Craig McKenzie

A dosimetric comparison of intensity-modulated proton therapy optimization techniques for pediatric craniopharyngiomas: a clinical case study.

To evaluate the dosimetric characteristics of intensity‐modulated proton therapy (IMPT) optimization techniques and pencil‐beam scanning (PBS) nozzle designs on pediatric craniopharyngiomas.

Image guidance based on prostate position for prostate cancer proton therapy.

PURPOSE To determine the target coverage for proton therapy with and without image guidance and daily prebeam reorientation. METHODS AND MATERIALS A total of 207 prostate positions were analyzed for 9 prostate cancer patients treated using our low-risk prostate proton therapy protocol (University of Florida Proton Therapy Institute 001). The planning target volume was defined as the prostate plus a 5-mm axial and 8-mm superoinferior extension. The prostate was repositioned using 5- and 10-mm shifts (anteriorly, inferiorly, posteriorly, and superiorly) and for Points A-D using a combination of 10-mm multidimensional movements (anteriorly or inferiorly; posteriorly or superiorly; and left or right). The beams were then realigned using the new prostate position. The prescription dose was 78 Gray equivalent (GE) to 95% of the planning target volume. RESULTS For small movements in the anterior, inferior, and posterior directions within the planning target volume (< or =5 mm), treatment realignment demonstrated small, but significant, improvements in the clinical target volume (CTV) coverage to the prescribed dose (78 GE). The anterior and posterior shifts also significantly increased the minimal CTV dose (Delta +1.59 GE). For prostate 10-mm movements in the inferior, posterior, and superior directions, the beam realignment produced larger and significant improvements for both the CTV V(78) (Delta +6.4%) and the CTV minimal dose (Delta +8.22 GE). For the compounded 10-mm multidimensional shifts, realignment significantly improved the CTV V(78) (Delta +11.8%) and CTV minimal dose (Delta +23.6 GE). After realignment, the CTV minimal dose was >76.6 GE (>98%) for all points (A-D). CONCLUSION Proton beam realignment after target shift will enhance CTV coverage for different prostate positions.

PROTON THERAPY COVERAGE FOR PROSTATE CANCER TREATMENT

PURPOSE To determine the impact of prostate motion on dose coverage in proton therapy. METHODS AND MATERIALS A total of 120 prostate positions were analyzed on 10 treatment plans for 10 prostate patients treated using our low-risk proton therapy prostate protocol (University of Florida Proton Therapy Institute 001). Computed tomography and magnetic resonance imaging T(2)-weighted turbo spin-echo scans were registered for all cases. The planning target volume included the prostate with a 5-mm axial and 8-mm superoinferior expansion. The prostate was repositioned using 5- and 10-mm one-dimensional vectors and 10-mm multidimensional vectors (Points A-D). The beam was realigned for the 5- and 10-mm displacements. The prescription dose was 78 Gy equivalent (GE). RESULTS The mean percentage of rectum receiving 70 Gy (V(70)) was 7.9%, the bladder V(70) was 14.0%, and the femoral head/neck V(50) was 0.1%, and the mean pelvic dose was 4.6 GE. The percentage of prostate receiving 78 Gy (V(78)) with the 5-mm movements changed by -0.2% (range, 0.006-0.5%, p > 0.7). However, the prostate V(78) after a 10-mm displacement changed significantly (p < 0.003) with different movements: 3.4% (superior), -5.6% (inferior), and -10.2% (posterior). The corresponding minimal doses were also reduced: 4.5 GE, -4.7 GE, and -11.7 GE (p < or = 0.003). For displacement points A-D, the clinical target volume V(78) coverage had a large and significant reduction of 17.4% (range, 13.5-17.4%, p < 0.001) in V(78) coverage of the clinical target volume. The minimal prostate dose was reduced 33% (25.8 GE), on average, for Points A-D. The prostate minimal dose improved from 69.3 GE to 78.2 GE (p < 0.001) with realignment for 10-mm movements. CONCLUSION The good dose coverage and low normal doses achieved for the initial plan was maintained with movements of < or = 5 mm. Beam realignment improved coverage for 10-mm displacements.

SU-GG-T-532: Dosimetric Comparison of Proton Delivery Techniques: Double-Scattering and Uniform-Scanning

Purpose: To compare proton dose distributions generated with double‐scattering to uniform‐scanning for different clinical sites. Method and Materials: The ‘universal nozzle’ developed by IBA incorporates several delivery modes. In double‐scattering (DS) a flattening filter scatters the proton beam into a flat circular profile. In uniform‐scanning(US) two dipole magnets scan the beam into a rectangular profile. US covers larger volumes both laterally and in depth, and has dosimetric characteristics that are different from US. This study deals with cases that can be treated with either US or DS. Eclipse (Varian) treatment planning is commissioned for both delivery modes. Comparison of water‐phantom calculations to measurements validates the treatment‐planning algorithm. We compare the dose for the following sites: prostate (2 cases), head‐and‐neck (4 cases), cranio‐spinal (3 cases). Dose‐volume‐histograms are used to evaluate target coverage and dose to critical structures. Results: The US in‐air penumbra is typically smaller because of less scattering material in the beam path. For a range of 15.0g/cm2, modulation of 8.0g/cm2, and air gap of 12.0cm, the 80%–20% penumbra at 11.0cm depth is 4.4mm in US and 6.9mm in DS. In addition, the US distal fall‐off is sharper because of reduced energy straggling in the treatment head. For a range of 5.0 (28.0) cm in water the 80%–20% fall‐off is 2.7mm (5.5mm) in US, compared to 4.0mm (6.0mm) in DS. For deep seated tumors (prostate) the sharper in‐air dose distribution in US is washed out by in‐patient scatter, resulting in no significant benefit. For targets at shallow and intermediate depth, located next to a critical structure, the sharper fall‐off in US allows for better target coverage and less dose to the critical structure. Conclusion: The sharper lateral and distal penumbra in uniform scanning are beneficial when the target volume abuts a critical structure. For deep‐seated tumors this advantage diminishes.