ESTRO 2021 Abstract Book

S1597

ESTRO 2021

PO-1875 Commissioning of an MR only dose planning paradigm for use with pelvic cancer patients S. McIlroy 1 , M. Berg 1 , H. Nissen 1 1 Sygehus Lillebælt, Oncology, Vejle, Denmark

Purpose or Objective MR-only based treatment planning minimizes total scan time and registration errors. As part of implementing a MR-only based workflow for curative pelvic RT, a study was undertaken to validate the dosimetric calculations on the MR derived simulated CT (s-CT) for rectal, anal and prostate cancers including irradiation of full lymph node regions. The aim was to determine if VMAT treatment plans optimized on s-CT differs from plans optimized on regular CT scans. Materials and Methods 23 patients encompassing prostate, rectum, and anal cancers were CT- and MR scanned as part of standard treatment planning. From a commercially available MR-only system (MRCAT prostate/MRCAT general pelvis, Philips, the Netherlands), s-CT’s were created. Treatment plans were generated on both CT and s-CT with a dual 6MV arc. Dose from each generated plan was then recalculated on the opposite imaging modality. To avoid differences in radiological depth between the two imaging modalities related to patient repositioning between scans, the external contour of the s-CT along with density correction ROI’s were used as the external for both image sets. The reported values are dose differences between the calculated dose on the s-CT and the CT (Ds-CT – DCT). For CTV mean dose and minimum dose (D100%) was used. For PTV D99% and D98% was used. Differences were evaluated using the Kolmogorov-Smirnov (K-S) test for two samples, with one sample being the differences for parameters listed above for the plan generated on the s-CT then recalculated on the CT, and the other sample being the corresponding values for plan generated on the CT then recalculated on the s-CT. Statistical significance was set at p < 0.05. Results Mean and standard deviation of the dose differences are listed in table 1. Differences are small compared to actual target doses, which are in the range of 50,4–78 Gy. Scaling individual plans to prescribed dose (data not shown) the difference is ~0.5%. This shows good agreement between the accuracy of the dose calculation on both CT’s, and is not a clinically relevant difference based on the prescription dose between 50.4 Gy and 78 Gy. Testing for differences between plans optimized on either CT or s-CT yields p > 0.24 for all parameters meaning plan evaluation parameters are insensitive to whether the plan stems from CT or s-CT. CT s-CT mean stdev mean stdev CTV_tumor Mean (Gy) 0.32 0.2 0.33 0.2 PTV_tumor Mean (Gy) 0.31 0.2 0.33 0.2

CTV_lymphnodes Mean (Gy) .026 0.1 0.23 0.2 PTV_lymphnodes Mean (Gy) .025 0.1 0.24 0.1 PTV_tumor D99% (Gy) 0.37 0.4 0.37 0.3 PTV_lymphnodes D98% (Gy) 0.27 0.2 0.28 0.2 Table 1: Mean and standard deviation for selected target dose parameter differences.

Conclusion Conclusion

MR-only based treatment planning for pelvic RT shows a higher calculated dose on the s-CT compared to regular CT, but we consider the difference clinically irrelevant given prescribed doses of 50-78 Gy. Target dose parameters showed no dependence on treatment plan imaging modality.

PO-1876 Procedure for total body irradiation (TBI) with Helical Tomotherapy C. Ferrer 1 , C. Huertas 1 , R. Plaza 1 , P. de la Monja 1 , C. Mínguez 1 , D. García 1 , A. Aznar 1 , M. Sáez 1 1 H.U. La Paz, Medical Physics Department, Madrid, Spain Purpose or Objective The procedure for performing a total body irradiation (TBI) treatment in a tomotherapy unit is described. Materials and Methods Three TBI treatments were performed with TomoHD™ tomotherapy equipment. The immobilization consists of two indexed moldcare couches and a thermoplastic mask. Due to limitations in the movement of the table the whole body image is acquired in two series, a head gantry CT and a feet to gantry CT. In addition, a 4D CT is performed to contour the lungs. All with a slice thickness of 5mm. In both series, the target volume is the external contour of the patient, expanded by 5 mm to ensure that the skin receives the prescribed dose (Dp). It is contoured to the overlapping area of both plans, where 5 gradient volumes of 2 cm thickness are generated. Between successive gradient volumes the dose is reduced by 2 Gy, so that positioning errors do not lead to large dose variations. In the region prior to the gradient, an overdosage is generated by adding up the plans. To avoid this, 2 to 4 cm of the patient's contour are prescribed at 95% Dp. The lungs are delimited on the MIP image and reduced by 1 cm in all directions allowing a gradient zone from the costal wall, which is also contoured to verify that it is not underdosed. The treatment schedule is 12 Gy in 6 fractions, twice a day, separated by more than 6 hours. The treatment goals are at least 95% volume coverage at 95% Dp , dose as homogeneous as possible and maximum lung dose of 10 Gy and minimum of 7 Gy. The length of both upper and lower plans should be less than 120-126cm due to limitations of table movement and immobilization indexations. The parameters of the upper plan are: field width=5 cm, pitch=0.287, modulation factor=2 till 3. The parameters of the lower plan are: field width=5cm, pitch=4, modulation factor=2.

Both plans are verified with ArcCheck, the gamma criterion (3 %, 3 mm) is applied. The overlap region is verified with both ArcCheck and radiochromic, both plans are irradiated consecutively by rotating the equipment. During treatment,