IMRT FOR CRANIOSPINAL IRRADIATION: CHALLENGES AND RESULTS A. Miller, L. Kasulaitytė Institute of Oncolygy, Vilnius University
Content 1.Introduction 2.Methods and materials 3.Results and discussion 4.Conclusion
1.Introduction Craniospinal irradiation is very important part in the treatment of patients with primitive neuroectodermal tumors such as medulloblastoma. Craniospinal irradiation (CSI) is technically very challenging because of the difficult geometry of the treatment fields, high radiosensitivity of the spinal cord and other critical structures around.
Introduction (cont) The mechanical properties of available equipment make possible treatment only with treatment field junctions and several isocenters. Thus causing potential problems under/over dosage of the spinal cord in the areas of field junction and over dosage of adjacent structures. IMRT technique can be used in craniospinal and extra-cranial tumor treatment due to better dose coverage, more homogenous dose distribution and fewer doses to adjacent normal tissues.
1.1 Aim of the study The aim of our study was to develop IMRT technique for CS irradiation, to measure patient set-up errors during treatment and to compare possible affection of these uncertainties to the dose statistics of 3D CRT and IMRT treatment plans.
2. Methods and materials Two young patients (age 31-35 years) with medulloblastomas were treated between 2009-2010. One patient had craniospinal treatment administered, and the second only extracranial (spinal) treatment. A dose of 32,4 Gy was prescribed to the PTV with 18 fractions. The PTV was created by adding 1,0 cm margin to CTV in all directions. Outlined organs at risk (OAR) included eyes, lenses and kidneys.
2.1 Treatment plans There were three plans generated for each patient - two 3D CRT and one IMRT treatment plans. Two 3DCRT plans had differences in field junction. First plan had field junction with 3 mm gaps in cranial-spinal area and spinal-spinal area. It was set on even days. Second treatment plan had no gaps (exact match) in cranial-spinal and spinal-spinal junction areas, and it was set on odd days.
3D CRT treatment plan consisted of 2 opposing cranial fields for craniospinal case and three spinal fields one upper and two lower fields. All spinal treatment fields had couch rotation to 90 degrees. Extracranial treatment plan consisted of tree spinal treatment fields.
IMRT treatment plan consisted of 2 cranial fields and 6 spinal fields. No spinal field had couch rotation.
2.2 Treatment plan verification Patient related IMRT QA was performed before treatment. Verification plan created within the treatment planning system with portal dose prediction algorithm was verified on linac portal imaging system. During the treatment on-line verification was performed before every session, and subsequently required adjustment of the patient position was made before plan delivery.
3.Results and discussion The IMRT and 3D CRT treatment plans comparison was performed and it revealed the apparent advantage of IMRT plan in terms of dose distribution homogeneity in the target volume.
3.1 Dose distribution homogeneity Extracranial 3D CRT treatment plan.
3.2 Dose distribution homogeneity Extracranial (spinal) and CSI IMRT treatment cases.
3.3 Set up verification It has been proven that IMRT method, rightfully applied, provide noticeably better dose distribution for targets and critical organs for many different sites. It is very important to demonstrate the IMRT plan stability and robustness to every day variation in patient position.
Therefore, the data of on-line patient setup errors was analyzed. Set up verification (cont)
3.4 Evaluation of set up verification Maximum setup error was found to be in our case 3mm in LNG and VRT directions. Influence of these maximum Original plan Cranial isocenter 3 mm lateral shift 3mm vertical shift Upper spinal isocenter 3 mm lateral shift 3 mm vertical shift Lower spinal isocenter 3 mm lateral shift 3 mm vertical shift Max dose Mean dose 108% 96 % 111,3% 99% 120% 94,6% 109,7% 94% 108,4% 94,6% 109,5% 93% 107,5% 94,3%
Evaluation of set up verification (cont) Simulated change in isocenter position for 3D CRT plans in order of 3mm magnitude gives rise of up to 147% maximum dose in the junction area. Additionally, 3D CRT plans might have more than 3 mm isocenter move as soon as field setup involves table rotation to 90-degree angle.
3.5 Whole body dose This issue is of paramount importance in case of medulloblastoma treatments, because of young patient age and relatively good prognosis. The dose to the normal surrounding tissues strongly depends on the number of fields involved and because of the interleaf leakage, partially on the number of monitor units for the particular plan. It is possible to produce sufficiently good IMRT plan with number of
Dose to the surrounding tissue is in all cases much better than for the 3D CRT plans. Whole body dose (cont)
Whole body dose (cont) Thought, the body dose from 1 to approximately 5 Gy is slightly lower for 3D CRT plan, the difference is small and the risk of secondary cancer incidence in this dose range is still unclear. The difference in body dose up from this point, however, is around two fold in favor of IMRT plan. The risk of secondary cancer development in this dose range (up to 4 cm from the field edges) is well known and depends strongly on the dose-volume parameter
4. Conclusions The target volume of medulloblastoma is usually very complex. Due to target long shape and different distance from skin surface it is possible to achieve homogenous dose distribution in the target only using IMRT treatment technique. IMRT technique also enables to ensure good treatment accuracy as this technique seems to be less sensitive to patient set-up uncertainties and there is no need for couch
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