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Feasibility of Extreme Dose Escalation for Glioblastoma Multiforme Using 4Ï€ Radiotherapy

D Nguyen

D Nguyen*, J Rwigema , V Yu , T Kaprealian , P Kupelian , M Selch , D Low , K Sheng , Department of Radiation Oncology, UCLA, Los Angeles, CA


SU-E-T-183 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose:GBM recurrence primarily occurs inside or near the high-dose radiation field of original tumor site requiring greater than 100 Gy to significantly improve local control. We utilize 4Ï€ non-coplanar radiotherapy to test the feasibility of planning target volume (PTV) margin expansions or extreme dose escalations without incurring additional radiation toxicities.

Methods:11 GBM patients treated with VMAT to a prescription dose of 59.4 Gy or 60 Gy were replanned with 4Ï€. Original VMAT plans were created with 2 to 4 coplanar or non-coplanar arcs using 3 mm hi-res MLC. The 4Ï€ optimization, using 5 mm MLC, selected and inverse optimized 30 beams from a candidate pool of 1162 beams evenly distributed through 4Ï€ steradians. 4Ï€ plans were first compared to clinical plans using the same prescription dose. Two more studies were then performed to respectively escalate the GTV and PTV doses to 100 Gy, followed by a fourth plan expanding the PTV by 5 mm and maintaining the prescription dose.

Results:The standard 4Ï€ plan significantly reduced (p<0.01) max and mean doses to critical structures by a range of 47.0-98.4% and 61.0-99.2%, respectively. The high dose PTV/high dose GTV/expanded PTV studies showed a reduction (p<0.05) or unchanged* (p>0.05) maximum dose of 72.1%/86.7%/77.1% (chiasm), 7.2%*/27.7%*/30.7% (brainstem), 39.8%*/84.2%/51.9%* (spinal cord), 69.0%/87.0%/66.9% (L eye), 76.2%/88.1%/84.1% (R eye), 95.0%/98.6%/97.5% (L lens), 93.9%/98.8%/97.6% (R lens), 74.3%/88.5%/72.4% (L optical nerve), 80.4%/91.3%/75.7% (R optical nerve), 64.8%/84.2%/44.9%* (L cochlea), and 85.2%/93.0%/78.0% (R cochlea), respectively. V30 and V36 for both brain and (brain - PTV) were reduced for all cases except the high dose PTV plan. PTV dose coverage increased for all 4Ï€ plans.

Conclusion:Extreme dose escalation or further margin expansion is achievable using 4Ï€, maintaining or reducing OAR doses. This study indicates that clinical trials employing 4Ï€ delivery using prescription doses up to 100 Gy are feasible.

Funding Support, Disclosures, and Conflict of Interest: Funding support partially contributed by Varian

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