Radiation therapy for deep periocular cancer treatments when protons are unavailable: is combining electrons and orthovoltage therapy beneficial?

Deep periocular cancers can be difficult to plan and treat with radiation, given the difficulties in apposing bolus to skin, and the proximity to the retina and other optic structures. We sought to compare the combination of electrons and orthovoltage therapy (OBE) with existing modalities for these lesions. Four cases-a retro-orbital melanoma (Case 1) and basal cell carcinomas, extending across the eyelid (Case 2) or along the medial canthus (Cases 3-4)-were selected for comparison. In each case, radiotherapy plans for electron only, 70% electron and 30% orthovoltage (OBE), volumetric-modulated arc therapy (VMAT), conformal arc, and protons were compared. Dose-volume histograms for planning target volume coverage and selected organs at risk (OARs) were then calculated. The V90% coverage of the planning target volume was >98% for electrons, VMAT, conformal arc and proton plans and 90.2% and 89.5% in OBE plans for Cases 2 and 3, respectively. The retinal V80% was >98% in electron, VMAT and proton plans and 79.4%; and 87.1% in OBE and conformal arcs for Case 2 and 91.3%, 36.4%, 56.9%, 52.4% and 43.7% for Case 3 in electrons, OBE, VMAT, conformal arc and proton plans, respectively. Protons provided superior coverage, homogeneity and OAR sparing, compared with all other modalities. However, given its simplicity and widespread availability, OBE is a potential alternative treatment option for moderately deep lesions where bolus placement is difficult.

Assessing the deviation from the inverse square law for orthovoltage beams with closed-ended applicators.

In this report, we quantify the divergence from the inverse square law (ISL) of the beam output as a function of distance (standoff) from closed-ended applicators for a modern clinical orthovoltage unit. The divergence is clinically significant exceeding 3% at a 1.2 cm distance for 4 × 4 and 10 × 10 cm2 closed-ended applicators. For all investigated cases, the measured dose falloff is more rapid than that predicted by the ISL and, therefore, causes a systematic underdose when using the ISL for dose calculations at extended SSD. The observed divergence from the ISL in closed-ended applicators can be explained by the end-plate scattering contribution not accounted for in the ISL calculation. The standoff measurements were also compared to the predictions from a home-built kV dose computation algorithm, kVDoseCalc. The kVDoseCalc computation predicted a more rapid falloff with distance than observed experimentally. The computation and measurements agree to within 1.1% for standoff distances of 3 cm or less for 4 × 4 cm2 and 10 × 10 cm2 field sizes. The overall agreement is within 2.3% for all field sizes and standoff distances measured. No significant deviation from the ISL was observed for open-ended applicators for standoff distances up to 10 cm.

Development and evaluation of an ultrasound-guided tracking and gating system for hepatic radiotherapy.

PURPOSE: Respiratory motion must be accounted for daily in order to permit optimum radiotherapy of hepatic malignancies. However, existing tracking systems are often invasive or poorly tolerated by patients. The authors describe the development and validation of an ultrasound-guided tracking and gating system for stereotactic body radiation therapy of the liver. METHODS: This noninvasive system is designed to determine the correlation between tumor and external fiducial motion and to verify the optimum gating level for treatment delivery daily. A tracked ultrasound probe moves with patient respiration, obtaining 2D ultrasound images of tumor motion throughout the respiratory cycle. The target volume is registered to the static radiotherapy treatment beams in order to verify optimum gating levels. These gating levels are then transferred to an existing gating system for treatment delivery. The authors examined the temporal and spatial accuracy of this system using a custom-built phantom and verified the accuracy of gating level transfer and delivery. RESULTS: The temporal accuracy of the ultrasound-guided system was shown to be comparable to the existing clinical x-ray imaging system. Using ultrasound rather than x-rays to image internal targets provides good soft-tissue contrast without the invasiveness of implanting fiducial markers. High frame rates enable continuous monitoring of the target throughout the respiratory cycle. The authors anticipate this passive monitoring system should be well tolerated by patients. CONCLUSIONS: The system developed provides good quality video of the laboratory motion phantom and can be successfully used in gated beam delivery.