Dosimetric Analysis of Planning Target Volume Margins and Prescription Isodose Levels for the Treatment Planning of Stereotactic Radiosurgery Treatments for Single Brain Metastatic Tumor: A Phantom-Based Study

Abstract

The purpose of this study was to evaluate the influence of the choice of planning target volume margins and prescription isodose levels on the dosimetry indices used to assess the quality of treatment plans of stereotactic radiosurgery treatments for single brain metastatic tumor on both the Gamma Knife and LINAC-based Systems SRS, respectively using the STEEV Phantom. The research aim was further developed into five specific objectives. Computed Tomography scan images of the STEEV anthropomorphic phantom were transferred into the GAMMA PLAN Treatment planning System at Queen Square Gamma Knife Unit of the University College London Hospital, UK. A target measuring 4.9 cm3 was contoured centrally with organs at risk. Plans without a planning target volume margin for five prescription isodose levels from 50% to 70% at 5% increments were created. Afterwards, the Planning target volume margin was adjusted to 0.5, 1, 1.5, and 2mm, and identical plans having the same prescription isodose levels as the previous ones were generated. Adjustments were made to each plan to achieve same target coverage. One-way ANOVA test was used to analyse the influence of planning target volume margins on the plan quality dosimetric indices whilst organs at risk were evaluated in terms of maximum and minimum doses. The CT images of the STEEV Phantom with contours were later transferred into the Eclipse treatment planning system at the Centro riferimento di Oncologico (CRO), in Aviano Italy where the impact of a set of prescription isodose levels (50, 55, 60, 65, 70, and 80%) were assessed using commonly used planning target margins (0, 1, and 2 mm) in SRS planning of brain metastasis and compared against constraints used at CRO. The accuracy of this set of plans was further evaluated using the new wireless delta 4 phantom+ with a gamma-index passing rate of 2 mm/2% (distance to agreement/dose deviation). The opinions of planning medical physicists with experience in SRS treatment planning on margin additions in the SRS treatment of single brain metastasis were also collected through a simple survey made up of five questions. The last objective was done by exploring the geographical distribution of radiotherapy machines for brain metastasis treatment in Africa from the DIRAC database and relevant literature and some guidelines/models for the implementations of the SRS program developed. The findings indicated that, increases in PTV margins target expansion for Gamma knife radiosurgery though a relatively novel concept, it influences all dosimetric parameters as well as organs at risk. The PTV margin of 2.0 mm exhibited the highest mean selectivity of (0.93 ± 0.00), PCI (0.92 ± 0.01), GI (2.50 ± 0 .04), V12 (16.17 ± 0.38) and treatment time (118.32 ±2.91 min). The 0.0 mm PTV margin had the lowest mean value for all the parameters except for the treatment time (105.58 ± 3.48 min) which was slightly higher compared to the 0.5 mm PTV margin (M= 86.36 ± 4.13 min). LINAC-based SRS planning reveals that using prescription isodose levels at 50 and 55% with 0 mm margins and 70 and 80% for higher margins like 1 mm or 2 mm can minimize dose to organs at risk while maintaining adequate tumour coverage. The 2 mm margin leads to increased exposure across all structures, particularly the brain stem and optic nerves at SRS80. This larger margin results in less sparing of critical structures. The calculated treatment planning system (TPS) dose and the measurement with the Delta4 phantom were in excellent agreement. The minimum gamma pass rate was 99.6% and the maximum 100%. The gamma passing rate above 95% for all plans and dose goals were achieved. In the survey, the response rate to whether or not to use margins in SRS, which was the most critical question, was 50%, thereby making no consensus to be achieved. In all of Africa, only two Gamma Knife machines were found. Two Cyberknifes; one in Egypt and one in Kenya and 432 other megavoltage units (366 LINAC s, 66 cobalt-60) distributed across the continent can be used for the treatment of brain metastasis although the level of cancer burden was found to be increasing. The GDP per capita of a nation was a significant predictor of the availability of these machines in countries, but it did not account for the heterogeneity in number. Based on the results of this study, planning target volume margins beyond 0 mm should not be used with the Gamma Knife, as they increase all dosimetric metrics associated with radionecrosis. For LINAC-based SRS, higher prescription isodose levels may be combined with larger margin sizes, but this approach requires careful evaluation. The expansion of radiotherapy through the use of linear accelerators suggests a promising future for the continent in enhancing its stereotactic radiosurgery treatment centres by adopting LINAC based SRS in accordance with appropriate guidelines. This will assist in delivering thorough care to individuals and enhance their overall well-being.