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Purpose Day-to-day shape variation in the rectum CTV results in considerable geometric uncertainties during rectal cancer radiotherapy. To ensure coverage a large CTV-to-PTV margin is required. The purpose of this study was to increase the accuracy of treatment delivery by building a population based library of planning CTVs for rectal cancer patients and to evaluate its potential for rectum PTV margin and PTV volume reduction. Methods Analysis was done retrospectively on 33 early-stage rectal cancer patients with daily repeat CTs who received short-course pre-operative radiotherapy in 5 fractions of 5 Gy. We created signed distance maps from the planning rectum CTV to each of the repeat CTVs, from which we calculated the group mean, systematic and random error. The correlation between different regions of the rectum CTV was analyzed and used in combination with the distance maps to create the library of nine planning CTVs. For each of the repeat CTVs the best fitting CTV structure in the library was automatically selected defined by the plan that minimized the mean absolute distance between the repeat and library CTV. Residual distance maps were calculated from which a new PTV margin was constructed. Bootstrapping was performed on the margin difference to assess its significance. Results Residual errors were found to decrease with the number of plans in the library, but adding more than five plans yields negligible further error reduction. Margin reduction of up to 50% was achieved at the upper-anterior site of the mesorectum. The average PTV volume decreased by 15.5% when a library is introduced. Conclusions A library of plans strategy for rectal cancer based on population statistics is feasible and results in a considerably reduced average rectum PTV volume compared to conventional radiotherapy.

Purpose: MRI guidance during radiotherapy has the potential to enable more accurate dose delivery, optimizing the balance between local control and treatment related toxicity. However, the presence of a permanent magnetic field influences the dose delivery, especially around air cavities. Here, electrons are able to return to the surface through which they entered the air cavity (electron return effect, ERE) locally resulting in dose hot- and cold-spots. Where RT of rectal cancer patients might benefit from MRI guidance for margin reduction, air cavities in and around the target volume are frequently present. The purpose of this research is to evaluate the impact of the presence of a 1.5 T transverse magnetic field on dose delivery in patients with rectal cancer. Methods: Ten patients treated with 5 × 5 Gy RT having large changes in pelvic air content were selected out of a cohort of 33 patients. On the planning CT, a 1.5 T, 6 MV, 7-field intensity modulated radiotherapy (IMRT) plan was created. This plan was subsequently recalculated on daily CT scans. For each daily CT, the CTV V{sub 95%} and V{sub 107%} and bowel area V{sub 5Gy}, V{sub 10Gy}, V{sub 15Gy}, V{sub 20Gy}, and V{sub 25Gy} were calculated tomore » evaluate the changes in dose distribution from fraction to fraction. For comparison, the authors repeated this procedure for the 0 T situation. To study the effect of changing air cavities separate from other anatomical changes, the authors also generated artificial air cavities in the CTV of one patient (2 and 5 cm diameter), in the high dose gradient region (2 cm), and in the low dose area (2 cm). Treatment plans were optimized without and with each simulated air cavity. For appearing and disappearing air cavities, the CTV V{sub 95%} and V{sub 107%} were evaluated. The authors also evaluated the ERE separate from attenuation changes locally around appearing gas pockets. Results: For the ten patients, at 1.5 T, the V{sub 95%} was influenced by both appearing and disappearing air, and dropped to

Purpose: Hydrogel injected between the rectum and prostate prior to radiotherapy provides a possible means of increased dose sparing to the rectum. Here the authors evaluate the overlap volume histogram (OVH) metric as a means to predict the rectal dose following hydrogel injection. Whether OVH predicted dose can serve as the dose objective or constraint for automated treatment planning was also investigated. Methods: Treatment planning was performed on 21 prostate cancer patients both pre- and posthydrogel injection, with five-field IMRT delivering 78 Gy to the planning target volume (PTV). The authors quantify the geometrical relationship between the rectum and the prostate PTV using an OVH metric which determines the fractional volume of the rectum that is within a specified distance of the PTV. For an OVH distance the authors selected, L20, the PTV expansion distance at which 20% of the rectum overlaps. The authors calculated the rectal dose, D20, received by 20% of the rectum volume on the dose volume histogram. Linear regression was used to examine the correlation between the L20 and D20, and between ΔL20 and ΔD20 (i.e., the change of L20 and D20 posthydrogel injection). Additionally, rectal dose D15, D25, D35, D50, and bladder dose D15 were predicted from the OVH (L15, L25, L35, L50, for rectum and L15 for bladder) by the Lx-Dx linear regression. The predicted doses were applied to the objectives for automated treatment planning of ten plans from five patients. Automatically generated plans were compared with plans manually generated on trial-and-error basis. Results: The rectal L20 was increased and dose D20 decreased due to the enlarged separation of rectum caused by the hydrogel injection. Linear regression showed an inverse linear correlation between L20 and D20, and between ΔL20 and ΔD20 (r2 = 0.77, 0.60, respectively; p < 0.0001). The increase in rectal sparing (ΔD20) is only weakly correlated with the volume of injected hydrogel (r2 = 0.17; p = 0.07), indicating OVH is a more predictive indicator of rectal sparing than the volume of hydrogel itself. Application of the predicted rectum and bladder doses to automated planning produced acceptable treatment plans, with rectal dose reduced for eight of ten plans. Conclusions: The OVH metric can predict the rectal dose in the external beam prostate radiotherapy for patients with hydrogel injection. The predicted doses can be applied to the objectives of optimization in automated treatment planning to produce acceptable treatment plans.