Estimated limits of IMRT dose escalation using varied planning target volume margins

Abstract

To estimate the limits of dose escalation for prostate cancer as a function of planning target volume (PTV) margins, the maximum achievable dose (MAD) was determined through iterative plan optimizations from data sets of 18 patients until the dose constraints for rectum, bladder and PTV could no longer be met. PTV margins of 10, 5 and 3 mm yielded a mean MAD of 83.0 Gy (range, 73.8-108.0 Gy), 113.1 Gy (range, 90.0-151.2 Gy) and 135.9 Gy (range, 102.6-189.0 Gy), respectively. All comparisons of MAD among margin groups were statistically significant (P < 0.001). Comparison of prostate volumes of 30-50 mL (n = 8) with volumes of 51-70 mL (n = 7) and 71-105 mL (n = 3) showed an inverse relationship with MAD. Decreases in PTV margin significantly decreased the PTV overlap of the rectum (P < 0.001 for all margin comparisons). With decreases in the PTV margin and maintenance of identical dose constraints, doses well above those currently prescribed for treatment of localized prostate cancer appear feasible. However, the dose escalation suggested by these findings is a theoretical estimate, and additional dose constraints will likely be necessary to limit toxicity to normal tissue.

Comparison of the effectiveness of different immobilization systems in different body regions using daily megavoltage CT in helical tomotherapy

“…This will facilitate dose escalation and better tumour control. 17,18 It is expected that the magnitudes of the CTV/PTV margin will follow a similar pattern as the set-up deviations obtained from this study.…”

Determination of the photon beam attenuation by the BrainLAB imaging couch: angular and field size dependence

“…However, to limit normal tissue toxicity, treatment margins have to be decreased. ( 2 , 3 ) In order to reduce treatment margin, the issue of organ motion needed to be addressed, so that the location of the target could be accurately determined during treatment. Target motion, for example, can cause a geometric miss in dose delivery; (4) hence, accurate radiation therapy requires knowledge of the exact location of the tumor at the time of treatment.…”

A predictive model to guide management of the overlap region between target volume and organs at risk in prostate cancer volumetric modulated arc therapy

“…In prostate cancer radiotherapy, the degree of overlap between the rectum and PTV has been shown to be the most important predictor of benefit from IMRT as compared to 3D-CRT [1,2,3], determines the limits of dose escalation possible [6,24], and can predict the degree of rectal sparing after hydrogel injection [18]. Similarly, in this study we have shown that the degree of overlap between PTV and bladder or rectum adversely effects sparing of that organ not only in the high-dose region of the dose-volume histogram, but also in the low-dose region as well.…”

“…Volumetric-modulated arc therapy (VMAT), the next generation of IMRT, allows for variation in the dose rate, speed of gantry rotation, and multi-leaf positions during rotation of the gantry in a full 360 degree arc, and has been shown to yield even further OAR sparing as compared to fixed-beam IMRT in prostate cancer [4,5]. However, regardless of the treatment delivery technique, if a large amount of overlap between PTV and OAR exist it may be physically impossible to achieve uniform coverage of the PTV with the prescription dose while also adequately sparing that OAR [6]. …”