Purpose
To evaluate the pattern of failure after limited margin radiotherapy for glioblastoma.
Methods and Materials
We analyzed 62 consecutive patients with newly diagnosed glioblastoma treated between 2006 and 2008 with standard fractionation to a total dose of 60Gy with concurrent temozolomide (97 % ) or arsenic trioxide (3 % ). The initial clinical target volume included postoperative T2 abnormality with a median margin of 0.7cm. The boost clinical target volume included residual T1-enhancing tumor and resection cavity with a median margin of 0.5cm. Planning target volumes added a 0.3- or 0.5-cm margin to clinical target volumes. The total boost planning target volume (PTVboost) margin was 1cm or less in 92 % of patients. The volume of recurrent tumor (new T1 enhancement) was categorized by the percent within the 60-Gy isodose line as central (>95 % ), infield (81–95 % ), marginal (20–80 % ), or distant (<20 % ). For comparison, an initial planning target volume with a 2-cm margin and PTVboost with a 2.5-cm margin were created for each patient.
Results
With a median follow-up of 12 months, radiographic tumor progression developed in 43 of 62 patients. Imaging was available for analysis in 41: 38 (93 % ) had central or infield failure, 2 (5 % ) had marginal failure, and 1 (2 % ) had distant failure relative to the 60-Gy isodose line. The treated PTVboost (median, 140cm3) was, on average, 70 % less than the PTVboost with a 2.5-cm margin (median, 477cm3) (p < 0.001).
Conclusions
A PTVboost margin of 1cm or less did not appear to increase the risk of marginal and/or distant tumor failures compared with other published series. With careful radiation planning and delivery, it appears that treatment margins for glioblastoma can be reduced.
