This work investigates advanced mixed beam radiation therapy (MBRT) of breast and head & neck cancer using energy and intensity modulated electron (MERT) and photon (IMRT) beams.
Materials/Methods
The advanced MBRT system consists of novel treatment planning software combined with multileaf collimators for both photon and electron beam modulation. The planning software is Monte Carlo based for dose calculation, treatment optimization, and beam delivery. Efficient and accurate beam delivery for advanced MERT is achieved using an electron-specific multileaf collimator (eMLC), which is an add-on device to existing clinical accelerators. This technique is being implemented clinically for breast and head & neck treatment through pilot studies and clinical trials that are specially designed for dose escalation and hypofractionation.
Results
MBRT uses IMRT to achieve lateral dose conformity and MERT for conformity in the depth direction, which provides excellent target coverage for treatments involving shallow target volumes such as breast and head & neck. Our preliminary results in 75 patients treated with hypofractionation and incorporated boost showed that acute grade II skin complications were comparable or lower than standard fractionation. The whole breast received 20 fractions of 2.25 Gy and the tumor bed an additional 0.55 Gy/day concurrent electron boost. The elimination of 10% hot spots in the whole breast volume ensures the whole breast dose to be under 2.5 Gy beyond which significant skin complications and long-term fibrosis have been reported in the literature. Optimized MERT dose distributions are significantly more conformal to the tumor bed. These conformal dose distributions are also achievable for coverage of superficial lymph nodes in head & neck cancer or when partial breast radiation limited to the tumor bed is clinically indicated.
Conclusions
A set of software and hardware tools have been developed for conformal radiation therapy of shallow targets with much improved target dose conformity and uniformity, adequate skin coverage/avoidance and significant reduction in the dose to the adjacent normal tissues. These techniques allowed for advanced dose fractionation schemes and dose escalation aiming at improved local control and/or reduced normal tissue complications. The availability of a motorized eMLC (see figure) allows for efficient and effective beam delivery for advanced MERT that added new dimensions in external beam radiotherapy for shallow tumors. Future applications of MBRT for breast and head & neck cancer include protocols using hypofractionation or incorporated boost radiation or partial breast radiation.
Author Disclosure: C. Ma, None; J. Li, None; L. Jin, None; A. ElDib, None; J. Fan, None; R.A. Price, None; G. Freedman, None; P. Anderson, None; N. Nicolaou, None.
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