Optimal matchline blocking and matchline dosimetry for lymphoma patients

Patient setup and geometrical factors can potentially lead to excessive spinal cord doses when treating lymphoma patients with matched mantle and para-aortic (PAN) fields. To ensure cord protection, partial transmission blocks are often used at the matchline. This study evaluates optimal placement of these blocks. Three patients were planned using a variety of mantle/PAN field geometries, including significantly different field lengths and isocenter vertical positions. Field matching in all cases was based on a standard gap calculation. Half-transmission blocks (HTB) were positioned on (a) both posterior fields, (b) both anterior fields, and (c) the posterior PAN field only. Doses were calculated with and without effects of setup uncertainty. HTB on both posterior fields yielded a lower maximum cord dose but slightly poorer target coverage than HTB on both anterior fields. HTB on the posterior PAN field alone resulted in a maximum cord dose similar to HTB on both anterior fields. Setup uncertainty decreased but did not eliminate the differences between block placement methods. HTB of sufficient size on both posterior fields ensures a maximum cord dose less than the prescription without significant compromise of target coverage.