gy 21.8 Gy 36.5 Gy

3 Spinal Canal

Max Dose

-4.5 Gy

4 Brainstem

Max Dose

-7.3 Gy

Mean Dose Mean Dose Mean Dose

Parotid Gland Left Parotid Gland Right Submandibular Gland Left

-1.5 Gy

5

-1.9 Gy

6

7

-3.0 Gy

48.4 43.4 -5.0 Gy

8 Submandibular Mean