ESTRO 2024 - Abstract Book

S4802

Physics - Quality assurance and auditing

ESTRO 2024

were carefully optimized so as not to have any cold spots. HFS plans were treated first using VMAT while the FFS plan are treated with 3DCRT technique using appropriate on-board images acquired during setup verification. 1 To validate the uncertainty in the delivered dose, the treatment plan was recalculated applying the actual shifts for each of the eight fractions. Sum of these 8 plans were considered as delivered plan. 2 The planned dose and the delivered dose were compared for the targets, Organs at risk (OARs), using various dosimetric parameters. OARs included the eyes, parotids, lungs, heart, kidneys, and bowels. Dose prescription was 13.2Gy/8# delivered twice daily. Plans were generated to achieve the ALARA doses to the OAR’s without compromising the doses to the target volume. Plans were generated using 8 overlapping 360° arcs optimized simultaneously using TrueBeam Linac using Eclipse treatment planning system .6MV photon beam at dose rate 600MU/min was used. The isocenters were placed at the overlapping area(5cm) between two adjacent arcs. The plans were validated using two different techniques. Lithium fluoride thermoluminescent dosimeters (LiF TLD), Ionization chambers and Arc CHECK phantom (M/S SUN NUCLEAR CORPORATION) . TLD and Ionization chambers were used for point dose measurements. Arc CHECK phantom was used for planar dose verification. Dose distributions were analysed using gamma criteria of 3% dose and 3 mm distance to agreement.

Results:

Percentage variation in point dose measurements with Ionization chambers and TLDs were found to be within 5% of the TPS calculated dose. Planer dose verification was within 95% of gamma index with 3%-3 mm criteria. In both the treatment plan and delivered plan, volume of PTV encompassed by 90 % Isodose line was 92.3 % of PTV volume and D2 (mean dose of 2cc volume of PTV) was 109% of prescribed dose.There was no difference between the planned and delivered dose to OARs viz Kidney, lungs, heart, and the bowels whereas the dose to eyes and oral cavity was slightly higher in the planned dose as compared to the delivered dose (~0.6 Gy). Even the Conformity index and Homogeneity index to the target volume were similar in the planned and the delivered dose. Maximum dose to body (D2 cc) at the junction area of HFS and FFS reduced from 123% to 114% in delivered plan from actual TPS plan.

Conclusion:

Favourable dosimetry of VMAT based TMLI and its validation using two independent methods makes this technique robust enough to potentially replace the conventional TBI technique with adequate PTV coverage and superior OAR sparing effect. Planned dose and delivered dose were comparable. Although, proper setup and feathering are essential to take care of the junctional area between the upper TMLI and the lower TMLI, one needs to be vigilant of it to properly implement this complex technique.

Keywords: TMLI,VMAT,Dose validation

References:

1.Godson HF, Raj JS, Sebastian P, Ponmalar RY, Babu ES, Paul I, Krishna R, Backianathan S, George B, Ravindran PB, Balakrishnan R. Feasibility study of total marrow lymphoid irradiation with volumetric modulated arc therapy: clinical implementation in a tertiary care center. Strahlenther Onkol. 2023 Oct;199(10):922-935. doi: 10.1007/s00066-023 02100-x. Epub 2023 Jun 6. PMID: 37278833.